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	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 // SPDX-License-Identifier: GPL-2.0
 /*
  * INET     An implementation of the TCP/IP protocol suite for the LINUX
  *      operating system.  INET is implemented using the  BSD Socket
  *      interface as the means of communication with the user level.
  *
  *      Implementation of the Transmission Control Protocol(TCP).
  *
  * Authors: Ross Biro
  *      Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
  *      Mark Evans, <evansmp@uhura.aston.ac.uk>
  *      Corey Minyard <wf-rch!minyard@relay.EU.net>
  *      Florian La Roche, <flla@stud.uni-sb.de>
  *      Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
  *      Linus Torvalds, <torvalds@cs.helsinki.fi>
  *      Alan Cox, <gw4pts@gw4pts.ampr.org>
  *      Matthew Dillon, <dillon@apollo.west.oic.com>
  *      Arnt Gulbrandsen, <agulbra@nvg.unit.no>
  *      Jorge Cwik, <jorge@laser.satlink.net>
  */
 
 /*
  * Changes:
  *      Pedro Roque :   Fast Retransmit/Recovery.
  *                  Two receive queues.
  *                  Retransmit queue handled by TCP.
  *                  Better retransmit timer handling.
  *                  New congestion avoidance.
  *                  Header prediction.
  *                  Variable renaming.
  *
  *      Eric        :   Fast Retransmit.
  *      Randy Scott :   MSS option defines.
  *      Eric Schenk :   Fixes to slow start algorithm.
  *      Eric Schenk :   Yet another double ACK bug.
  *      Eric Schenk :   Delayed ACK bug fixes.
  *      Eric Schenk :   Floyd style fast retrans war avoidance.
  *      David S. Miller :   Don't allow zero congestion window.
  *      Eric Schenk :   Fix retransmitter so that it sends
  *                  next packet on ack of previous packet.
  *      Andi Kleen  :   Moved open_request checking here
  *                  and process RSTs for open_requests.
  *      Andi Kleen  :   Better prune_queue, and other fixes.
  *      Andrey Savochkin:   Fix RTT measurements in the presence of
  *                  timestamps.
  *      Andrey Savochkin:   Check sequence numbers correctly when
  *                  removing SACKs due to in sequence incoming
  *                  data segments.
  *      Andi Kleen:     Make sure we never ack data there is not
  *                  enough room for. Also make this condition
  *                  a fatal error if it might still happen.
  *      Andi Kleen:     Add tcp_measure_rcv_mss to make
  *                  connections with MSS<min(MTU,ann. MSS)
  *                  work without delayed acks.
  *      Andi Kleen:     Process packets with PSH set in the
  *                  fast path.
  *      J Hadi Salim:       ECN support
  *      Andrei Gurtov,
  *      Pasi Sarolahti,
  *      Panu Kuhlberg:      Experimental audit of TCP (re)transmission
  *                  engine. Lots of bugs are found.
  *      Pasi Sarolahti:     F-RTO for dealing with spurious RTOs
  */
 
 #define pr_fmt(fmt) "TCP: " fmt
 
 #include <linux/mm.h>
 #include <linux/slab.h>
 #include <linux/module.h>
 #include <linux/sysctl.h>
 #include <linux/kernel.h>
 #include <linux/prefetch.h>
 #include <net/dst.h>
 #include <net/tcp.h>
 #include <net/inet_common.h>
 #include <linux/ipsec.h>
 #include <asm/unaligned.h>
 #include <linux/errqueue.h>
 #include <trace/events/tcp.h>
 #include <linux/jump_label_ratelimit.h>
 #include <net/busy_poll.h>
 #include <net/mptcp.h>
 
 int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
 
 #define FLAG_DATA       0x01 /* Incoming frame contained data.      */
 #define FLAG_WIN_UPDATE     0x02 /* Incoming ACK was a window update.   */
 #define FLAG_DATA_ACKED     0x04 /* This ACK acknowledged new data.     */
 #define FLAG_RETRANS_DATA_ACKED 0x08 /* "" "" some of which was retransmitted.  */
 #define FLAG_SYN_ACKED      0x10 /* This ACK acknowledged SYN.      */
 #define FLAG_DATA_SACKED    0x20 /* New SACK.               */
 #define FLAG_ECE        0x40 /* ECE in this ACK             */
 #define FLAG_LOST_RETRANS   0x80 /* This ACK marks some retransmission lost */
 #define FLAG_SLOWPATH       0x100 /* Do not skip RFC checks for window update.*/
 #define FLAG_ORIG_SACK_ACKED    0x200 /* Never retransmitted data are (s)acked  */
 #define FLAG_SND_UNA_ADVANCED   0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
 #define FLAG_DSACKING_ACK   0x800 /* SACK blocks contained D-SACK info */
 #define FLAG_SET_XMIT_TIMER 0x1000 /* Set TLP or RTO timer */
 #define FLAG_SACK_RENEGING  0x2000 /* snd_una advanced to a sacked seq */
 #define FLAG_UPDATE_TS_RECENT   0x4000 /* tcp_replace_ts_recent() */
 #define FLAG_NO_CHALLENGE_ACK   0x8000 /* do not call tcp_send_challenge_ack()  */
 #define FLAG_ACK_MAYBE_DELAYED  0x10000 /* Likely a delayed ACK */
 #define FLAG_DSACK_TLP      0x20000 /* DSACK for tail loss probe */
 
 #define FLAG_ACKED      (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
 #define FLAG_NOT_DUP        (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
 #define FLAG_CA_ALERT       (FLAG_DATA_SACKED|FLAG_ECE|FLAG_DSACKING_ACK)
 #define FLAG_FORWARD_PROGRESS   (FLAG_ACKED|FLAG_DATA_SACKED)
 
 #define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
 #define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))
 
 #define REXMIT_NONE 0 /* no loss recovery to do */
 #define REXMIT_LOST 1 /* retransmit packets marked lost */
 #define REXMIT_NEW  2 /* FRTO-style transmit of unsent/new packets */
 
 #if IS_ENABLED(CONFIG_TLS_DEVICE)
 static DEFINE_STATIC_KEY_DEFERRED_FALSE(clean_acked_data_enabled, HZ);
 
 void clean_acked_data_enable(struct inet_connection_sock *icsk,
                  void (*cad)(struct sock *sk, u32 ack_seq))
 {
     icsk->icsk_clean_acked = cad;
     static_branch_deferred_inc(&clean_acked_data_enabled);
 }
 EXPORT_SYMBOL_GPL(clean_acked_data_enable);
 
 void clean_acked_data_disable(struct inet_connection_sock *icsk)
 {
     static_branch_slow_dec_deferred(&clean_acked_data_enabled);
     icsk->icsk_clean_acked = NULL;
 }
 EXPORT_SYMBOL_GPL(clean_acked_data_disable);
 
 void clean_acked_data_flush(void)
 {
     static_key_deferred_flush(&clean_acked_data_enabled);
 }
 EXPORT_SYMBOL_GPL(clean_acked_data_flush);
 #endif
 
 #ifdef CONFIG_CGROUP_BPF
 static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
 {
     bool unknown_opt = tcp_sk(sk)->rx_opt.saw_unknown &&
         BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                        BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG);
     bool parse_all_opt = BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                             BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG);
     struct bpf_sock_ops_kern sock_ops;
 
     if (likely(!unknown_opt && !parse_all_opt))
         return;
 
     /* The skb will be handled in the
      * bpf_skops_established() or
      * bpf_skops_write_hdr_opt().
      */
     switch (sk->sk_state) {
     case TCP_SYN_RECV:
     case TCP_SYN_SENT:
     case TCP_LISTEN:
         return;
     }
 
     sock_owned_by_me(sk);
 
     memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
     sock_ops.op = BPF_SOCK_OPS_PARSE_HDR_OPT_CB;
     sock_ops.is_fullsock = 1;
     sock_ops.sk = sk;
     bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));
 
     BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
 }
 
 static void bpf_skops_established(struct sock *sk, int bpf_op,
                   struct sk_buff *skb)
 {
     struct bpf_sock_ops_kern sock_ops;
 
     sock_owned_by_me(sk);
 
     memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
     sock_ops.op = bpf_op;
     sock_ops.is_fullsock = 1;
     sock_ops.sk = sk;
     /* sk with TCP_REPAIR_ON does not have skb in tcp_finish_connect */
     if (skb)
         bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));
 
     BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
 }
 #else
 static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
 {
 }
 
 static void bpf_skops_established(struct sock *sk, int bpf_op,
                   struct sk_buff *skb)
 {
 }
 #endif
 
 static void tcp_gro_dev_warn(struct sock *sk, const struct sk_buff *skb,
                  unsigned int len)
 {
     static bool __once __read_mostly;
 
     if (!__once) {
         struct net_device *dev;
 
         __once = true;
 
         rcu_read_lock();
         dev = dev_get_by_index_rcu(sock_net(sk), skb->skb_iif);
         if (!dev || len >= dev->mtu)
             pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised.\n",
                 dev ? dev->name : "Unknown driver");
         rcu_read_unlock();
     }
 }
 
 /* Adapt the MSS value used to make delayed ack decision to the
  * real world.
  */
 static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     const unsigned int lss = icsk->icsk_ack.last_seg_size;
     unsigned int len;
 
     icsk->icsk_ack.last_seg_size = 0;
 
     /* skb->len may jitter because of SACKs, even if peer
      * sends good full-sized frames.
      */
     len = skb_shinfo(skb)->gso_size ? : skb->len;
     if (len >= icsk->icsk_ack.rcv_mss) {
         icsk->icsk_ack.rcv_mss = min_t(unsigned int, len,
                            tcp_sk(sk)->advmss);
         /* Account for possibly-removed options */
         if (unlikely(len > icsk->icsk_ack.rcv_mss +
                    MAX_TCP_OPTION_SPACE))
             tcp_gro_dev_warn(sk, skb, len);
     } else {
         /* Otherwise, we make more careful check taking into account,
          * that SACKs block is variable.
          *
          * "len" is invariant segment length, including TCP header.
          */
         len += skb->data - skb_transport_header(skb);
         if (len >= TCP_MSS_DEFAULT + sizeof(struct tcphdr) ||
             /* If PSH is not set, packet should be
              * full sized, provided peer TCP is not badly broken.
              * This observation (if it is correct 8)) allows
              * to handle super-low mtu links fairly.
              */
             (len >= TCP_MIN_MSS + sizeof(struct tcphdr) &&
              !(tcp_flag_word(tcp_hdr(skb)) & TCP_REMNANT))) {
             /* Subtract also invariant (if peer is RFC compliant),
              * tcp header plus fixed timestamp option length.
              * Resulting "len" is MSS free of SACK jitter.
              */
             len -= tcp_sk(sk)->tcp_header_len;
             icsk->icsk_ack.last_seg_size = len;
             if (len == lss) {
                 icsk->icsk_ack.rcv_mss = len;
                 return;
             }
         }
         if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
             icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
         icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
     }
 }
 
 static void tcp_incr_quickack(struct sock *sk, unsigned int max_quickacks)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     unsigned int quickacks = tcp_sk(sk)->rcv_wnd / (2 * icsk->icsk_ack.rcv_mss);
 
     if (quickacks == 0)
         quickacks = 2;
     quickacks = min(quickacks, max_quickacks);
     if (quickacks > icsk->icsk_ack.quick)
         icsk->icsk_ack.quick = quickacks;
 }
 
 void tcp_enter_quickack_mode(struct sock *sk, unsigned int max_quickacks)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
 
     tcp_incr_quickack(sk, max_quickacks);
     inet_csk_exit_pingpong_mode(sk);
     icsk->icsk_ack.ato = TCP_ATO_MIN;
 }
 EXPORT_SYMBOL(tcp_enter_quickack_mode);
 
 /* Send ACKs quickly, if "quick" count is not exhausted
  * and the session is not interactive.
  */
 
 static bool tcp_in_quickack_mode(struct sock *sk)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     const struct dst_entry *dst = __sk_dst_get(sk);
 
     return (dst && dst_metric(dst, RTAX_QUICKACK)) ||
         (icsk->icsk_ack.quick && !inet_csk_in_pingpong_mode(sk));
 }
 
 static void tcp_ecn_queue_cwr(struct tcp_sock *tp)
 {
     if (tp->ecn_flags & TCP_ECN_OK)
         tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
 }
 
 static void tcp_ecn_accept_cwr(struct sock *sk, const struct sk_buff *skb)
 {
     if (tcp_hdr(skb)->cwr) {
         tcp_sk(sk)->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
 
         /* If the sender is telling us it has entered CWR, then its
          * cwnd may be very low (even just 1 packet), so we should ACK
          * immediately.
          */
         if (TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq)
             inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
     }
 }
 
 static void tcp_ecn_withdraw_cwr(struct tcp_sock *tp)
 {
     tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
 }
 
 static void __tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     switch (TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK) {
     case INET_ECN_NOT_ECT:
         /* Funny extension: if ECT is not set on a segment,
          * and we already seen ECT on a previous segment,
          * it is probably a retransmit.
          */
         if (tp->ecn_flags & TCP_ECN_SEEN)
             tcp_enter_quickack_mode(sk, 2);
         break;
     case INET_ECN_CE:
         if (tcp_ca_needs_ecn(sk))
             tcp_ca_event(sk, CA_EVENT_ECN_IS_CE);
 
         if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
             /* Better not delay acks, sender can have a very low cwnd */
             tcp_enter_quickack_mode(sk, 2);
             tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
         }
         tp->ecn_flags |= TCP_ECN_SEEN;
         break;
     default:
         if (tcp_ca_needs_ecn(sk))
             tcp_ca_event(sk, CA_EVENT_ECN_NO_CE);
         tp->ecn_flags |= TCP_ECN_SEEN;
         break;
     }
 }
 
 static void tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
 {
     if (tcp_sk(sk)->ecn_flags & TCP_ECN_OK)
         __tcp_ecn_check_ce(sk, skb);
 }
 
 static void tcp_ecn_rcv_synack(struct tcp_sock *tp, const struct tcphdr *th)
 {
     if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || th->cwr))
         tp->ecn_flags &= ~TCP_ECN_OK;
 }
 
 static void tcp_ecn_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th)
 {
     if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || !th->cwr))
         tp->ecn_flags &= ~TCP_ECN_OK;
 }
 
 static bool tcp_ecn_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr *th)
 {
     if (th->ece && !th->syn && (tp->ecn_flags & TCP_ECN_OK))
         return true;
     return false;
 }
 
 /* Buffer size and advertised window tuning.
  *
  * 1. Tuning sk->sk_sndbuf, when connection enters established state.
  */
 
 static void tcp_sndbuf_expand(struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
     int sndmem, per_mss;
     u32 nr_segs;
 
     /* Worst case is non GSO/TSO : each frame consumes one skb
      * and skb->head is kmalloced using power of two area of memory
      */
     per_mss = max_t(u32, tp->rx_opt.mss_clamp, tp->mss_cache) +
           MAX_TCP_HEADER +
           SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 
     per_mss = roundup_pow_of_two(per_mss) +
           SKB_DATA_ALIGN(sizeof(struct sk_buff));
 
     nr_segs = max_t(u32, TCP_INIT_CWND, tcp_snd_cwnd(tp));
     nr_segs = max_t(u32, nr_segs, tp->reordering + 1);
 
     /* Fast Recovery (RFC 5681 3.2) :
      * Cubic needs 1.7 factor, rounded to 2 to include
      * extra cushion (application might react slowly to EPOLLOUT)
      */
     sndmem = ca_ops->sndbuf_expand ? ca_ops->sndbuf_expand(sk) : 2;
     sndmem *= nr_segs * per_mss;
 
     if (sk->sk_sndbuf < sndmem)
         WRITE_ONCE(sk->sk_sndbuf,
                min(sndmem, READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_wmem[2])));
 }
 
 /* 2. Tuning advertised window (window_clamp, rcv_ssthresh)
  *
  * All tcp_full_space() is split to two parts: "network" buffer, allocated
  * forward and advertised in receiver window (tp->rcv_wnd) and
  * "application buffer", required to isolate scheduling/application
  * latencies from network.
  * window_clamp is maximal advertised window. It can be less than
  * tcp_full_space(), in this case tcp_full_space() - window_clamp
  * is reserved for "application" buffer. The less window_clamp is
  * the smoother our behaviour from viewpoint of network, but the lower
  * throughput and the higher sensitivity of the connection to losses. 8)
  *
  * rcv_ssthresh is more strict window_clamp used at "slow start"
  * phase to predict further behaviour of this connection.
  * It is used for two goals:
  * - to enforce header prediction at sender, even when application
  *   requires some significant "application buffer". It is check #1.
  * - to prevent pruning of receive queue because of misprediction
  *   of receiver window. Check #2.
  *
  * The scheme does not work when sender sends good segments opening
  * window and then starts to feed us spaghetti. But it should work
  * in common situations. Otherwise, we have to rely on queue collapsing.
  */
 
 /* Slow part of check#2. */
 static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb,
                  unsigned int skbtruesize)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     /* Optimize this! */
     int truesize = tcp_win_from_space(sk, skbtruesize) >> 1;
     int window = tcp_win_from_space(sk, READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2])) >> 1;
 
     while (tp->rcv_ssthresh <= window) {
         if (truesize <= skb->len)
             return 2 * inet_csk(sk)->icsk_ack.rcv_mss;
 
         truesize >>= 1;
         window >>= 1;
     }
     return 0;
 }
 
 /* Even if skb appears to have a bad len/truesize ratio, TCP coalescing
  * can play nice with us, as sk_buff and skb->head might be either
  * freed or shared with up to MAX_SKB_FRAGS segments.
  * Only give a boost to drivers using page frag(s) to hold the frame(s),
  * and if no payload was pulled in skb->head before reaching us.
  */
 static u32 truesize_adjust(bool adjust, const struct sk_buff *skb)
 {
     u32 truesize = skb->truesize;
 
     if (adjust && !skb_headlen(skb)) {
         truesize -= SKB_TRUESIZE(skb_end_offset(skb));
         /* paranoid check, some drivers might be buggy */
         if (unlikely((int)truesize < (int)skb->len))
             truesize = skb->truesize;
     }
     return truesize;
 }
 
 static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb,
                 bool adjust)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     int room;
 
     room = min_t(int, tp->window_clamp, tcp_space(sk)) - tp->rcv_ssthresh;
 
     if (room <= 0)
         return;
 
     /* Check #1 */
     if (!tcp_under_memory_pressure(sk)) {
         unsigned int truesize = truesize_adjust(adjust, skb);
         int incr;
 
         /* Check #2. Increase window, if skb with such overhead
          * will fit to rcvbuf in future.
          */
         if (tcp_win_from_space(sk, truesize) <= skb->len)
             incr = 2 * tp->advmss;
         else
             incr = __tcp_grow_window(sk, skb, truesize);
 
         if (incr) {
             incr = max_t(int, incr, 2 * skb->len);
             tp->rcv_ssthresh += min(room, incr);
             inet_csk(sk)->icsk_ack.quick |= 1;
         }
     } else {
         /* Under pressure:
          * Adjust rcv_ssthresh according to reserved mem
          */
         tcp_adjust_rcv_ssthresh(sk);
     }
 }
 
 /* 3. Try to fixup all. It is made immediately after connection enters
  *    established state.
  */
 static void tcp_init_buffer_space(struct sock *sk)
 {
     int tcp_app_win = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_app_win);
     struct tcp_sock *tp = tcp_sk(sk);
     int maxwin;
 
     if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
         tcp_sndbuf_expand(sk);
 
     tcp_mstamp_refresh(tp);
     tp->rcvq_space.time = tp->tcp_mstamp;
     tp->rcvq_space.seq = tp->copied_seq;
 
     maxwin = tcp_full_space(sk);
 
     if (tp->window_clamp >= maxwin) {
         tp->window_clamp = maxwin;
 
         if (tcp_app_win && maxwin > 4 * tp->advmss)
             tp->window_clamp = max(maxwin -
                            (maxwin >> tcp_app_win),
                            4 * tp->advmss);
     }
 
     /* Force reservation of one segment. */
     if (tcp_app_win &&
         tp->window_clamp > 2 * tp->advmss &&
         tp->window_clamp + tp->advmss > maxwin)
         tp->window_clamp = max(2 * tp->advmss, maxwin - tp->advmss);
 
     tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
     tp->snd_cwnd_stamp = tcp_jiffies32;
     tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd,
                     (u32)TCP_INIT_CWND * tp->advmss);
 }
 
 /* 4. Recalculate window clamp after socket hit its memory bounds. */
 static void tcp_clamp_window(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct net *net = sock_net(sk);
     int rmem2;
 
     icsk->icsk_ack.quick = 0;
     rmem2 = READ_ONCE(net->ipv4.sysctl_tcp_rmem[2]);
 
     if (sk->sk_rcvbuf < rmem2 &&
         !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
         !tcp_under_memory_pressure(sk) &&
         sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 0)) {
         WRITE_ONCE(sk->sk_rcvbuf,
                min(atomic_read(&sk->sk_rmem_alloc), rmem2));
     }
     if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
         tp->rcv_ssthresh = min(tp->window_clamp, 2U * tp->advmss);
 }
 
 /* Initialize RCV_MSS value.
  * RCV_MSS is an our guess about MSS used by the peer.
  * We haven't any direct information about the MSS.
  * It's better to underestimate the RCV_MSS rather than overestimate.
  * Overestimations make us ACKing less frequently than needed.
  * Underestimations are more easy to detect and fix by tcp_measure_rcv_mss().
  */
 void tcp_initialize_rcv_mss(struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     unsigned int hint = min_t(unsigned int, tp->advmss, tp->mss_cache);
 
     hint = min(hint, tp->rcv_wnd / 2);
     hint = min(hint, TCP_MSS_DEFAULT);
     hint = max(hint, TCP_MIN_MSS);
 
     inet_csk(sk)->icsk_ack.rcv_mss = hint;
 }
 EXPORT_SYMBOL(tcp_initialize_rcv_mss);
 
 /* Receiver "autotuning" code.
  *
  * The algorithm for RTT estimation w/o timestamps is based on
  * Dynamic Right-Sizing (DRS) by Wu Feng and Mike Fisk of LANL.
  * <https://public.lanl.gov/radiant/pubs.html#DRS>
  *
  * More detail on this code can be found at
  * <http://staff.psc.edu/jheffner/>,
  * though this reference is out of date.  A new paper
  * is pending.
  */
 static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
 {
     u32 new_sample = tp->rcv_rtt_est.rtt_us;
     long m = sample;
 
     if (new_sample != 0) {
         /* If we sample in larger samples in the non-timestamp
          * case, we could grossly overestimate the RTT especially
          * with chatty applications or bulk transfer apps which
          * are stalled on filesystem I/O.
          *
          * Also, since we are only going for a minimum in the
          * non-timestamp case, we do not smooth things out
          * else with timestamps disabled convergence takes too
          * long.
          */
         if (!win_dep) {
             m -= (new_sample >> 3);
             new_sample += m;
         } else {
             m <<= 3;
             if (m < new_sample)
                 new_sample = m;
         }
     } else {
         /* No previous measure. */
         new_sample = m << 3;
     }
 
     tp->rcv_rtt_est.rtt_us = new_sample;
 }
 
 static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
 {
     u32 delta_us;
 
     if (tp->rcv_rtt_est.time == 0)
         goto new_measure;
     if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
         return;
     delta_us = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcv_rtt_est.time);
     if (!delta_us)
         delta_us = 1;
     tcp_rcv_rtt_update(tp, delta_us, 1);
 
 new_measure:
     tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
     tp->rcv_rtt_est.time = tp->tcp_mstamp;
 }
 
 static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
                       const struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tp->rx_opt.rcv_tsecr == tp->rcv_rtt_last_tsecr)
         return;
     tp->rcv_rtt_last_tsecr = tp->rx_opt.rcv_tsecr;
 
     if (TCP_SKB_CB(skb)->end_seq -
         TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss) {
         u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
         u32 delta_us;
 
         if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
             if (!delta)
                 delta = 1;
             delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
             tcp_rcv_rtt_update(tp, delta_us, 0);
         }
     }
 }
 
 /*
  * This function should be called every time data is copied to user space.
  * It calculates the appropriate TCP receive buffer space.
  */
 void tcp_rcv_space_adjust(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 copied;
     int time;
 
     trace_tcp_rcv_space_adjust(sk);
 
     tcp_mstamp_refresh(tp);
     time = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcvq_space.time);
     if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)
         return;
 
     /* Number of bytes copied to user in last RTT */
     copied = tp->copied_seq - tp->rcvq_space.seq;
     if (copied <= tp->rcvq_space.space)
         goto new_measure;
 
     /* A bit of theory :
      * copied = bytes received in previous RTT, our base window
      * To cope with packet losses, we need a 2x factor
      * To cope with slow start, and sender growing its cwin by 100 %
      * every RTT, we need a 4x factor, because the ACK we are sending
      * now is for the next RTT, not the current one :
      * <prev RTT . ><current RTT .. ><next RTT .... >
      */
 
     if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf) &&
         !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
         int rcvmem, rcvbuf;
         u64 rcvwin, grow;
 
         /* minimal window to cope with packet losses, assuming
          * steady state. Add some cushion because of small variations.
          */
         rcvwin = ((u64)copied << 1) + 16 * tp->advmss;
 
         /* Accommodate for sender rate increase (eg. slow start) */
         grow = rcvwin * (copied - tp->rcvq_space.space);
         do_div(grow, tp->rcvq_space.space);
         rcvwin += (grow << 1);
 
         rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
         while (tcp_win_from_space(sk, rcvmem) < tp->advmss)
             rcvmem += 128;
 
         do_div(rcvwin, tp->advmss);
         rcvbuf = min_t(u64, rcvwin * rcvmem,
                    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
         if (rcvbuf > sk->sk_rcvbuf) {
             WRITE_ONCE(sk->sk_rcvbuf, rcvbuf);
 
             /* Make the window clamp follow along.  */
             tp->window_clamp = tcp_win_from_space(sk, rcvbuf);
         }
     }
     tp->rcvq_space.space = copied;
 
 new_measure:
     tp->rcvq_space.seq = tp->copied_seq;
     tp->rcvq_space.time = tp->tcp_mstamp;
 }
 
 /* There is something which you must keep in mind when you analyze the
  * behavior of the tp->ato delayed ack timeout interval.  When a
  * connection starts up, we want to ack as quickly as possible.  The
  * problem is that "good" TCP's do slow start at the beginning of data
  * transmission.  The means that until we send the first few ACK's the
  * sender will sit on his end and only queue most of his data, because
  * he can only send snd_cwnd unacked packets at any given time.  For
  * each ACK we send, he increments snd_cwnd and transmits more of his
  * queue.  -DaveM
  */
 static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_connection_sock *icsk = inet_csk(sk);
     u32 now;
 
     inet_csk_schedule_ack(sk);
 
     tcp_measure_rcv_mss(sk, skb);
 
     tcp_rcv_rtt_measure(tp);
 
     now = tcp_jiffies32;
 
     if (!icsk->icsk_ack.ato) {
         /* The _first_ data packet received, initialize
          * delayed ACK engine.
          */
         tcp_incr_quickack(sk, TCP_MAX_QUICKACKS);
         icsk->icsk_ack.ato = TCP_ATO_MIN;
     } else {
         int m = now - icsk->icsk_ack.lrcvtime;
 
         if (m <= TCP_ATO_MIN / 2) {
             /* The fastest case is the first. */
             icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + TCP_ATO_MIN / 2;
         } else if (m < icsk->icsk_ack.ato) {
             icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + m;
             if (icsk->icsk_ack.ato > icsk->icsk_rto)
                 icsk->icsk_ack.ato = icsk->icsk_rto;
         } else if (m > icsk->icsk_rto) {
             /* Too long gap. Apparently sender failed to
              * restart window, so that we send ACKs quickly.
              */
             tcp_incr_quickack(sk, TCP_MAX_QUICKACKS);
         }
     }
     icsk->icsk_ack.lrcvtime = now;
 
     tcp_ecn_check_ce(sk, skb);
 
     if (skb->len >= 128)
         tcp_grow_window(sk, skb, true);
 }
 
 /* Called to compute a smoothed rtt estimate. The data fed to this
  * routine either comes from timestamps, or from segments that were
  * known _not_ to have been retransmitted [see Karn/Partridge
  * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
  * piece by Van Jacobson.
  * NOTE: the next three routines used to be one big routine.
  * To save cycles in the RFC 1323 implementation it was better to break
  * it up into three procedures. -- erics
  */
 static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     long m = mrtt_us; /* RTT */
     u32 srtt = tp->srtt_us;
 
     /*  The following amusing code comes from Jacobson's
      *  article in SIGCOMM '88.  Note that rtt and mdev
      *  are scaled versions of rtt and mean deviation.
      *  This is designed to be as fast as possible
      *  m stands for "measurement".
      *
      *  On a 1990 paper the rto value is changed to:
      *  RTO = rtt + 4 * mdev
      *
      * Funny. This algorithm seems to be very broken.
      * These formulae increase RTO, when it should be decreased, increase
      * too slowly, when it should be increased quickly, decrease too quickly
      * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
      * does not matter how to _calculate_ it. Seems, it was trap
      * that VJ failed to avoid. 8)
      */
     if (srtt != 0) {
         m -= (srtt >> 3);   /* m is now error in rtt est */
         srtt += m;      /* rtt = 7/8 rtt + 1/8 new */
         if (m < 0) {
             m = -m;     /* m is now abs(error) */
             m -= (tp->mdev_us >> 2);   /* similar update on mdev */
             /* This is similar to one of Eifel findings.
              * Eifel blocks mdev updates when rtt decreases.
              * This solution is a bit different: we use finer gain
              * for mdev in this case (alpha*beta).
              * Like Eifel it also prevents growth of rto,
              * but also it limits too fast rto decreases,
              * happening in pure Eifel.
              */
             if (m > 0)
                 m >>= 3;
         } else {
             m -= (tp->mdev_us >> 2);   /* similar update on mdev */
         }
         tp->mdev_us += m;       /* mdev = 3/4 mdev + 1/4 new */
         if (tp->mdev_us > tp->mdev_max_us) {
             tp->mdev_max_us = tp->mdev_us;
             if (tp->mdev_max_us > tp->rttvar_us)
                 tp->rttvar_us = tp->mdev_max_us;
         }
         if (after(tp->snd_una, tp->rtt_seq)) {
             if (tp->mdev_max_us < tp->rttvar_us)
                 tp->rttvar_us -= (tp->rttvar_us - tp->mdev_max_us) >> 2;
             tp->rtt_seq = tp->snd_nxt;
             tp->mdev_max_us = tcp_rto_min_us(sk);
 
             tcp_bpf_rtt(sk);
         }
     } else {
         /* no previous measure. */
         srtt = m << 3;      /* take the measured time to be rtt */
         tp->mdev_us = m << 1;   /* make sure rto = 3*rtt */
         tp->rttvar_us = max(tp->mdev_us, tcp_rto_min_us(sk));
         tp->mdev_max_us = tp->rttvar_us;
         tp->rtt_seq = tp->snd_nxt;
 
         tcp_bpf_rtt(sk);
     }
     tp->srtt_us = max(1U, srtt);
 }
 
 static void tcp_update_pacing_rate(struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     u64 rate;
 
     /* set sk_pacing_rate to 200 % of current rate (mss * cwnd / srtt) */
     rate = (u64)tp->mss_cache * ((USEC_PER_SEC / 100) << 3);
 
     /* current rate is (cwnd * mss) / srtt
      * In Slow Start [1], set sk_pacing_rate to 200 % the current rate.
      * In Congestion Avoidance phase, set it to 120 % the current rate.
      *
      * [1] : Normal Slow Start condition is (tp->snd_cwnd < tp->snd_ssthresh)
      *   If snd_cwnd >= (tp->snd_ssthresh / 2), we are approaching
      *   end of slow start and should slow down.
      */
     if (tcp_snd_cwnd(tp) < tp->snd_ssthresh / 2)
         rate *= READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_pacing_ss_ratio);
     else
         rate *= READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_pacing_ca_ratio);
 
     rate *= max(tcp_snd_cwnd(tp), tp->packets_out);
 
     if (likely(tp->srtt_us))
         do_div(rate, tp->srtt_us);
 
     /* WRITE_ONCE() is needed because sch_fq fetches sk_pacing_rate
      * without any lock. We want to make sure compiler wont store
      * intermediate values in this location.
      */
     WRITE_ONCE(sk->sk_pacing_rate, min_t(u64, rate,
                          sk->sk_max_pacing_rate));
 }
 
 /* Calculate rto without backoff.  This is the second half of Van Jacobson's
  * routine referred to above.
  */
 static void tcp_set_rto(struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     /* Old crap is replaced with new one. 8)
      *
      * More seriously:
      * 1. If rtt variance happened to be less 50msec, it is hallucination.
      *    It cannot be less due to utterly erratic ACK generation made
      *    at least by solaris and freebsd. "Erratic ACKs" has _nothing_
      *    to do with delayed acks, because at cwnd>2 true delack timeout
      *    is invisible. Actually, Linux-2.4 also generates erratic
      *    ACKs in some circumstances.
      */
     inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);
 
     /* 2. Fixups made earlier cannot be right.
      *    If we do not estimate RTO correctly without them,
      *    all the algo is pure shit and should be replaced
      *    with correct one. It is exactly, which we pretend to do.
      */
 
     /* NOTE: clamping at TCP_RTO_MIN is not required, current algo
      * guarantees that rto is higher.
      */
     tcp_bound_rto(sk);
 }
 
 __u32 tcp_init_cwnd(const struct tcp_sock *tp, const struct dst_entry *dst)
 {
     __u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);
 
     if (!cwnd)
         cwnd = TCP_INIT_CWND;
     return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
 }
 
 struct tcp_sacktag_state {
     /* Timestamps for earliest and latest never-retransmitted segment
      * that was SACKed. RTO needs the earliest RTT to stay conservative,
      * but congestion control should still get an accurate delay signal.
      */
     u64 first_sackt;
     u64 last_sackt;
     u32 reord;
     u32 sack_delivered;
     int flag;
     unsigned int mss_now;
     struct rate_sample *rate;
 };
 
 /* Take a notice that peer is sending D-SACKs. Skip update of data delivery
  * and spurious retransmission information if this DSACK is unlikely caused by
  * sender's action:
  * - DSACKed sequence range is larger than maximum receiver's window.
  * - Total no. of DSACKed segments exceed the total no. of retransmitted segs.
  */
 static u32 tcp_dsack_seen(struct tcp_sock *tp, u32 start_seq,
               u32 end_seq, struct tcp_sacktag_state *state)
 {
     u32 seq_len, dup_segs = 1;
 
     if (!before(start_seq, end_seq))
         return 0;
 
     seq_len = end_seq - start_seq;
     /* Dubious DSACK: DSACKed range greater than maximum advertised rwnd */
     if (seq_len > tp->max_window)
         return 0;
     if (seq_len > tp->mss_cache)
         dup_segs = DIV_ROUND_UP(seq_len, tp->mss_cache);
     else if (tp->tlp_high_seq && tp->tlp_high_seq == end_seq)
         state->flag |= FLAG_DSACK_TLP;
 
     tp->dsack_dups += dup_segs;
     /* Skip the DSACK if dup segs weren't retransmitted by sender */
     if (tp->dsack_dups > tp->total_retrans)
         return 0;
 
     tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
     /* We increase the RACK ordering window in rounds where we receive
      * DSACKs that may have been due to reordering causing RACK to trigger
      * a spurious fast recovery. Thus RACK ignores DSACKs that happen
      * without having seen reordering, or that match TLP probes (TLP
      * is timer-driven, not triggered by RACK).
      */
     if (tp->reord_seen && !(state->flag & FLAG_DSACK_TLP))
         tp->rack.dsack_seen = 1;
 
     state->flag |= FLAG_DSACKING_ACK;
     /* A spurious retransmission is delivered */
     state->sack_delivered += dup_segs;
 
     return dup_segs;
 }
 
 /* It's reordering when higher sequence was delivered (i.e. sacked) before
  * some lower never-retransmitted sequence ("low_seq"). The maximum reordering
  * distance is approximated in full-mss packet distance ("reordering").
  */
 static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
                       const int ts)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     const u32 mss = tp->mss_cache;
     u32 fack, metric;
 
     fack = tcp_highest_sack_seq(tp);
     if (!before(low_seq, fack))
         return;
 
     metric = fack - low_seq;
     if ((metric > tp->reordering * mss) && mss) {
 #if FASTRETRANS_DEBUG > 1
         pr_debug("Disorder%d %d %u f%u s%u rr%d\n",
              tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
              tp->reordering,
              0,
              tp->sacked_out,
              tp->undo_marker ? tp->undo_retrans : 0);
 #endif
         tp->reordering = min_t(u32, (metric + mss - 1) / mss,
                        READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_max_reordering));
     }
 
     /* This exciting event is worth to be remembered. 8) */
     tp->reord_seen++;
     NET_INC_STATS(sock_net(sk),
               ts ? LINUX_MIB_TCPTSREORDER : LINUX_MIB_TCPSACKREORDER);
 }
 
  /* This must be called before lost_out or retrans_out are updated
   * on a new loss, because we want to know if all skbs previously
   * known to be lost have already been retransmitted, indicating
   * that this newly lost skb is our next skb to retransmit.
   */
 static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
 {
     if ((!tp->retransmit_skb_hint && tp->retrans_out >= tp->lost_out) ||
         (tp->retransmit_skb_hint &&
          before(TCP_SKB_CB(skb)->seq,
             TCP_SKB_CB(tp->retransmit_skb_hint)->seq)))
         tp->retransmit_skb_hint = skb;
 }
 
 /* Sum the number of packets on the wire we have marked as lost, and
  * notify the congestion control module that the given skb was marked lost.
  */
 static void tcp_notify_skb_loss_event(struct tcp_sock *tp, const struct sk_buff *skb)
 {
     tp->lost += tcp_skb_pcount(skb);
 }
 
 void tcp_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
 {
     __u8 sacked = TCP_SKB_CB(skb)->sacked;
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (sacked & TCPCB_SACKED_ACKED)
         return;
 
     tcp_verify_retransmit_hint(tp, skb);
     if (sacked & TCPCB_LOST) {
         if (sacked & TCPCB_SACKED_RETRANS) {
             /* Account for retransmits that are lost again */
             TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
             tp->retrans_out -= tcp_skb_pcount(skb);
             NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
                       tcp_skb_pcount(skb));
             tcp_notify_skb_loss_event(tp, skb);
         }
     } else {
         tp->lost_out += tcp_skb_pcount(skb);
         TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
         tcp_notify_skb_loss_event(tp, skb);
     }
 }
 
 /* Updates the delivered and delivered_ce counts */
 static void tcp_count_delivered(struct tcp_sock *tp, u32 delivered,
                 bool ece_ack)
 {
     tp->delivered += delivered;
     if (ece_ack)
         tp->delivered_ce += delivered;
 }
 
 /* This procedure tags the retransmission queue when SACKs arrive.
  *
  * We have three tag bits: SACKED(S), RETRANS(R) and LOST(L).
  * Packets in queue with these bits set are counted in variables
  * sacked_out, retrans_out and lost_out, correspondingly.
  *
  * Valid combinations are:
  * Tag  InFlight    Description
  * 0    1       - orig segment is in flight.
  * S    0       - nothing flies, orig reached receiver.
  * L    0       - nothing flies, orig lost by net.
  * R    2       - both orig and retransmit are in flight.
  * L|R  1       - orig is lost, retransmit is in flight.
  * S|R  1       - orig reached receiver, retrans is still in flight.
  * (L|S|R is logically valid, it could occur when L|R is sacked,
  *  but it is equivalent to plain S and code short-curcuits it to S.
  *  L|S is logically invalid, it would mean -1 packet in flight 8))
  *
  * These 6 states form finite state machine, controlled by the following events:
  * 1. New ACK (+SACK) arrives. (tcp_sacktag_write_queue())
  * 2. Retransmission. (tcp_retransmit_skb(), tcp_xmit_retransmit_queue())
  * 3. Loss detection event of two flavors:
  *  A. Scoreboard estimator decided the packet is lost.
  *     A'. Reno "three dupacks" marks head of queue lost.
  *  B. SACK arrives sacking SND.NXT at the moment, when the
  *     segment was retransmitted.
  * 4. D-SACK added new rule: D-SACK changes any tag to S.
  *
  * It is pleasant to note, that state diagram turns out to be commutative,
  * so that we are allowed not to be bothered by order of our actions,
  * when multiple events arrive simultaneously. (see the function below).
  *
  * Reordering detection.
  * --------------------
  * Reordering metric is maximal distance, which a packet can be displaced
  * in packet stream. With SACKs we can estimate it:
  *
  * 1. SACK fills old hole and the corresponding segment was not
  *    ever retransmitted -> reordering. Alas, we cannot use it
  *    when segment was retransmitted.
  * 2. The last flaw is solved with D-SACK. D-SACK arrives
  *    for retransmitted and already SACKed segment -> reordering..
  * Both of these heuristics are not used in Loss state, when we cannot
  * account for retransmits accurately.
  *
  * SACK block validation.
  * ----------------------
  *
  * SACK block range validation checks that the received SACK block fits to
  * the expected sequence limits, i.e., it is between SND.UNA and SND.NXT.
  * Note that SND.UNA is not included to the range though being valid because
  * it means that the receiver is rather inconsistent with itself reporting
  * SACK reneging when it should advance SND.UNA. Such SACK block this is
  * perfectly valid, however, in light of RFC2018 which explicitly states
  * that "SACK block MUST reflect the newest segment.  Even if the newest
  * segment is going to be discarded ...", not that it looks very clever
  * in case of head skb. Due to potentional receiver driven attacks, we
  * choose to avoid immediate execution of a walk in write queue due to
  * reneging and defer head skb's loss recovery to standard loss recovery
  * procedure that will eventually trigger (nothing forbids us doing this).
  *
  * Implements also blockage to start_seq wrap-around. Problem lies in the
  * fact that though start_seq (s) is before end_seq (i.e., not reversed),
  * there's no guarantee that it will be before snd_nxt (n). The problem
  * happens when start_seq resides between end_seq wrap (e_w) and snd_nxt
  * wrap (s_w):
  *
  *         <- outs wnd ->                          <- wrapzone ->
  *         u     e      n                         u_w   e_w  s n_w
  *         |     |      |                          |     |   |  |
  * |<------------+------+----- TCP seqno space --------------+---------->|
  * ...-- <2^31 ->|                                           |<--------...
  * ...---- >2^31 ------>|                                    |<--------...
  *
  * Current code wouldn't be vulnerable but it's better still to discard such
  * crazy SACK blocks. Doing this check for start_seq alone closes somewhat
  * similar case (end_seq after snd_nxt wrap) as earlier reversed check in
  * snd_nxt wrap -> snd_una region will then become "well defined", i.e.,
  * equal to the ideal case (infinite seqno space without wrap caused issues).
  *
  * With D-SACK the lower bound is extended to cover sequence space below
  * SND.UNA down to undo_marker, which is the last point of interest. Yet
  * again, D-SACK block must not to go across snd_una (for the same reason as
  * for the normal SACK blocks, explained above). But there all simplicity
  * ends, TCP might receive valid D-SACKs below that. As long as they reside
  * fully below undo_marker they do not affect behavior in anyway and can
  * therefore be safely ignored. In rare cases (which are more or less
  * theoretical ones), the D-SACK will nicely cross that boundary due to skb
  * fragmentation and packet reordering past skb's retransmission. To consider
  * them correctly, the acceptable range must be extended even more though
  * the exact amount is rather hard to quantify. However, tp->max_window can
  * be used as an exaggerated estimate.
  */
 static bool tcp_is_sackblock_valid(struct tcp_sock *tp, bool is_dsack,
                    u32 start_seq, u32 end_seq)
 {
     /* Too far in future, or reversed (interpretation is ambiguous) */
     if (after(end_seq, tp->snd_nxt) || !before(start_seq, end_seq))
         return false;
 
     /* Nasty start_seq wrap-around check (see comments above) */
     if (!before(start_seq, tp->snd_nxt))
         return false;
 
     /* In outstanding window? ...This is valid exit for D-SACKs too.
      * start_seq == snd_una is non-sensical (see comments above)
      */
     if (after(start_seq, tp->snd_una))
         return true;
 
     if (!is_dsack || !tp->undo_marker)
         return false;
 
     /* ...Then it's D-SACK, and must reside below snd_una completely */
     if (after(end_seq, tp->snd_una))
         return false;
 
     if (!before(start_seq, tp->undo_marker))
         return true;
 
     /* Too old */
     if (!after(end_seq, tp->undo_marker))
         return false;
 
     /* Undo_marker boundary crossing (overestimates a lot). Known already:
      *   start_seq < undo_marker and end_seq >= undo_marker.
      */
     return !before(start_seq, end_seq - tp->max_window);
 }
 
 static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
                 struct tcp_sack_block_wire *sp, int num_sacks,
                 u32 prior_snd_una, struct tcp_sacktag_state *state)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 start_seq_0 = get_unaligned_be32(&sp[0].start_seq);
     u32 end_seq_0 = get_unaligned_be32(&sp[0].end_seq);
     u32 dup_segs;
 
     if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKRECV);
     } else if (num_sacks > 1) {
         u32 end_seq_1 = get_unaligned_be32(&sp[1].end_seq);
         u32 start_seq_1 = get_unaligned_be32(&sp[1].start_seq);
 
         if (after(end_seq_0, end_seq_1) || before(start_seq_0, start_seq_1))
             return false;
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKOFORECV);
     } else {
         return false;
     }
 
     dup_segs = tcp_dsack_seen(tp, start_seq_0, end_seq_0, state);
     if (!dup_segs) {    /* Skip dubious DSACK */
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKIGNOREDDUBIOUS);
         return false;
     }
 
     NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPDSACKRECVSEGS, dup_segs);
 
     /* D-SACK for already forgotten data... Do dumb counting. */
     if (tp->undo_marker && tp->undo_retrans > 0 &&
         !after(end_seq_0, prior_snd_una) &&
         after(end_seq_0, tp->undo_marker))
         tp->undo_retrans = max_t(int, 0, tp->undo_retrans - dup_segs);
 
     return true;
 }
 
 /* Check if skb is fully within the SACK block. In presence of GSO skbs,
  * the incoming SACK may not exactly match but we can find smaller MSS
  * aligned portion of it that matches. Therefore we might need to fragment
  * which may fail and creates some hassle (caller must handle error case
  * returns).
  *
  * FIXME: this could be merged to shift decision code
  */
 static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
                   u32 start_seq, u32 end_seq)
 {
     int err;
     bool in_sack;
     unsigned int pkt_len;
     unsigned int mss;
 
     in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
           !before(end_seq, TCP_SKB_CB(skb)->end_seq);
 
     if (tcp_skb_pcount(skb) > 1 && !in_sack &&
         after(TCP_SKB_CB(skb)->end_seq, start_seq)) {
         mss = tcp_skb_mss(skb);
         in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
 
         if (!in_sack) {
             pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
             if (pkt_len < mss)
                 pkt_len = mss;
         } else {
             pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
             if (pkt_len < mss)
                 return -EINVAL;
         }
 
         /* Round if necessary so that SACKs cover only full MSSes
          * and/or the remaining small portion (if present)
          */
         if (pkt_len > mss) {
             unsigned int new_len = (pkt_len / mss) * mss;
             if (!in_sack && new_len < pkt_len)
                 new_len += mss;
             pkt_len = new_len;
         }
 
         if (pkt_len >= skb->len && !in_sack)
             return 0;
 
         err = tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
                    pkt_len, mss, GFP_ATOMIC);
         if (err < 0)
             return err;
     }
 
     return in_sack;
 }
 
 /* Mark the given newly-SACKed range as such, adjusting counters and hints. */
 static u8 tcp_sacktag_one(struct sock *sk,
               struct tcp_sacktag_state *state, u8 sacked,
               u32 start_seq, u32 end_seq,
               int dup_sack, int pcount,
               u64 xmit_time)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* Account D-SACK for retransmitted packet. */
     if (dup_sack && (sacked & TCPCB_RETRANS)) {
         if (tp->undo_marker && tp->undo_retrans > 0 &&
             after(end_seq, tp->undo_marker))
             tp->undo_retrans = max_t(int, 0, tp->undo_retrans - pcount);
         if ((sacked & TCPCB_SACKED_ACKED) &&
             before(start_seq, state->reord))
                 state->reord = start_seq;
     }
 
     /* Nothing to do; acked frame is about to be dropped (was ACKed). */
     if (!after(end_seq, tp->snd_una))
         return sacked;
 
     if (!(sacked & TCPCB_SACKED_ACKED)) {
         tcp_rack_advance(tp, sacked, end_seq, xmit_time);
 
         if (sacked & TCPCB_SACKED_RETRANS) {
             /* If the segment is not tagged as lost,
              * we do not clear RETRANS, believing
              * that retransmission is still in flight.
              */
             if (sacked & TCPCB_LOST) {
                 sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
                 tp->lost_out -= pcount;
                 tp->retrans_out -= pcount;
             }
         } else {
             if (!(sacked & TCPCB_RETRANS)) {
                 /* New sack for not retransmitted frame,
                  * which was in hole. It is reordering.
                  */
                 if (before(start_seq,
                        tcp_highest_sack_seq(tp)) &&
                     before(start_seq, state->reord))
                     state->reord = start_seq;
 
                 if (!after(end_seq, tp->high_seq))
                     state->flag |= FLAG_ORIG_SACK_ACKED;
                 if (state->first_sackt == 0)
                     state->first_sackt = xmit_time;
                 state->last_sackt = xmit_time;
             }
 
             if (sacked & TCPCB_LOST) {
                 sacked &= ~TCPCB_LOST;
                 tp->lost_out -= pcount;
             }
         }
 
         sacked |= TCPCB_SACKED_ACKED;
         state->flag |= FLAG_DATA_SACKED;
         tp->sacked_out += pcount;
         /* Out-of-order packets delivered */
         state->sack_delivered += pcount;
 
         /* Lost marker hint past SACKed? Tweak RFC3517 cnt */
         if (tp->lost_skb_hint &&
             before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
             tp->lost_cnt_hint += pcount;
     }
 
     /* D-SACK. We can detect redundant retransmission in S|R and plain R
      * frames and clear it. undo_retrans is decreased above, L|R frames
      * are accounted above as well.
      */
     if (dup_sack && (sacked & TCPCB_SACKED_RETRANS)) {
         sacked &= ~TCPCB_SACKED_RETRANS;
         tp->retrans_out -= pcount;
     }
 
     return sacked;
 }
 
 /* Shift newly-SACKed bytes from this skb to the immediately previous
  * already-SACKed sk_buff. Mark the newly-SACKed bytes as such.
  */
 static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *prev,
                 struct sk_buff *skb,
                 struct tcp_sacktag_state *state,
                 unsigned int pcount, int shifted, int mss,
                 bool dup_sack)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 start_seq = TCP_SKB_CB(skb)->seq;   /* start of newly-SACKed */
     u32 end_seq = start_seq + shifted;  /* end of newly-SACKed */
 
     BUG_ON(!pcount);
 
     /* Adjust counters and hints for the newly sacked sequence
      * range but discard the return value since prev is already
      * marked. We must tag the range first because the seq
      * advancement below implicitly advances
      * tcp_highest_sack_seq() when skb is highest_sack.
      */
     tcp_sacktag_one(sk, state, TCP_SKB_CB(skb)->sacked,
             start_seq, end_seq, dup_sack, pcount,
             tcp_skb_timestamp_us(skb));
     tcp_rate_skb_delivered(sk, skb, state->rate);
 
     if (skb == tp->lost_skb_hint)
         tp->lost_cnt_hint += pcount;
 
     TCP_SKB_CB(prev)->end_seq += shifted;
     TCP_SKB_CB(skb)->seq += shifted;
 
     tcp_skb_pcount_add(prev, pcount);
     WARN_ON_ONCE(tcp_skb_pcount(skb) < pcount);
     tcp_skb_pcount_add(skb, -pcount);
 
     /* When we're adding to gso_segs == 1, gso_size will be zero,
      * in theory this shouldn't be necessary but as long as DSACK
      * code can come after this skb later on it's better to keep
      * setting gso_size to something.
      */
     if (!TCP_SKB_CB(prev)->tcp_gso_size)
         TCP_SKB_CB(prev)->tcp_gso_size = mss;
 
     /* CHECKME: To clear or not to clear? Mimics normal skb currently */
     if (tcp_skb_pcount(skb) <= 1)
         TCP_SKB_CB(skb)->tcp_gso_size = 0;
 
     /* Difference in this won't matter, both ACKed by the same cumul. ACK */
     TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);
 
     if (skb->len > 0) {
         BUG_ON(!tcp_skb_pcount(skb));
         NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKSHIFTED);
         return false;
     }
 
     /* Whole SKB was eaten :-) */
 
     if (skb == tp->retransmit_skb_hint)
         tp->retransmit_skb_hint = prev;
     if (skb == tp->lost_skb_hint) {
         tp->lost_skb_hint = prev;
         tp->lost_cnt_hint -= tcp_skb_pcount(prev);
     }
 
     TCP_SKB_CB(prev)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
     TCP_SKB_CB(prev)->eor = TCP_SKB_CB(skb)->eor;
     if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
         TCP_SKB_CB(prev)->end_seq++;
 
     if (skb == tcp_highest_sack(sk))
         tcp_advance_highest_sack(sk, skb);
 
     tcp_skb_collapse_tstamp(prev, skb);
     if (unlikely(TCP_SKB_CB(prev)->tx.delivered_mstamp))
         TCP_SKB_CB(prev)->tx.delivered_mstamp = 0;
 
     tcp_rtx_queue_unlink_and_free(skb, sk);
 
     NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKMERGED);
 
     return true;
 }
 
 /* I wish gso_size would have a bit more sane initialization than
  * something-or-zero which complicates things
  */
 static int tcp_skb_seglen(const struct sk_buff *skb)
 {
     return tcp_skb_pcount(skb) == 1 ? skb->len : tcp_skb_mss(skb);
 }
 
 /* Shifting pages past head area doesn't work */
 static int skb_can_shift(const struct sk_buff *skb)
 {
     return !skb_headlen(skb) && skb_is_nonlinear(skb);
 }
 
 int tcp_skb_shift(struct sk_buff *to, struct sk_buff *from,
           int pcount, int shiftlen)
 {
     /* TCP min gso_size is 8 bytes (TCP_MIN_GSO_SIZE)
      * Since TCP_SKB_CB(skb)->tcp_gso_segs is 16 bits, we need
      * to make sure not storing more than 65535 * 8 bytes per skb,
      * even if current MSS is bigger.
      */
     if (unlikely(to->len + shiftlen >= 65535 * TCP_MIN_GSO_SIZE))
         return 0;
     if (unlikely(tcp_skb_pcount(to) + pcount > 65535))
         return 0;
     return skb_shift(to, from, shiftlen);
 }
 
 /* Try collapsing SACK blocks spanning across multiple skbs to a single
  * skb.
  */
 static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
                       struct tcp_sacktag_state *state,
                       u32 start_seq, u32 end_seq,
                       bool dup_sack)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *prev;
     int mss;
     int pcount = 0;
     int len;
     int in_sack;
 
     /* Normally R but no L won't result in plain S */
     if (!dup_sack &&
         (TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_RETRANS)) == TCPCB_SACKED_RETRANS)
         goto fallback;
     if (!skb_can_shift(skb))
         goto fallback;
     /* This frame is about to be dropped (was ACKed). */
     if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
         goto fallback;
 
     /* Can only happen with delayed DSACK + discard craziness */
     prev = skb_rb_prev(skb);
     if (!prev)
         goto fallback;
 
     if ((TCP_SKB_CB(prev)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED)
         goto fallback;
 
     if (!tcp_skb_can_collapse(prev, skb))
         goto fallback;
 
     in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
           !before(end_seq, TCP_SKB_CB(skb)->end_seq);
 
     if (in_sack) {
         len = skb->len;
         pcount = tcp_skb_pcount(skb);
         mss = tcp_skb_seglen(skb);
 
         /* TODO: Fix DSACKs to not fragment already SACKed and we can
          * drop this restriction as unnecessary
          */
         if (mss != tcp_skb_seglen(prev))
             goto fallback;
     } else {
         if (!after(TCP_SKB_CB(skb)->end_seq, start_seq))
             goto noop;
         /* CHECKME: This is non-MSS split case only?, this will
          * cause skipped skbs due to advancing loop btw, original
          * has that feature too
          */
         if (tcp_skb_pcount(skb) <= 1)
             goto noop;
 
         in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
         if (!in_sack) {
             /* TODO: head merge to next could be attempted here
              * if (!after(TCP_SKB_CB(skb)->end_seq, end_seq)),
              * though it might not be worth of the additional hassle
              *
              * ...we can probably just fallback to what was done
              * previously. We could try merging non-SACKed ones
              * as well but it probably isn't going to buy off
              * because later SACKs might again split them, and
              * it would make skb timestamp tracking considerably
              * harder problem.
              */
             goto fallback;
         }
 
         len = end_seq - TCP_SKB_CB(skb)->seq;
         BUG_ON(len < 0);
         BUG_ON(len > skb->len);
 
         /* MSS boundaries should be honoured or else pcount will
          * severely break even though it makes things bit trickier.
          * Optimize common case to avoid most of the divides
          */
         mss = tcp_skb_mss(skb);
 
         /* TODO: Fix DSACKs to not fragment already SACKed and we can
          * drop this restriction as unnecessary
          */
         if (mss != tcp_skb_seglen(prev))
             goto fallback;
 
         if (len == mss) {
             pcount = 1;
         } else if (len < mss) {
             goto noop;
         } else {
             pcount = len / mss;
             len = pcount * mss;
         }
     }
 
     /* tcp_sacktag_one() won't SACK-tag ranges below snd_una */
     if (!after(TCP_SKB_CB(skb)->seq + len, tp->snd_una))
         goto fallback;
 
     if (!tcp_skb_shift(prev, skb, pcount, len))
         goto fallback;
     if (!tcp_shifted_skb(sk, prev, skb, state, pcount, len, mss, dup_sack))
         goto out;
 
     /* Hole filled allows collapsing with the next as well, this is very
      * useful when hole on every nth skb pattern happens
      */
     skb = skb_rb_next(prev);
     if (!skb)
         goto out;
 
     if (!skb_can_shift(skb) ||
         ((TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED) ||
         (mss != tcp_skb_seglen(skb)))
         goto out;
 
     if (!tcp_skb_can_collapse(prev, skb))
         goto out;
     len = skb->len;
     pcount = tcp_skb_pcount(skb);
     if (tcp_skb_shift(prev, skb, pcount, len))
         tcp_shifted_skb(sk, prev, skb, state, pcount,
                 len, mss, 0);
 
 out:
     return prev;
 
 noop:
     return skb;
 
 fallback:
     NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKSHIFTFALLBACK);
     return NULL;
 }
 
 static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
                     struct tcp_sack_block *next_dup,
                     struct tcp_sacktag_state *state,
                     u32 start_seq, u32 end_seq,
                     bool dup_sack_in)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *tmp;
 
     skb_rbtree_walk_from(skb) {
         int in_sack = 0;
         bool dup_sack = dup_sack_in;
 
         /* queue is in-order => we can short-circuit the walk early */
         if (!before(TCP_SKB_CB(skb)->seq, end_seq))
             break;
 
         if (next_dup  &&
             before(TCP_SKB_CB(skb)->seq, next_dup->end_seq)) {
             in_sack = tcp_match_skb_to_sack(sk, skb,
                             next_dup->start_seq,
                             next_dup->end_seq);
             if (in_sack > 0)
                 dup_sack = true;
         }
 
         /* skb reference here is a bit tricky to get right, since
          * shifting can eat and free both this skb and the next,
          * so not even _safe variant of the loop is enough.
          */
         if (in_sack <= 0) {
             tmp = tcp_shift_skb_data(sk, skb, state,
                          start_seq, end_seq, dup_sack);
             if (tmp) {
                 if (tmp != skb) {
                     skb = tmp;
                     continue;
                 }
 
                 in_sack = 0;
             } else {
                 in_sack = tcp_match_skb_to_sack(sk, skb,
                                 start_seq,
                                 end_seq);
             }
         }
 
         if (unlikely(in_sack < 0))
             break;
 
         if (in_sack) {
             TCP_SKB_CB(skb)->sacked =
                 tcp_sacktag_one(sk,
                         state,
                         TCP_SKB_CB(skb)->sacked,
                         TCP_SKB_CB(skb)->seq,
                         TCP_SKB_CB(skb)->end_seq,
                         dup_sack,
                         tcp_skb_pcount(skb),
                         tcp_skb_timestamp_us(skb));
             tcp_rate_skb_delivered(sk, skb, state->rate);
             if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                 list_del_init(&skb->tcp_tsorted_anchor);
 
             if (!before(TCP_SKB_CB(skb)->seq,
                     tcp_highest_sack_seq(tp)))
                 tcp_advance_highest_sack(sk, skb);
         }
     }
     return skb;
 }
 
 static struct sk_buff *tcp_sacktag_bsearch(struct sock *sk, u32 seq)
 {
     struct rb_node *parent, **p = &sk->tcp_rtx_queue.rb_node;
     struct sk_buff *skb;
 
     while (*p) {
         parent = *p;
         skb = rb_to_skb(parent);
         if (before(seq, TCP_SKB_CB(skb)->seq)) {
             p = &parent->rb_left;
             continue;
         }
         if (!before(seq, TCP_SKB_CB(skb)->end_seq)) {
             p = &parent->rb_right;
             continue;
         }
         return skb;
     }
     return NULL;
 }
 
 static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
                     u32 skip_to_seq)
 {
     if (skb && after(TCP_SKB_CB(skb)->seq, skip_to_seq))
         return skb;
 
     return tcp_sacktag_bsearch(sk, skip_to_seq);
 }
 
 static struct sk_buff *tcp_maybe_skipping_dsack(struct sk_buff *skb,
                         struct sock *sk,
                         struct tcp_sack_block *next_dup,
                         struct tcp_sacktag_state *state,
                         u32 skip_to_seq)
 {
     if (!next_dup)
         return skb;
 
     if (before(next_dup->start_seq, skip_to_seq)) {
         skb = tcp_sacktag_skip(skb, sk, next_dup->start_seq);
         skb = tcp_sacktag_walk(skb, sk, NULL, state,
                        next_dup->start_seq, next_dup->end_seq,
                        1);
     }
 
     return skb;
 }
 
 static int tcp_sack_cache_ok(const struct tcp_sock *tp, const struct tcp_sack_block *cache)
 {
     return cache < tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
 }
 
 static int
 tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
             u32 prior_snd_una, struct tcp_sacktag_state *state)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     const unsigned char *ptr = (skb_transport_header(ack_skb) +
                     TCP_SKB_CB(ack_skb)->sacked);
     struct tcp_sack_block_wire *sp_wire = (struct tcp_sack_block_wire *)(ptr+2);
     struct tcp_sack_block sp[TCP_NUM_SACKS];
     struct tcp_sack_block *cache;
     struct sk_buff *skb;
     int num_sacks = min(TCP_NUM_SACKS, (ptr[1] - TCPOLEN_SACK_BASE) >> 3);
     int used_sacks;
     bool found_dup_sack = false;
     int i, j;
     int first_sack_index;
 
     state->flag = 0;
     state->reord = tp->snd_nxt;
 
     if (!tp->sacked_out)
         tcp_highest_sack_reset(sk);
 
     found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
                      num_sacks, prior_snd_una, state);
 
     /* Eliminate too old ACKs, but take into
      * account more or less fresh ones, they can
      * contain valid SACK info.
      */
     if (before(TCP_SKB_CB(ack_skb)->ack_seq, prior_snd_una - tp->max_window))
         return 0;
 
     if (!tp->packets_out)
         goto out;
 
     used_sacks = 0;
     first_sack_index = 0;
     for (i = 0; i < num_sacks; i++) {
         bool dup_sack = !i && found_dup_sack;
 
         sp[used_sacks].start_seq = get_unaligned_be32(&sp_wire[i].start_seq);
         sp[used_sacks].end_seq = get_unaligned_be32(&sp_wire[i].end_seq);
 
         if (!tcp_is_sackblock_valid(tp, dup_sack,
                         sp[used_sacks].start_seq,
                         sp[used_sacks].end_seq)) {
             int mib_idx;
 
             if (dup_sack) {
                 if (!tp->undo_marker)
                     mib_idx = LINUX_MIB_TCPDSACKIGNOREDNOUNDO;
                 else
                     mib_idx = LINUX_MIB_TCPDSACKIGNOREDOLD;
             } else {
                 /* Don't count olds caused by ACK reordering */
                 if ((TCP_SKB_CB(ack_skb)->ack_seq != tp->snd_una) &&
                     !after(sp[used_sacks].end_seq, tp->snd_una))
                     continue;
                 mib_idx = LINUX_MIB_TCPSACKDISCARD;
             }
 
             NET_INC_STATS(sock_net(sk), mib_idx);
             if (i == 0)
                 first_sack_index = -1;
             continue;
         }
 
         /* Ignore very old stuff early */
         if (!after(sp[used_sacks].end_seq, prior_snd_una)) {
             if (i == 0)
                 first_sack_index = -1;
             continue;
         }
 
         used_sacks++;
     }
 
     /* order SACK blocks to allow in order walk of the retrans queue */
     for (i = used_sacks - 1; i > 0; i--) {
         for (j = 0; j < i; j++) {
             if (after(sp[j].start_seq, sp[j + 1].start_seq)) {
                 swap(sp[j], sp[j + 1]);
 
                 /* Track where the first SACK block goes to */
                 if (j == first_sack_index)
                     first_sack_index = j + 1;
             }
         }
     }
 
     state->mss_now = tcp_current_mss(sk);
     skb = NULL;
     i = 0;
 
     if (!tp->sacked_out) {
         /* It's already past, so skip checking against it */
         cache = tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
     } else {
         cache = tp->recv_sack_cache;
         /* Skip empty blocks in at head of the cache */
         while (tcp_sack_cache_ok(tp, cache) && !cache->start_seq &&
                !cache->end_seq)
             cache++;
     }
 
     while (i < used_sacks) {
         u32 start_seq = sp[i].start_seq;
         u32 end_seq = sp[i].end_seq;
         bool dup_sack = (found_dup_sack && (i == first_sack_index));
         struct tcp_sack_block *next_dup = NULL;
 
         if (found_dup_sack && ((i + 1) == first_sack_index))
             next_dup = &sp[i + 1];
 
         /* Skip too early cached blocks */
         while (tcp_sack_cache_ok(tp, cache) &&
                !before(start_seq, cache->end_seq))
             cache++;
 
         /* Can skip some work by looking recv_sack_cache? */
         if (tcp_sack_cache_ok(tp, cache) && !dup_sack &&
             after(end_seq, cache->start_seq)) {
 
             /* Head todo? */
             if (before(start_seq, cache->start_seq)) {
                 skb = tcp_sacktag_skip(skb, sk, start_seq);
                 skb = tcp_sacktag_walk(skb, sk, next_dup,
                                state,
                                start_seq,
                                cache->start_seq,
                                dup_sack);
             }
 
             /* Rest of the block already fully processed? */
             if (!after(end_seq, cache->end_seq))
                 goto advance_sp;
 
             skb = tcp_maybe_skipping_dsack(skb, sk, next_dup,
                                state,
                                cache->end_seq);
 
             /* ...tail remains todo... */
             if (tcp_highest_sack_seq(tp) == cache->end_seq) {
                 /* ...but better entrypoint exists! */
                 skb = tcp_highest_sack(sk);
                 if (!skb)
                     break;
                 cache++;
                 goto walk;
             }
 
             skb = tcp_sacktag_skip(skb, sk, cache->end_seq);
             /* Check overlap against next cached too (past this one already) */
             cache++;
             continue;
         }
 
         if (!before(start_seq, tcp_highest_sack_seq(tp))) {
             skb = tcp_highest_sack(sk);
             if (!skb)
                 break;
         }
         skb = tcp_sacktag_skip(skb, sk, start_seq);
 
 walk:
         skb = tcp_sacktag_walk(skb, sk, next_dup, state,
                        start_seq, end_seq, dup_sack);
 
 advance_sp:
         i++;
     }
 
     /* Clear the head of the cache sack blocks so we can skip it next time */
     for (i = 0; i < ARRAY_SIZE(tp->recv_sack_cache) - used_sacks; i++) {
         tp->recv_sack_cache[i].start_seq = 0;
         tp->recv_sack_cache[i].end_seq = 0;
     }
     for (j = 0; j < used_sacks; j++)
         tp->recv_sack_cache[i++] = sp[j];
 
     if (inet_csk(sk)->icsk_ca_state != TCP_CA_Loss || tp->undo_marker)
         tcp_check_sack_reordering(sk, state->reord, 0);
 
     tcp_verify_left_out(tp);
 out:
 
 #if FASTRETRANS_DEBUG > 0
     WARN_ON((int)tp->sacked_out < 0);
     WARN_ON((int)tp->lost_out < 0);
     WARN_ON((int)tp->retrans_out < 0);
     WARN_ON((int)tcp_packets_in_flight(tp) < 0);
 #endif
     return state->flag;
 }
 
 /* Limits sacked_out so that sum with lost_out isn't ever larger than
  * packets_out. Returns false if sacked_out adjustement wasn't necessary.
  */
 static bool tcp_limit_reno_sacked(struct tcp_sock *tp)
 {
     u32 holes;
 
     holes = max(tp->lost_out, 1U);
     holes = min(holes, tp->packets_out);
 
     if ((tp->sacked_out + holes) > tp->packets_out) {
         tp->sacked_out = tp->packets_out - holes;
         return true;
     }
     return false;
 }
 
 /* If we receive more dupacks than we expected counting segments
  * in assumption of absent reordering, interpret this as reordering.
  * The only another reason could be bug in receiver TCP.
  */
 static void tcp_check_reno_reordering(struct sock *sk, const int addend)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (!tcp_limit_reno_sacked(tp))
         return;
 
     tp->reordering = min_t(u32, tp->packets_out + addend,
                    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_max_reordering));
     tp->reord_seen++;
     NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRENOREORDER);
 }
 
 /* Emulate SACKs for SACKless connection: account for a new dupack. */
 
 static void tcp_add_reno_sack(struct sock *sk, int num_dupack, bool ece_ack)
 {
     if (num_dupack) {
         struct tcp_sock *tp = tcp_sk(sk);
         u32 prior_sacked = tp->sacked_out;
         s32 delivered;
 
         tp->sacked_out += num_dupack;
         tcp_check_reno_reordering(sk, 0);
         delivered = tp->sacked_out - prior_sacked;
         if (delivered > 0)
             tcp_count_delivered(tp, delivered, ece_ack);
         tcp_verify_left_out(tp);
     }
 }
 
 /* Account for ACK, ACKing some data in Reno Recovery phase. */
 
 static void tcp_remove_reno_sacks(struct sock *sk, int acked, bool ece_ack)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (acked > 0) {
         /* One ACK acked hole. The rest eat duplicate ACKs. */
         tcp_count_delivered(tp, max_t(int, acked - tp->sacked_out, 1),
                     ece_ack);
         if (acked - 1 >= tp->sacked_out)
             tp->sacked_out = 0;
         else
             tp->sacked_out -= acked - 1;
     }
     tcp_check_reno_reordering(sk, acked);
     tcp_verify_left_out(tp);
 }
 
 static inline void tcp_reset_reno_sack(struct tcp_sock *tp)
 {
     tp->sacked_out = 0;
 }
 
 void tcp_clear_retrans(struct tcp_sock *tp)
 {
     tp->retrans_out = 0;
     tp->lost_out = 0;
     tp->undo_marker = 0;
     tp->undo_retrans = -1;
     tp->sacked_out = 0;
 }
 
 static inline void tcp_init_undo(struct tcp_sock *tp)
 {
     tp->undo_marker = tp->snd_una;
     /* Retransmission still in flight may cause DSACKs later. */
     tp->undo_retrans = tp->retrans_out ? : -1;
 }
 
 static bool tcp_is_rack(const struct sock *sk)
 {
     return READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) &
         TCP_RACK_LOSS_DETECTION;
 }
 
 /* If we detect SACK reneging, forget all SACK information
  * and reset tags completely, otherwise preserve SACKs. If receiver
  * dropped its ofo queue, we will know this due to reneging detection.
  */
 static void tcp_timeout_mark_lost(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb, *head;
     bool is_reneg;          /* is receiver reneging on SACKs? */
 
     head = tcp_rtx_queue_head(sk);
     is_reneg = head && (TCP_SKB_CB(head)->sacked & TCPCB_SACKED_ACKED);
     if (is_reneg) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);
         tp->sacked_out = 0;
         /* Mark SACK reneging until we recover from this loss event. */
         tp->is_sack_reneg = 1;
     } else if (tcp_is_reno(tp)) {
         tcp_reset_reno_sack(tp);
     }
 
     skb = head;
     skb_rbtree_walk_from(skb) {
         if (is_reneg)
             TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
         else if (tcp_is_rack(sk) && skb != head &&
              tcp_rack_skb_timeout(tp, skb, 0) > 0)
             continue; /* Don't mark recently sent ones lost yet */
         tcp_mark_skb_lost(sk, skb);
     }
     tcp_verify_left_out(tp);
     tcp_clear_all_retrans_hints(tp);
 }
 
 /* Enter Loss state. */
 void tcp_enter_loss(struct sock *sk)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct net *net = sock_net(sk);
     bool new_recovery = icsk->icsk_ca_state < TCP_CA_Recovery;
     u8 reordering;
 
     tcp_timeout_mark_lost(sk);
 
     /* Reduce ssthresh if it has not yet been made inside this window. */
     if (icsk->icsk_ca_state <= TCP_CA_Disorder ||
         !after(tp->high_seq, tp->snd_una) ||
         (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
         tp->prior_ssthresh = tcp_current_ssthresh(sk);
         tp->prior_cwnd = tcp_snd_cwnd(tp);
         tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
         tcp_ca_event(sk, CA_EVENT_LOSS);
         tcp_init_undo(tp);
     }
     tcp_snd_cwnd_set(tp, tcp_packets_in_flight(tp) + 1);
     tp->snd_cwnd_cnt   = 0;
     tp->snd_cwnd_stamp = tcp_jiffies32;
 
     /* Timeout in disordered state after receiving substantial DUPACKs
      * suggests that the degree of reordering is over-estimated.
      */
     reordering = READ_ONCE(net->ipv4.sysctl_tcp_reordering);
     if (icsk->icsk_ca_state <= TCP_CA_Disorder &&
         tp->sacked_out >= reordering)
         tp->reordering = min_t(unsigned int, tp->reordering,
                        reordering);
 
     tcp_set_ca_state(sk, TCP_CA_Loss);
     tp->high_seq = tp->snd_nxt;
     tcp_ecn_queue_cwr(tp);
 
     /* F-RTO RFC5682 sec 3.1 step 1: retransmit SND.UNA if no previous
      * loss recovery is underway except recurring timeout(s) on
      * the same SND.UNA (sec 3.2). Disable F-RTO on path MTU probing
      */
     tp->frto = READ_ONCE(net->ipv4.sysctl_tcp_frto) &&
            (new_recovery || icsk->icsk_retransmits) &&
            !inet_csk(sk)->icsk_mtup.probe_size;
 }
 
 /* If ACK arrived pointing to a remembered SACK, it means that our
  * remembered SACKs do not reflect real state of receiver i.e.
  * receiver _host_ is heavily congested (or buggy).
  *
  * To avoid big spurious retransmission bursts due to transient SACK
  * scoreboard oddities that look like reneging, we give the receiver a
  * little time (max(RTT/2, 10ms)) to send us some more ACKs that will
  * restore sanity to the SACK scoreboard. If the apparent reneging
  * persists until this RTO then we'll clear the SACK scoreboard.
  */
 static bool tcp_check_sack_reneging(struct sock *sk, int flag)
 {
     if (flag & FLAG_SACK_RENEGING) {
         struct tcp_sock *tp = tcp_sk(sk);
         unsigned long delay = max(usecs_to_jiffies(tp->srtt_us >> 4),
                       msecs_to_jiffies(10));
 
         inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                       delay, TCP_RTO_MAX);
         return true;
     }
     return false;
 }
 
 /* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
  * counter when SACK is enabled (without SACK, sacked_out is used for
  * that purpose).
  *
  * With reordering, holes may still be in flight, so RFC3517 recovery
  * uses pure sacked_out (total number of SACKed segments) even though
  * it violates the RFC that uses duplicate ACKs, often these are equal
  * but when e.g. out-of-window ACKs or packet duplication occurs,
  * they differ. Since neither occurs due to loss, TCP should really
  * ignore them.
  */
 static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
 {
     return tp->sacked_out + 1;
 }
 
 /* Linux NewReno/SACK/ECN state machine.
  * --------------------------------------
  *
  * "Open"   Normal state, no dubious events, fast path.
  * "Disorder"   In all the respects it is "Open",
  *      but requires a bit more attention. It is entered when
  *      we see some SACKs or dupacks. It is split of "Open"
  *      mainly to move some processing from fast path to slow one.
  * "CWR"    CWND was reduced due to some Congestion Notification event.
  *      It can be ECN, ICMP source quench, local device congestion.
  * "Recovery"   CWND was reduced, we are fast-retransmitting.
  * "Loss"   CWND was reduced due to RTO timeout or SACK reneging.
  *
  * tcp_fastretrans_alert() is entered:
  * - each incoming ACK, if state is not "Open"
  * - when arrived ACK is unusual, namely:
  *  * SACK
  *  * Duplicate ACK.
  *  * ECN ECE.
  *
  * Counting packets in flight is pretty simple.
  *
  *  in_flight = packets_out - left_out + retrans_out
  *
  *  packets_out is SND.NXT-SND.UNA counted in packets.
  *
  *  retrans_out is number of retransmitted segments.
  *
  *  left_out is number of segments left network, but not ACKed yet.
  *
  *      left_out = sacked_out + lost_out
  *
  *     sacked_out: Packets, which arrived to receiver out of order
  *         and hence not ACKed. With SACKs this number is simply
  *         amount of SACKed data. Even without SACKs
  *         it is easy to give pretty reliable estimate of this number,
  *         counting duplicate ACKs.
  *
  *       lost_out: Packets lost by network. TCP has no explicit
  *         "loss notification" feedback from network (for now).
  *         It means that this number can be only _guessed_.
  *         Actually, it is the heuristics to predict lossage that
  *         distinguishes different algorithms.
  *
  *  F.e. after RTO, when all the queue is considered as lost,
  *  lost_out = packets_out and in_flight = retrans_out.
  *
  *      Essentially, we have now a few algorithms detecting
  *      lost packets.
  *
  *      If the receiver supports SACK:
  *
  *      RFC6675/3517: It is the conventional algorithm. A packet is
  *      considered lost if the number of higher sequence packets
  *      SACKed is greater than or equal the DUPACK thoreshold
  *      (reordering). This is implemented in tcp_mark_head_lost and
  *      tcp_update_scoreboard.
  *
  *      RACK (draft-ietf-tcpm-rack-01): it is a newer algorithm
  *      (2017-) that checks timing instead of counting DUPACKs.
  *      Essentially a packet is considered lost if it's not S/ACKed
  *      after RTT + reordering_window, where both metrics are
  *      dynamically measured and adjusted. This is implemented in
  *      tcp_rack_mark_lost.
  *
  *      If the receiver does not support SACK:
  *
  *      NewReno (RFC6582): in Recovery we assume that one segment
  *      is lost (classic Reno). While we are in Recovery and
  *      a partial ACK arrives, we assume that one more packet
  *      is lost (NewReno). This heuristics are the same in NewReno
  *      and SACK.
  *
  * Really tricky (and requiring careful tuning) part of algorithm
  * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue().
  * The first determines the moment _when_ we should reduce CWND and,
  * hence, slow down forward transmission. In fact, it determines the moment
  * when we decide that hole is caused by loss, rather than by a reorder.
  *
  * tcp_xmit_retransmit_queue() decides, _what_ we should retransmit to fill
  * holes, caused by lost packets.
  *
  * And the most logically complicated part of algorithm is undo
  * heuristics. We detect false retransmits due to both too early
  * fast retransmit (reordering) and underestimated RTO, analyzing
  * timestamps and D-SACKs. When we detect that some segments were
  * retransmitted by mistake and CWND reduction was wrong, we undo
  * window reduction and abort recovery phase. This logic is hidden
  * inside several functions named tcp_try_undo_<something>.
  */
 
 /* This function decides, when we should leave Disordered state
  * and enter Recovery phase, reducing congestion window.
  *
  * Main question: may we further continue forward transmission
  * with the same cwnd?
  */
 static bool tcp_time_to_recover(struct sock *sk, int flag)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* Trick#1: The loss is proven. */
     if (tp->lost_out)
         return true;
 
     /* Not-A-Trick#2 : Classic rule... */
     if (!tcp_is_rack(sk) && tcp_dupack_heuristics(tp) > tp->reordering)
         return true;
 
     return false;
 }
 
 /* Detect loss in event "A" above by marking head of queue up as lost.
  * For RFC3517 SACK, a segment is considered lost if it
  * has at least tp->reordering SACKed seqments above it; "packets" refers to
  * the maximum SACKed segments to pass before reaching this limit.
  */
 static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb;
     int cnt;
     /* Use SACK to deduce losses of new sequences sent during recovery */
     const u32 loss_high = tp->snd_nxt;
 
     WARN_ON(packets > tp->packets_out);
     skb = tp->lost_skb_hint;
     if (skb) {
         /* Head already handled? */
         if (mark_head && after(TCP_SKB_CB(skb)->seq, tp->snd_una))
             return;
         cnt = tp->lost_cnt_hint;
     } else {
         skb = tcp_rtx_queue_head(sk);
         cnt = 0;
     }
 
     skb_rbtree_walk_from(skb) {
         /* TODO: do this better */
         /* this is not the most efficient way to do this... */
         tp->lost_skb_hint = skb;
         tp->lost_cnt_hint = cnt;
 
         if (after(TCP_SKB_CB(skb)->end_seq, loss_high))
             break;
 
         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
             cnt += tcp_skb_pcount(skb);
 
         if (cnt > packets)
             break;
 
         if (!(TCP_SKB_CB(skb)->sacked & TCPCB_LOST))
             tcp_mark_skb_lost(sk, skb);
 
         if (mark_head)
             break;
     }
     tcp_verify_left_out(tp);
 }
 
 /* Account newly detected lost packet(s) */
 
 static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tcp_is_sack(tp)) {
         int sacked_upto = tp->sacked_out - tp->reordering;
         if (sacked_upto >= 0)
             tcp_mark_head_lost(sk, sacked_upto, 0);
         else if (fast_rexmit)
             tcp_mark_head_lost(sk, 1, 1);
     }
 }
 
 static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when)
 {
     return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
            before(tp->rx_opt.rcv_tsecr, when);
 }
 
 /* skb is spurious retransmitted if the returned timestamp echo
  * reply is prior to the skb transmission time
  */
 static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp,
                      const struct sk_buff *skb)
 {
     return (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS) &&
            tcp_tsopt_ecr_before(tp, tcp_skb_timestamp(skb));
 }
 
 /* Nothing was retransmitted or returned timestamp is less
  * than timestamp of the first retransmission.
  */
 static inline bool tcp_packet_delayed(const struct tcp_sock *tp)
 {
     return tp->retrans_stamp &&
            tcp_tsopt_ecr_before(tp, tp->retrans_stamp);
 }
 
 /* Undo procedures. */
 
 /* We can clear retrans_stamp when there are no retransmissions in the
  * window. It would seem that it is trivially available for us in
  * tp->retrans_out, however, that kind of assumptions doesn't consider
  * what will happen if errors occur when sending retransmission for the
  * second time. ...It could the that such segment has only
  * TCPCB_EVER_RETRANS set at the present time. It seems that checking
  * the head skb is enough except for some reneging corner cases that
  * are not worth the effort.
  *
  * Main reason for all this complexity is the fact that connection dying
  * time now depends on the validity of the retrans_stamp, in particular,
  * that successive retransmissions of a segment must not advance
  * retrans_stamp under any conditions.
  */
 static bool tcp_any_retrans_done(const struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb;
 
     if (tp->retrans_out)
         return true;
 
     skb = tcp_rtx_queue_head(sk);
     if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS))
         return true;
 
     return false;
 }
 
 static void DBGUNDO(struct sock *sk, const char *msg)
 {
 #if FASTRETRANS_DEBUG > 1
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_sock *inet = inet_sk(sk);
 
     if (sk->sk_family == AF_INET) {
         pr_debug("Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
              msg,
              &inet->inet_daddr, ntohs(inet->inet_dport),
              tcp_snd_cwnd(tp), tcp_left_out(tp),
              tp->snd_ssthresh, tp->prior_ssthresh,
              tp->packets_out);
     }
 #if IS_ENABLED(CONFIG_IPV6)
     else if (sk->sk_family == AF_INET6) {
         pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
              msg,
              &sk->sk_v6_daddr, ntohs(inet->inet_dport),
              tcp_snd_cwnd(tp), tcp_left_out(tp),
              tp->snd_ssthresh, tp->prior_ssthresh,
              tp->packets_out);
     }
 #endif
 #endif
 }
 
 static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (unmark_loss) {
         struct sk_buff *skb;
 
         skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
             TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
         }
         tp->lost_out = 0;
         tcp_clear_all_retrans_hints(tp);
     }
 
     if (tp->prior_ssthresh) {
         const struct inet_connection_sock *icsk = inet_csk(sk);
 
         tcp_snd_cwnd_set(tp, icsk->icsk_ca_ops->undo_cwnd(sk));
 
         if (tp->prior_ssthresh > tp->snd_ssthresh) {
             tp->snd_ssthresh = tp->prior_ssthresh;
             tcp_ecn_withdraw_cwr(tp);
         }
     }
     tp->snd_cwnd_stamp = tcp_jiffies32;
     tp->undo_marker = 0;
     tp->rack.advanced = 1; /* Force RACK to re-exam losses */
 }
 
 static inline bool tcp_may_undo(const struct tcp_sock *tp)
 {
     return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp));
 }
 
 static bool tcp_is_non_sack_preventing_reopen(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
         /* Hold old state until something *above* high_seq
          * is ACKed. For Reno it is MUST to prevent false
          * fast retransmits (RFC2582). SACK TCP is safe. */
         if (!tcp_any_retrans_done(sk))
             tp->retrans_stamp = 0;
         return true;
     }
     return false;
 }
 
 /* People celebrate: "We love our President!" */
 static bool tcp_try_undo_recovery(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tcp_may_undo(tp)) {
         int mib_idx;
 
         /* Happy end! We did not retransmit anything
          * or our original transmission succeeded.
          */
         DBGUNDO(sk, inet_csk(sk)->icsk_ca_state == TCP_CA_Loss ? "loss" : "retrans");
         tcp_undo_cwnd_reduction(sk, false);
         if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
             mib_idx = LINUX_MIB_TCPLOSSUNDO;
         else
             mib_idx = LINUX_MIB_TCPFULLUNDO;
 
         NET_INC_STATS(sock_net(sk), mib_idx);
     } else if (tp->rack.reo_wnd_persist) {
         tp->rack.reo_wnd_persist--;
     }
     if (tcp_is_non_sack_preventing_reopen(sk))
         return true;
     tcp_set_ca_state(sk, TCP_CA_Open);
     tp->is_sack_reneg = 0;
     return false;
 }
 
 /* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
 static bool tcp_try_undo_dsack(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tp->undo_marker && !tp->undo_retrans) {
         tp->rack.reo_wnd_persist = min(TCP_RACK_RECOVERY_THRESH,
                            tp->rack.reo_wnd_persist + 1);
         DBGUNDO(sk, "D-SACK");
         tcp_undo_cwnd_reduction(sk, false);
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
         return true;
     }
     return false;
 }
 
 /* Undo during loss recovery after partial ACK or using F-RTO. */
 static bool tcp_try_undo_loss(struct sock *sk, bool frto_undo)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (frto_undo || tcp_may_undo(tp)) {
         tcp_undo_cwnd_reduction(sk, true);
 
         DBGUNDO(sk, "partial loss");
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
         if (frto_undo)
             NET_INC_STATS(sock_net(sk),
                     LINUX_MIB_TCPSPURIOUSRTOS);
         inet_csk(sk)->icsk_retransmits = 0;
         if (tcp_is_non_sack_preventing_reopen(sk))
             return true;
         if (frto_undo || tcp_is_sack(tp)) {
             tcp_set_ca_state(sk, TCP_CA_Open);
             tp->is_sack_reneg = 0;
         }
         return true;
     }
     return false;
 }
 
 /* The cwnd reduction in CWR and Recovery uses the PRR algorithm in RFC 6937.
  * It computes the number of packets to send (sndcnt) based on packets newly
  * delivered:
  *   1) If the packets in flight is larger than ssthresh, PRR spreads the
  *  cwnd reductions across a full RTT.
  *   2) Otherwise PRR uses packet conservation to send as much as delivered.
  *      But when SND_UNA is acked without further losses,
  *      slow starts cwnd up to ssthresh to speed up the recovery.
  */
 static void tcp_init_cwnd_reduction(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     tp->high_seq = tp->snd_nxt;
     tp->tlp_high_seq = 0;
     tp->snd_cwnd_cnt = 0;
     tp->prior_cwnd = tcp_snd_cwnd(tp);
     tp->prr_delivered = 0;
     tp->prr_out = 0;
     tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
     tcp_ecn_queue_cwr(tp);
 }
 
 void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost, int flag)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     int sndcnt = 0;
     int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
 
     if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
         return;
 
     tp->prr_delivered += newly_acked_sacked;
     if (delta < 0) {
         u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
                    tp->prior_cwnd - 1;
         sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
     } else {
         sndcnt = max_t(int, tp->prr_delivered - tp->prr_out,
                    newly_acked_sacked);
         if (flag & FLAG_SND_UNA_ADVANCED && !newly_lost)
             sndcnt++;
         sndcnt = min(delta, sndcnt);
     }
     /* Force a fast retransmit upon entering fast recovery */
     sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1));
     tcp_snd_cwnd_set(tp, tcp_packets_in_flight(tp) + sndcnt);
 }
 
 static inline void tcp_end_cwnd_reduction(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (inet_csk(sk)->icsk_ca_ops->cong_control)
         return;
 
     /* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
     if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH &&
         (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR || tp->undo_marker)) {
         tcp_snd_cwnd_set(tp, tp->snd_ssthresh);
         tp->snd_cwnd_stamp = tcp_jiffies32;
     }
     tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
 }
 
 /* Enter CWR state. Disable cwnd undo since congestion is proven with ECN */
 void tcp_enter_cwr(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     tp->prior_ssthresh = 0;
     if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
         tp->undo_marker = 0;
         tcp_init_cwnd_reduction(sk);
         tcp_set_ca_state(sk, TCP_CA_CWR);
     }
 }
 EXPORT_SYMBOL(tcp_enter_cwr);
 
 static void tcp_try_keep_open(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     int state = TCP_CA_Open;
 
     if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
         state = TCP_CA_Disorder;
 
     if (inet_csk(sk)->icsk_ca_state != state) {
         tcp_set_ca_state(sk, state);
         tp->high_seq = tp->snd_nxt;
     }
 }
 
 static void tcp_try_to_open(struct sock *sk, int flag)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     tcp_verify_left_out(tp);
 
     if (!tcp_any_retrans_done(sk))
         tp->retrans_stamp = 0;
 
     if (flag & FLAG_ECE)
         tcp_enter_cwr(sk);
 
     if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
         tcp_try_keep_open(sk);
     }
 }
 
 static void tcp_mtup_probe_failed(struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
 
     icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
     icsk->icsk_mtup.probe_size = 0;
     NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPFAIL);
 }
 
 static void tcp_mtup_probe_success(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_connection_sock *icsk = inet_csk(sk);
     u64 val;
 
     tp->prior_ssthresh = tcp_current_ssthresh(sk);
 
     val = (u64)tcp_snd_cwnd(tp) * tcp_mss_to_mtu(sk, tp->mss_cache);
     do_div(val, icsk->icsk_mtup.probe_size);
     DEBUG_NET_WARN_ON_ONCE((u32)val != val);
     tcp_snd_cwnd_set(tp, max_t(u32, 1U, val));
 
     tp->snd_cwnd_cnt = 0;
     tp->snd_cwnd_stamp = tcp_jiffies32;
     tp->snd_ssthresh = tcp_current_ssthresh(sk);
 
     icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
     icsk->icsk_mtup.probe_size = 0;
     tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
     NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPSUCCESS);
 }
 
 /* Do a simple retransmit without using the backoff mechanisms in
  * tcp_timer. This is used for path mtu discovery.
  * The socket is already locked here.
  */
 void tcp_simple_retransmit(struct sock *sk)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb;
     int mss;
 
     /* A fastopen SYN request is stored as two separate packets within
      * the retransmit queue, this is done by tcp_send_syn_data().
      * As a result simply checking the MSS of the frames in the queue
      * will not work for the SYN packet.
      *
      * Us being here is an indication of a path MTU issue so we can
      * assume that the fastopen SYN was lost and just mark all the
      * frames in the retransmit queue as lost. We will use an MSS of
      * -1 to mark all frames as lost, otherwise compute the current MSS.
      */
     if (tp->syn_data && sk->sk_state == TCP_SYN_SENT)
         mss = -1;
     else
         mss = tcp_current_mss(sk);
 
     skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
         if (tcp_skb_seglen(skb) > mss)
             tcp_mark_skb_lost(sk, skb);
     }
 
     tcp_clear_retrans_hints_partial(tp);
 
     if (!tp->lost_out)
         return;
 
     if (tcp_is_reno(tp))
         tcp_limit_reno_sacked(tp);
 
     tcp_verify_left_out(tp);
 
     /* Don't muck with the congestion window here.
      * Reason is that we do not increase amount of _data_
      * in network, but units changed and effective
      * cwnd/ssthresh really reduced now.
      */
     if (icsk->icsk_ca_state != TCP_CA_Loss) {
         tp->high_seq = tp->snd_nxt;
         tp->snd_ssthresh = tcp_current_ssthresh(sk);
         tp->prior_ssthresh = 0;
         tp->undo_marker = 0;
         tcp_set_ca_state(sk, TCP_CA_Loss);
     }
     tcp_xmit_retransmit_queue(sk);
 }
 EXPORT_SYMBOL(tcp_simple_retransmit);
 
 void tcp_enter_recovery(struct sock *sk, bool ece_ack)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     int mib_idx;
 
     if (tcp_is_reno(tp))
         mib_idx = LINUX_MIB_TCPRENORECOVERY;
     else
         mib_idx = LINUX_MIB_TCPSACKRECOVERY;
 
     NET_INC_STATS(sock_net(sk), mib_idx);
 
     tp->prior_ssthresh = 0;
     tcp_init_undo(tp);
 
     if (!tcp_in_cwnd_reduction(sk)) {
         if (!ece_ack)
             tp->prior_ssthresh = tcp_current_ssthresh(sk);
         tcp_init_cwnd_reduction(sk);
     }
     tcp_set_ca_state(sk, TCP_CA_Recovery);
 }
 
 /* Process an ACK in CA_Loss state. Move to CA_Open if lost data are
  * recovered or spurious. Otherwise retransmits more on partial ACKs.
  */
 static void tcp_process_loss(struct sock *sk, int flag, int num_dupack,
                  int *rexmit)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     bool recovered = !before(tp->snd_una, tp->high_seq);
 
     if ((flag & FLAG_SND_UNA_ADVANCED || rcu_access_pointer(tp->fastopen_rsk)) &&
         tcp_try_undo_loss(sk, false))
         return;
 
     if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
         /* Step 3.b. A timeout is spurious if not all data are
          * lost, i.e., never-retransmitted data are (s)acked.
          */
         if ((flag & FLAG_ORIG_SACK_ACKED) &&
             tcp_try_undo_loss(sk, true))
             return;
 
         if (after(tp->snd_nxt, tp->high_seq)) {
             if (flag & FLAG_DATA_SACKED || num_dupack)
                 tp->frto = 0; /* Step 3.a. loss was real */
         } else if (flag & FLAG_SND_UNA_ADVANCED && !recovered) {
             tp->high_seq = tp->snd_nxt;
             /* Step 2.b. Try send new data (but deferred until cwnd
              * is updated in tcp_ack()). Otherwise fall back to
              * the conventional recovery.
              */
             if (!tcp_write_queue_empty(sk) &&
                 after(tcp_wnd_end(tp), tp->snd_nxt)) {
                 *rexmit = REXMIT_NEW;
                 return;
             }
             tp->frto = 0;
         }
     }
 
     if (recovered) {
         /* F-RTO RFC5682 sec 3.1 step 2.a and 1st part of step 3.a */
         tcp_try_undo_recovery(sk);
         return;
     }
     if (tcp_is_reno(tp)) {
         /* A Reno DUPACK means new data in F-RTO step 2.b above are
          * delivered. Lower inflight to clock out (re)tranmissions.
          */
         if (after(tp->snd_nxt, tp->high_seq) && num_dupack)
             tcp_add_reno_sack(sk, num_dupack, flag & FLAG_ECE);
         else if (flag & FLAG_SND_UNA_ADVANCED)
             tcp_reset_reno_sack(tp);
     }
     *rexmit = REXMIT_LOST;
 }
 
 static bool tcp_force_fast_retransmit(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     return after(tcp_highest_sack_seq(tp),
              tp->snd_una + tp->reordering * tp->mss_cache);
 }
 
 /* Undo during fast recovery after partial ACK. */
 static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una,
                  bool *do_lost)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tp->undo_marker && tcp_packet_delayed(tp)) {
         /* Plain luck! Hole if filled with delayed
          * packet, rather than with a retransmit. Check reordering.
          */
         tcp_check_sack_reordering(sk, prior_snd_una, 1);
 
         /* We are getting evidence that the reordering degree is higher
          * than we realized. If there are no retransmits out then we
          * can undo. Otherwise we clock out new packets but do not
          * mark more packets lost or retransmit more.
          */
         if (tp->retrans_out)
             return true;
 
         if (!tcp_any_retrans_done(sk))
             tp->retrans_stamp = 0;
 
         DBGUNDO(sk, "partial recovery");
         tcp_undo_cwnd_reduction(sk, true);
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);
         tcp_try_keep_open(sk);
     } else {
         /* Partial ACK arrived. Force fast retransmit. */
         *do_lost = tcp_force_fast_retransmit(sk);
     }
     return false;
 }
 
 static void tcp_identify_packet_loss(struct sock *sk, int *ack_flag)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tcp_rtx_queue_empty(sk))
         return;
 
     if (unlikely(tcp_is_reno(tp))) {
         tcp_newreno_mark_lost(sk, *ack_flag & FLAG_SND_UNA_ADVANCED);
     } else if (tcp_is_rack(sk)) {
         u32 prior_retrans = tp->retrans_out;
 
         if (tcp_rack_mark_lost(sk))
             *ack_flag &= ~FLAG_SET_XMIT_TIMER;
         if (prior_retrans > tp->retrans_out)
             *ack_flag |= FLAG_LOST_RETRANS;
     }
 }
 
 /* Process an event, which can update packets-in-flight not trivially.
  * Main goal of this function is to calculate new estimate for left_out,
  * taking into account both packets sitting in receiver's buffer and
  * packets lost by network.
  *
  * Besides that it updates the congestion state when packet loss or ECN
  * is detected. But it does not reduce the cwnd, it is done by the
  * congestion control later.
  *
  * It does _not_ decide what to send, it is made in function
  * tcp_xmit_retransmit_queue().
  */
 static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
                   int num_dupack, int *ack_flag, int *rexmit)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     int fast_rexmit = 0, flag = *ack_flag;
     bool ece_ack = flag & FLAG_ECE;
     bool do_lost = num_dupack || ((flag & FLAG_DATA_SACKED) &&
                       tcp_force_fast_retransmit(sk));
 
     if (!tp->packets_out && tp->sacked_out)
         tp->sacked_out = 0;
 
     /* Now state machine starts.
      * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
     if (ece_ack)
         tp->prior_ssthresh = 0;
 
     /* B. In all the states check for reneging SACKs. */
     if (tcp_check_sack_reneging(sk, flag))
         return;
 
     /* C. Check consistency of the current state. */
     tcp_verify_left_out(tp);
 
     /* D. Check state exit conditions. State can be terminated
      *    when high_seq is ACKed. */
     if (icsk->icsk_ca_state == TCP_CA_Open) {
         WARN_ON(tp->retrans_out != 0 && !tp->syn_data);
         tp->retrans_stamp = 0;
     } else if (!before(tp->snd_una, tp->high_seq)) {
         switch (icsk->icsk_ca_state) {
         case TCP_CA_CWR:
             /* CWR is to be held something *above* high_seq
              * is ACKed for CWR bit to reach receiver. */
             if (tp->snd_una != tp->high_seq) {
                 tcp_end_cwnd_reduction(sk);
                 tcp_set_ca_state(sk, TCP_CA_Open);
             }
             break;
 
         case TCP_CA_Recovery:
             if (tcp_is_reno(tp))
                 tcp_reset_reno_sack(tp);
             if (tcp_try_undo_recovery(sk))
                 return;
             tcp_end_cwnd_reduction(sk);
             break;
         }
     }
 
     /* E. Process state. */
     switch (icsk->icsk_ca_state) {
     case TCP_CA_Recovery:
         if (!(flag & FLAG_SND_UNA_ADVANCED)) {
             if (tcp_is_reno(tp))
                 tcp_add_reno_sack(sk, num_dupack, ece_ack);
         } else if (tcp_try_undo_partial(sk, prior_snd_una, &do_lost))
             return;
 
         if (tcp_try_undo_dsack(sk))
             tcp_try_keep_open(sk);
 
         tcp_identify_packet_loss(sk, ack_flag);
         if (icsk->icsk_ca_state != TCP_CA_Recovery) {
             if (!tcp_time_to_recover(sk, flag))
                 return;
             /* Undo reverts the recovery state. If loss is evident,
              * starts a new recovery (e.g. reordering then loss);
              */
             tcp_enter_recovery(sk, ece_ack);
         }
         break;
     case TCP_CA_Loss:
         tcp_process_loss(sk, flag, num_dupack, rexmit);
         tcp_identify_packet_loss(sk, ack_flag);
         if (!(icsk->icsk_ca_state == TCP_CA_Open ||
               (*ack_flag & FLAG_LOST_RETRANS)))
             return;
         /* Change state if cwnd is undone or retransmits are lost */
         fallthrough;
     default:
         if (tcp_is_reno(tp)) {
             if (flag & FLAG_SND_UNA_ADVANCED)
                 tcp_reset_reno_sack(tp);
             tcp_add_reno_sack(sk, num_dupack, ece_ack);
         }
 
         if (icsk->icsk_ca_state <= TCP_CA_Disorder)
             tcp_try_undo_dsack(sk);
 
         tcp_identify_packet_loss(sk, ack_flag);
         if (!tcp_time_to_recover(sk, flag)) {
             tcp_try_to_open(sk, flag);
             return;
         }
 
         /* MTU probe failure: don't reduce cwnd */
         if (icsk->icsk_ca_state < TCP_CA_CWR &&
             icsk->icsk_mtup.probe_size &&
             tp->snd_una == tp->mtu_probe.probe_seq_start) {
             tcp_mtup_probe_failed(sk);
             /* Restores the reduction we did in tcp_mtup_probe() */
             tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
             tcp_simple_retransmit(sk);
             return;
         }
 
         /* Otherwise enter Recovery state */
         tcp_enter_recovery(sk, ece_ack);
         fast_rexmit = 1;
     }
 
     if (!tcp_is_rack(sk) && do_lost)
         tcp_update_scoreboard(sk, fast_rexmit);
     *rexmit = REXMIT_LOST;
 }
 
 static void tcp_update_rtt_min(struct sock *sk, u32 rtt_us, const int flag)
 {
     u32 wlen = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_rtt_wlen) * HZ;
     struct tcp_sock *tp = tcp_sk(sk);
 
     if ((flag & FLAG_ACK_MAYBE_DELAYED) && rtt_us > tcp_min_rtt(tp)) {
         /* If the remote keeps returning delayed ACKs, eventually
          * the min filter would pick it up and overestimate the
          * prop. delay when it expires. Skip suspected delayed ACKs.
          */
         return;
     }
     minmax_running_min(&tp->rtt_min, wlen, tcp_jiffies32,
                rtt_us ? : jiffies_to_usecs(1));
 }
 
 static bool tcp_ack_update_rtt(struct sock *sk, const int flag,
                    long seq_rtt_us, long sack_rtt_us,
                    long ca_rtt_us, struct rate_sample *rs)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
 
     /* Prefer RTT measured from ACK's timing to TS-ECR. This is because
      * broken middle-boxes or peers may corrupt TS-ECR fields. But
      * Karn's algorithm forbids taking RTT if some retransmitted data
      * is acked (RFC6298).
      */
     if (seq_rtt_us < 0)
         seq_rtt_us = sack_rtt_us;
 
     /* RTTM Rule: A TSecr value received in a segment is used to
      * update the averaged RTT measurement only if the segment
      * acknowledges some new data, i.e., only if it advances the
      * left edge of the send window.
      * See draft-ietf-tcplw-high-performance-00, section 3.3.
      */
     if (seq_rtt_us < 0 && tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
         flag & FLAG_ACKED) {
         u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
 
         if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
             if (!delta)
                 delta = 1;
             seq_rtt_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
             ca_rtt_us = seq_rtt_us;
         }
     }
     rs->rtt_us = ca_rtt_us; /* RTT of last (S)ACKed packet (or -1) */
     if (seq_rtt_us < 0)
         return false;
 
     /* ca_rtt_us >= 0 is counting on the invariant that ca_rtt_us is
      * always taken together with ACK, SACK, or TS-opts. Any negative
      * values will be skipped with the seq_rtt_us < 0 check above.
      */
     tcp_update_rtt_min(sk, ca_rtt_us, flag);
     tcp_rtt_estimator(sk, seq_rtt_us);
     tcp_set_rto(sk);
 
     /* RFC6298: only reset backoff on valid RTT measurement. */
     inet_csk(sk)->icsk_backoff = 0;
     return true;
 }
 
 /* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
 void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
 {
     struct rate_sample rs;
     long rtt_us = -1L;
 
     if (req && !req->num_retrans && tcp_rsk(req)->snt_synack)
         rtt_us = tcp_stamp_us_delta(tcp_clock_us(), tcp_rsk(req)->snt_synack);
 
     tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, rtt_us, -1L, rtt_us, &rs);
 }
 
 
 static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
 
     icsk->icsk_ca_ops->cong_avoid(sk, ack, acked);
     tcp_sk(sk)->snd_cwnd_stamp = tcp_jiffies32;
 }
 
 /* Restart timer after forward progress on connection.
  * RFC2988 recommends to restart timer to now+rto.
  */
 void tcp_rearm_rto(struct sock *sk)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* If the retrans timer is currently being used by Fast Open
      * for SYN-ACK retrans purpose, stay put.
      */
     if (rcu_access_pointer(tp->fastopen_rsk))
         return;
 
     if (!tp->packets_out) {
         inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
     } else {
         u32 rto = inet_csk(sk)->icsk_rto;
         /* Offset the time elapsed after installing regular RTO */
         if (icsk->icsk_pending == ICSK_TIME_REO_TIMEOUT ||
             icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
             s64 delta_us = tcp_rto_delta_us(sk);
             /* delta_us may not be positive if the socket is locked
              * when the retrans timer fires and is rescheduled.
              */
             rto = usecs_to_jiffies(max_t(int, delta_us, 1));
         }
         tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
                      TCP_RTO_MAX);
     }
 }
 
 /* Try to schedule a loss probe; if that doesn't work, then schedule an RTO. */
 static void tcp_set_xmit_timer(struct sock *sk)
 {
     if (!tcp_schedule_loss_probe(sk, true))
         tcp_rearm_rto(sk);
 }
 
 /* If we get here, the whole TSO packet has not been acked. */
 static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 packets_acked;
 
     BUG_ON(!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una));
 
     packets_acked = tcp_skb_pcount(skb);
     if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
         return 0;
     packets_acked -= tcp_skb_pcount(skb);
 
     if (packets_acked) {
         BUG_ON(tcp_skb_pcount(skb) == 0);
         BUG_ON(!before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq));
     }
 
     return packets_acked;
 }
 
 static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb,
                const struct sk_buff *ack_skb, u32 prior_snd_una)
 {
     const struct skb_shared_info *shinfo;
 
     /* Avoid cache line misses to get skb_shinfo() and shinfo->tx_flags */
     if (likely(!TCP_SKB_CB(skb)->txstamp_ack))
         return;
 
     shinfo = skb_shinfo(skb);
     if (!before(shinfo->tskey, prior_snd_una) &&
         before(shinfo->tskey, tcp_sk(sk)->snd_una)) {
         tcp_skb_tsorted_save(skb) {
             __skb_tstamp_tx(skb, ack_skb, NULL, sk, SCM_TSTAMP_ACK);
         } tcp_skb_tsorted_restore(skb);
     }
 }
 
 /* Remove acknowledged frames from the retransmission queue. If our packet
  * is before the ack sequence we can discard it as it's confirmed to have
  * arrived at the other end.
  */
 static int tcp_clean_rtx_queue(struct sock *sk, const struct sk_buff *ack_skb,
                    u32 prior_fack, u32 prior_snd_una,
                    struct tcp_sacktag_state *sack, bool ece_ack)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     u64 first_ackt, last_ackt;
     struct tcp_sock *tp = tcp_sk(sk);
     u32 prior_sacked = tp->sacked_out;
     u32 reord = tp->snd_nxt; /* lowest acked un-retx un-sacked seq */
     struct sk_buff *skb, *next;
     bool fully_acked = true;
     long sack_rtt_us = -1L;
     long seq_rtt_us = -1L;
     long ca_rtt_us = -1L;
     u32 pkts_acked = 0;
     bool rtt_update;
     int flag = 0;
 
     first_ackt = 0;
 
     for (skb = skb_rb_first(&sk->tcp_rtx_queue); skb; skb = next) {
         struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
         const u32 start_seq = scb->seq;
         u8 sacked = scb->sacked;
         u32 acked_pcount;
 
         /* Determine how many packets and what bytes were acked, tso and else */
         if (after(scb->end_seq, tp->snd_una)) {
             if (tcp_skb_pcount(skb) == 1 ||
                 !after(tp->snd_una, scb->seq))
                 break;
 
             acked_pcount = tcp_tso_acked(sk, skb);
             if (!acked_pcount)
                 break;
             fully_acked = false;
         } else {
             acked_pcount = tcp_skb_pcount(skb);
         }
 
         if (unlikely(sacked & TCPCB_RETRANS)) {
             if (sacked & TCPCB_SACKED_RETRANS)
                 tp->retrans_out -= acked_pcount;
             flag |= FLAG_RETRANS_DATA_ACKED;
         } else if (!(sacked & TCPCB_SACKED_ACKED)) {
             last_ackt = tcp_skb_timestamp_us(skb);
             WARN_ON_ONCE(last_ackt == 0);
             if (!first_ackt)
                 first_ackt = last_ackt;
 
             if (before(start_seq, reord))
                 reord = start_seq;
             if (!after(scb->end_seq, tp->high_seq))
                 flag |= FLAG_ORIG_SACK_ACKED;
         }
 
         if (sacked & TCPCB_SACKED_ACKED) {
             tp->sacked_out -= acked_pcount;
         } else if (tcp_is_sack(tp)) {
             tcp_count_delivered(tp, acked_pcount, ece_ack);
             if (!tcp_skb_spurious_retrans(tp, skb))
                 tcp_rack_advance(tp, sacked, scb->end_seq,
                          tcp_skb_timestamp_us(skb));
         }
         if (sacked & TCPCB_LOST)
             tp->lost_out -= acked_pcount;
 
         tp->packets_out -= acked_pcount;
         pkts_acked += acked_pcount;
         tcp_rate_skb_delivered(sk, skb, sack->rate);
 
         /* Initial outgoing SYN's get put onto the write_queue
          * just like anything else we transmit.  It is not
          * true data, and if we misinform our callers that
          * this ACK acks real data, we will erroneously exit
          * connection startup slow start one packet too
          * quickly.  This is severely frowned upon behavior.
          */
         if (likely(!(scb->tcp_flags & TCPHDR_SYN))) {
             flag |= FLAG_DATA_ACKED;
         } else {
             flag |= FLAG_SYN_ACKED;
             tp->retrans_stamp = 0;
         }
 
         if (!fully_acked)
             break;
 
         tcp_ack_tstamp(sk, skb, ack_skb, prior_snd_una);
 
         next = skb_rb_next(skb);
         if (unlikely(skb == tp->retransmit_skb_hint))
             tp->retransmit_skb_hint = NULL;
         if (unlikely(skb == tp->lost_skb_hint))
             tp->lost_skb_hint = NULL;
         tcp_highest_sack_replace(sk, skb, next);
         tcp_rtx_queue_unlink_and_free(skb, sk);
     }
 
     if (!skb)
         tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
 
     if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una)))
         tp->snd_up = tp->snd_una;
 
     if (skb) {
         tcp_ack_tstamp(sk, skb, ack_skb, prior_snd_una);
         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
             flag |= FLAG_SACK_RENEGING;
     }
 
     if (likely(first_ackt) && !(flag & FLAG_RETRANS_DATA_ACKED)) {
         seq_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, first_ackt);
         ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, last_ackt);
 
         if (pkts_acked == 1 && fully_acked && !prior_sacked &&
             (tp->snd_una - prior_snd_una) < tp->mss_cache &&
             sack->rate->prior_delivered + 1 == tp->delivered &&
             !(flag & (FLAG_CA_ALERT | FLAG_SYN_ACKED))) {
             /* Conservatively mark a delayed ACK. It's typically
              * from a lone runt packet over the round trip to
              * a receiver w/o out-of-order or CE events.
              */
             flag |= FLAG_ACK_MAYBE_DELAYED;
         }
     }
     if (sack->first_sackt) {
         sack_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->first_sackt);
         ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->last_sackt);
     }
     rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us,
                     ca_rtt_us, sack->rate);
 
     if (flag & FLAG_ACKED) {
         flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
         if (unlikely(icsk->icsk_mtup.probe_size &&
                  !after(tp->mtu_probe.probe_seq_end, tp->snd_una))) {
             tcp_mtup_probe_success(sk);
         }
 
         if (tcp_is_reno(tp)) {
             tcp_remove_reno_sacks(sk, pkts_acked, ece_ack);
 
             /* If any of the cumulatively ACKed segments was
              * retransmitted, non-SACK case cannot confirm that
              * progress was due to original transmission due to
              * lack of TCPCB_SACKED_ACKED bits even if some of
              * the packets may have been never retransmitted.
              */
             if (flag & FLAG_RETRANS_DATA_ACKED)
                 flag &= ~FLAG_ORIG_SACK_ACKED;
         } else {
             int delta;
 
             /* Non-retransmitted hole got filled? That's reordering */
             if (before(reord, prior_fack))
                 tcp_check_sack_reordering(sk, reord, 0);
 
             delta = prior_sacked - tp->sacked_out;
             tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
         }
     } else if (skb && rtt_update && sack_rtt_us >= 0 &&
            sack_rtt_us > tcp_stamp_us_delta(tp->tcp_mstamp,
                             tcp_skb_timestamp_us(skb))) {
         /* Do not re-arm RTO if the sack RTT is measured from data sent
          * after when the head was last (re)transmitted. Otherwise the
          * timeout may continue to extend in loss recovery.
          */
         flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
     }
 
     if (icsk->icsk_ca_ops->pkts_acked) {
         struct ack_sample sample = { .pkts_acked = pkts_acked,
                          .rtt_us = sack->rate->rtt_us };
 
         sample.in_flight = tp->mss_cache *
             (tp->delivered - sack->rate->prior_delivered);
         icsk->icsk_ca_ops->pkts_acked(sk, &sample);
     }
 
 #if FASTRETRANS_DEBUG > 0
     WARN_ON((int)tp->sacked_out < 0);
     WARN_ON((int)tp->lost_out < 0);
     WARN_ON((int)tp->retrans_out < 0);
     if (!tp->packets_out && tcp_is_sack(tp)) {
         icsk = inet_csk(sk);
         if (tp->lost_out) {
             pr_debug("Leak l=%u %d\n",
                  tp->lost_out, icsk->icsk_ca_state);
             tp->lost_out = 0;
         }
         if (tp->sacked_out) {
             pr_debug("Leak s=%u %d\n",
                  tp->sacked_out, icsk->icsk_ca_state);
             tp->sacked_out = 0;
         }
         if (tp->retrans_out) {
             pr_debug("Leak r=%u %d\n",
                  tp->retrans_out, icsk->icsk_ca_state);
             tp->retrans_out = 0;
         }
     }
 #endif
     return flag;
 }
 
 static void tcp_ack_probe(struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct sk_buff *head = tcp_send_head(sk);
     const struct tcp_sock *tp = tcp_sk(sk);
 
     /* Was it a usable window open? */
     if (!head)
         return;
     if (!after(TCP_SKB_CB(head)->end_seq, tcp_wnd_end(tp))) {
         icsk->icsk_backoff = 0;
         icsk->icsk_probes_tstamp = 0;
         inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
         /* Socket must be waked up by subsequent tcp_data_snd_check().
          * This function is not for random using!
          */
     } else {
         unsigned long when = tcp_probe0_when(sk, TCP_RTO_MAX);
 
         when = tcp_clamp_probe0_to_user_timeout(sk, when);
         tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, when, TCP_RTO_MAX);
     }
 }
 
 static inline bool tcp_ack_is_dubious(const struct sock *sk, const int flag)
 {
     return !(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
         inet_csk(sk)->icsk_ca_state != TCP_CA_Open;
 }
 
 /* Decide wheather to run the increase function of congestion control. */
 static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
 {
     /* If reordering is high then always grow cwnd whenever data is
      * delivered regardless of its ordering. Otherwise stay conservative
      * and only grow cwnd on in-order delivery (RFC5681). A stretched ACK w/
      * new SACK or ECE mark may first advance cwnd here and later reduce
      * cwnd in tcp_fastretrans_alert() based on more states.
      */
     if (tcp_sk(sk)->reordering >
         READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_reordering))
         return flag & FLAG_FORWARD_PROGRESS;
 
     return flag & FLAG_DATA_ACKED;
 }
 
 /* The "ultimate" congestion control function that aims to replace the rigid
  * cwnd increase and decrease control (tcp_cong_avoid,tcp_*cwnd_reduction).
  * It's called toward the end of processing an ACK with precise rate
  * information. All transmission or retransmission are delayed afterwards.
  */
 static void tcp_cong_control(struct sock *sk, u32 ack, u32 acked_sacked,
                  int flag, const struct rate_sample *rs)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
 
     if (icsk->icsk_ca_ops->cong_control) {
         icsk->icsk_ca_ops->cong_control(sk, rs);
         return;
     }
 
     if (tcp_in_cwnd_reduction(sk)) {
         /* Reduce cwnd if state mandates */
         tcp_cwnd_reduction(sk, acked_sacked, rs->losses, flag);
     } else if (tcp_may_raise_cwnd(sk, flag)) {
         /* Advance cwnd if state allows */
         tcp_cong_avoid(sk, ack, acked_sacked);
     }
     tcp_update_pacing_rate(sk);
 }
 
 /* Check that window update is acceptable.
  * The function assumes that snd_una<=ack<=snd_next.
  */
 static inline bool tcp_may_update_window(const struct tcp_sock *tp,
                     const u32 ack, const u32 ack_seq,
                     const u32 nwin)
 {
     return  after(ack, tp->snd_una) ||
         after(ack_seq, tp->snd_wl1) ||
         (ack_seq == tp->snd_wl1 && nwin > tp->snd_wnd);
 }
 
 /* If we update tp->snd_una, also update tp->bytes_acked */
 static void tcp_snd_una_update(struct tcp_sock *tp, u32 ack)
 {
     u32 delta = ack - tp->snd_una;
 
     sock_owned_by_me((struct sock *)tp);
     tp->bytes_acked += delta;
     tp->snd_una = ack;
 }
 
 /* If we update tp->rcv_nxt, also update tp->bytes_received */
 static void tcp_rcv_nxt_update(struct tcp_sock *tp, u32 seq)
 {
     u32 delta = seq - tp->rcv_nxt;
 
     sock_owned_by_me((struct sock *)tp);
     tp->bytes_received += delta;
     WRITE_ONCE(tp->rcv_nxt, seq);
 }
 
 /* Update our send window.
  *
  * Window update algorithm, described in RFC793/RFC1122 (used in linux-2.2
  * and in FreeBSD. NetBSD's one is even worse.) is wrong.
  */
 static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32 ack,
                  u32 ack_seq)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     int flag = 0;
     u32 nwin = ntohs(tcp_hdr(skb)->window);
 
     if (likely(!tcp_hdr(skb)->syn))
         nwin <<= tp->rx_opt.snd_wscale;
 
     if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
         flag |= FLAG_WIN_UPDATE;
         tcp_update_wl(tp, ack_seq);
 
         if (tp->snd_wnd != nwin) {
             tp->snd_wnd = nwin;
 
             /* Note, it is the only place, where
              * fast path is recovered for sending TCP.
              */
             tp->pred_flags = 0;
             tcp_fast_path_check(sk);
 
             if (!tcp_write_queue_empty(sk))
                 tcp_slow_start_after_idle_check(sk);
 
             if (nwin > tp->max_window) {
                 tp->max_window = nwin;
                 tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
             }
         }
     }
 
     tcp_snd_una_update(tp, ack);
 
     return flag;
 }
 
 static bool __tcp_oow_rate_limited(struct net *net, int mib_idx,
                    u32 *last_oow_ack_time)
 {
     if (*last_oow_ack_time) {
         s32 elapsed = (s32)(tcp_jiffies32 - *last_oow_ack_time);
 
         if (0 <= elapsed &&
             elapsed < READ_ONCE(net->ipv4.sysctl_tcp_invalid_ratelimit)) {
             NET_INC_STATS(net, mib_idx);
             return true;    /* rate-limited: don't send yet! */
         }
     }
 
     *last_oow_ack_time = tcp_jiffies32;
 
     return false;   /* not rate-limited: go ahead, send dupack now! */
 }
 
 /* Return true if we're currently rate-limiting out-of-window ACKs and
  * thus shouldn't send a dupack right now. We rate-limit dupacks in
  * response to out-of-window SYNs or ACKs to mitigate ACK loops or DoS
  * attacks that send repeated SYNs or ACKs for the same connection. To
  * do this, we do not send a duplicate SYNACK or ACK if the remote
  * endpoint is sending out-of-window SYNs or pure ACKs at a high rate.
  */
 bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,
               int mib_idx, u32 *last_oow_ack_time)
 {
     /* Data packets without SYNs are not likely part of an ACK loop. */
     if ((TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq) &&
         !tcp_hdr(skb)->syn)
         return false;
 
     return __tcp_oow_rate_limited(net, mib_idx, last_oow_ack_time);
 }
 
 /* RFC 5961 7 [ACK Throttling] */
 static void tcp_send_challenge_ack(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct net *net = sock_net(sk);
     u32 count, now, ack_limit;
 
     /* First check our per-socket dupack rate limit. */
     if (__tcp_oow_rate_limited(net,
                    LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
                    &tp->last_oow_ack_time))
         return;
 
     ack_limit = READ_ONCE(net->ipv4.sysctl_tcp_challenge_ack_limit);
     if (ack_limit == INT_MAX)
         goto send_ack;
 
     /* Then check host-wide RFC 5961 rate limit. */
     now = jiffies / HZ;
     if (now != READ_ONCE(net->ipv4.tcp_challenge_timestamp)) {
         u32 half = (ack_limit + 1) >> 1;
 
         WRITE_ONCE(net->ipv4.tcp_challenge_timestamp, now);
         WRITE_ONCE(net->ipv4.tcp_challenge_count, half + prandom_u32_max(ack_limit));
     }
     count = READ_ONCE(net->ipv4.tcp_challenge_count);
     if (count > 0) {
         WRITE_ONCE(net->ipv4.tcp_challenge_count, count - 1);
 send_ack:
         NET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);
         tcp_send_ack(sk);
     }
 }
 
 static void tcp_store_ts_recent(struct tcp_sock *tp)
 {
     tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
     tp->rx_opt.ts_recent_stamp = ktime_get_seconds();
 }
 
 static void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
 {
     if (tp->rx_opt.saw_tstamp && !after(seq, tp->rcv_wup)) {
         /* PAWS bug workaround wrt. ACK frames, the PAWS discard
          * extra check below makes sure this can only happen
          * for pure ACK frames.  -DaveM
          *
          * Not only, also it occurs for expired timestamps.
          */
 
         if (tcp_paws_check(&tp->rx_opt, 0))
             tcp_store_ts_recent(tp);
     }
 }
 
 /* This routine deals with acks during a TLP episode and ends an episode by
  * resetting tlp_high_seq. Ref: TLP algorithm in draft-ietf-tcpm-rack
  */
 static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (before(ack, tp->tlp_high_seq))
         return;
 
     if (!tp->tlp_retrans) {
         /* TLP of new data has been acknowledged */
         tp->tlp_high_seq = 0;
     } else if (flag & FLAG_DSACK_TLP) {
         /* This DSACK means original and TLP probe arrived; no loss */
         tp->tlp_high_seq = 0;
     } else if (after(ack, tp->tlp_high_seq)) {
         /* ACK advances: there was a loss, so reduce cwnd. Reset
          * tlp_high_seq in tcp_init_cwnd_reduction()
          */
         tcp_init_cwnd_reduction(sk);
         tcp_set_ca_state(sk, TCP_CA_CWR);
         tcp_end_cwnd_reduction(sk);
         tcp_try_keep_open(sk);
         NET_INC_STATS(sock_net(sk),
                 LINUX_MIB_TCPLOSSPROBERECOVERY);
     } else if (!(flag & (FLAG_SND_UNA_ADVANCED |
                  FLAG_NOT_DUP | FLAG_DATA_SACKED))) {
         /* Pure dupack: original and TLP probe arrived; no loss */
         tp->tlp_high_seq = 0;
     }
 }
 
 static inline void tcp_in_ack_event(struct sock *sk, u32 flags)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
 
     if (icsk->icsk_ca_ops->in_ack_event)
         icsk->icsk_ca_ops->in_ack_event(sk, flags);
 }
 
 /* Congestion control has updated the cwnd already. So if we're in
  * loss recovery then now we do any new sends (for FRTO) or
  * retransmits (for CA_Loss or CA_recovery) that make sense.
  */
 static void tcp_xmit_recovery(struct sock *sk, int rexmit)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (rexmit == REXMIT_NONE || sk->sk_state == TCP_SYN_SENT)
         return;
 
     if (unlikely(rexmit == REXMIT_NEW)) {
         __tcp_push_pending_frames(sk, tcp_current_mss(sk),
                       TCP_NAGLE_OFF);
         if (after(tp->snd_nxt, tp->high_seq))
             return;
         tp->frto = 0;
     }
     tcp_xmit_retransmit_queue(sk);
 }
 
 /* Returns the number of packets newly acked or sacked by the current ACK */
 static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered, int flag)
 {
     const struct net *net = sock_net(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     u32 delivered;
 
     delivered = tp->delivered - prior_delivered;
     NET_ADD_STATS(net, LINUX_MIB_TCPDELIVERED, delivered);
     if (flag & FLAG_ECE)
         NET_ADD_STATS(net, LINUX_MIB_TCPDELIVEREDCE, delivered);
 
     return delivered;
 }
 
 /* This routine deals with incoming acks, but not outgoing ones. */
 static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct tcp_sacktag_state sack_state;
     struct rate_sample rs = { .prior_delivered = 0 };
     u32 prior_snd_una = tp->snd_una;
     bool is_sack_reneg = tp->is_sack_reneg;
     u32 ack_seq = TCP_SKB_CB(skb)->seq;
     u32 ack = TCP_SKB_CB(skb)->ack_seq;
     int num_dupack = 0;
     int prior_packets = tp->packets_out;
     u32 delivered = tp->delivered;
     u32 lost = tp->lost;
     int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
     u32 prior_fack;
 
     sack_state.first_sackt = 0;
     sack_state.rate = &rs;
     sack_state.sack_delivered = 0;
 
     /* We very likely will need to access rtx queue. */
     prefetch(sk->tcp_rtx_queue.rb_node);
 
     /* If the ack is older than previous acks
      * then we can probably ignore it.
      */
     if (before(ack, prior_snd_una)) {
         /* RFC 5961 5.2 [Blind Data Injection Attack].[Mitigation] */
         if (before(ack, prior_snd_una - tp->max_window)) {
             if (!(flag & FLAG_NO_CHALLENGE_ACK))
                 tcp_send_challenge_ack(sk);
             return -SKB_DROP_REASON_TCP_TOO_OLD_ACK;
         }
         goto old_ack;
     }
 
     /* If the ack includes data we haven't sent yet, discard
      * this segment (RFC793 Section 3.9).
      */
     if (after(ack, tp->snd_nxt))
         return -SKB_DROP_REASON_TCP_ACK_UNSENT_DATA;
 
     if (after(ack, prior_snd_una)) {
         flag |= FLAG_SND_UNA_ADVANCED;
         icsk->icsk_retransmits = 0;
 
 #if IS_ENABLED(CONFIG_TLS_DEVICE)
         if (static_branch_unlikely(&clean_acked_data_enabled.key))
             if (icsk->icsk_clean_acked)
                 icsk->icsk_clean_acked(sk, ack);
 #endif
     }
 
     prior_fack = tcp_is_sack(tp) ? tcp_highest_sack_seq(tp) : tp->snd_una;
     rs.prior_in_flight = tcp_packets_in_flight(tp);
 
     /* ts_recent update must be made after we are sure that the packet
      * is in window.
      */
     if (flag & FLAG_UPDATE_TS_RECENT)
         tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
 
     if ((flag & (FLAG_SLOWPATH | FLAG_SND_UNA_ADVANCED)) ==
         FLAG_SND_UNA_ADVANCED) {
         /* Window is constant, pure forward advance.
          * No more checks are required.
          * Note, we use the fact that SND.UNA>=SND.WL2.
          */
         tcp_update_wl(tp, ack_seq);
         tcp_snd_una_update(tp, ack);
         flag |= FLAG_WIN_UPDATE;
 
         tcp_in_ack_event(sk, CA_ACK_WIN_UPDATE);
 
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPACKS);
     } else {
         u32 ack_ev_flags = CA_ACK_SLOWPATH;
 
         if (ack_seq != TCP_SKB_CB(skb)->end_seq)
             flag |= FLAG_DATA;
         else
             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPUREACKS);
 
         flag |= tcp_ack_update_window(sk, skb, ack, ack_seq);
 
         if (TCP_SKB_CB(skb)->sacked)
             flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
                             &sack_state);
 
         if (tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb))) {
             flag |= FLAG_ECE;
             ack_ev_flags |= CA_ACK_ECE;
         }
 
         if (sack_state.sack_delivered)
             tcp_count_delivered(tp, sack_state.sack_delivered,
                         flag & FLAG_ECE);
 
         if (flag & FLAG_WIN_UPDATE)
             ack_ev_flags |= CA_ACK_WIN_UPDATE;
 
         tcp_in_ack_event(sk, ack_ev_flags);
     }
 
     /* This is a deviation from RFC3168 since it states that:
      * "When the TCP data sender is ready to set the CWR bit after reducing
      * the congestion window, it SHOULD set the CWR bit only on the first
      * new data packet that it transmits."
      * We accept CWR on pure ACKs to be more robust
      * with widely-deployed TCP implementations that do this.
      */
     tcp_ecn_accept_cwr(sk, skb);
 
     /* We passed data and got it acked, remove any soft error
      * log. Something worked...
      */
     sk->sk_err_soft = 0;
     icsk->icsk_probes_out = 0;
     tp->rcv_tstamp = tcp_jiffies32;
     if (!prior_packets)
         goto no_queue;
 
     /* See if we can take anything off of the retransmit queue. */
     flag |= tcp_clean_rtx_queue(sk, skb, prior_fack, prior_snd_una,
                     &sack_state, flag & FLAG_ECE);
 
     tcp_rack_update_reo_wnd(sk, &rs);
 
     if (tp->tlp_high_seq)
         tcp_process_tlp_ack(sk, ack, flag);
 
     if (tcp_ack_is_dubious(sk, flag)) {
         if (!(flag & (FLAG_SND_UNA_ADVANCED |
                   FLAG_NOT_DUP | FLAG_DSACKING_ACK))) {
             num_dupack = 1;
             /* Consider if pure acks were aggregated in tcp_add_backlog() */
             if (!(flag & FLAG_DATA))
                 num_dupack = max_t(u16, 1, skb_shinfo(skb)->gso_segs);
         }
         tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
                       &rexmit);
     }
 
     /* If needed, reset TLP/RTO timer when RACK doesn't set. */
     if (flag & FLAG_SET_XMIT_TIMER)
         tcp_set_xmit_timer(sk);
 
     if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
         sk_dst_confirm(sk);
 
     delivered = tcp_newly_delivered(sk, delivered, flag);
     lost = tp->lost - lost;         /* freshly marked lost */
     rs.is_ack_delayed = !!(flag & FLAG_ACK_MAYBE_DELAYED);
     tcp_rate_gen(sk, delivered, lost, is_sack_reneg, sack_state.rate);
     tcp_cong_control(sk, ack, delivered, flag, sack_state.rate);
     tcp_xmit_recovery(sk, rexmit);
     return 1;
 
 no_queue:
     /* If data was DSACKed, see if we can undo a cwnd reduction. */
     if (flag & FLAG_DSACKING_ACK) {
         tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
                       &rexmit);
         tcp_newly_delivered(sk, delivered, flag);
     }
     /* If this ack opens up a zero window, clear backoff.  It was
      * being used to time the probes, and is probably far higher than
      * it needs to be for normal retransmission.
      */
     tcp_ack_probe(sk);
 
     if (tp->tlp_high_seq)
         tcp_process_tlp_ack(sk, ack, flag);
     return 1;
 
 old_ack:
     /* If data was SACKed, tag it and see if we should send more data.
      * If data was DSACKed, see if we can undo a cwnd reduction.
      */
     if (TCP_SKB_CB(skb)->sacked) {
         flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
                         &sack_state);
         tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
                       &rexmit);
         tcp_newly_delivered(sk, delivered, flag);
         tcp_xmit_recovery(sk, rexmit);
     }
 
     return 0;
 }
 
 static void tcp_parse_fastopen_option(int len, const unsigned char *cookie,
                       bool syn, struct tcp_fastopen_cookie *foc,
                       bool exp_opt)
 {
     /* Valid only in SYN or SYN-ACK with an even length.  */
     if (!foc || !syn || len < 0 || (len & 1))
         return;
 
     if (len >= TCP_FASTOPEN_COOKIE_MIN &&
         len <= TCP_FASTOPEN_COOKIE_MAX)
         memcpy(foc->val, cookie, len);
     else if (len != 0)
         len = -1;
     foc->len = len;
     foc->exp = exp_opt;
 }
 
 static bool smc_parse_options(const struct tcphdr *th,
                   struct tcp_options_received *opt_rx,
                   const unsigned char *ptr,
                   int opsize)
 {
 #if IS_ENABLED(CONFIG_SMC)
     if (static_branch_unlikely(&tcp_have_smc)) {
         if (th->syn && !(opsize & 1) &&
             opsize >= TCPOLEN_EXP_SMC_BASE &&
             get_unaligned_be32(ptr) == TCPOPT_SMC_MAGIC) {
             opt_rx->smc_ok = 1;
             return true;
         }
     }
 #endif
     return false;
 }
 
 /* Try to parse the MSS option from the TCP header. Return 0 on failure, clamped
  * value on success.
  */
 u16 tcp_parse_mss_option(const struct tcphdr *th, u16 user_mss)
 {
     const unsigned char *ptr = (const unsigned char *)(th + 1);
     int length = (th->doff * 4) - sizeof(struct tcphdr);
     u16 mss = 0;
 
     while (length > 0) {
         int opcode = *ptr++;
         int opsize;
 
         switch (opcode) {
         case TCPOPT_EOL:
             return mss;
         case TCPOPT_NOP:    /* Ref: RFC 793 section 3.1 */
             length--;
             continue;
         default:
             if (length < 2)
                 return mss;
             opsize = *ptr++;
             if (opsize < 2) /* "silly options" */
                 return mss;
             if (opsize > length)
                 return mss; /* fail on partial options */
             if (opcode == TCPOPT_MSS && opsize == TCPOLEN_MSS) {
                 u16 in_mss = get_unaligned_be16(ptr);
 
                 if (in_mss) {
                     if (user_mss && user_mss < in_mss)
                         in_mss = user_mss;
                     mss = in_mss;
                 }
             }
             ptr += opsize - 2;
             length -= opsize;
         }
     }
     return mss;
 }
 EXPORT_SYMBOL_GPL(tcp_parse_mss_option);
 
 /* Look for tcp options. Normally only called on SYN and SYNACK packets.
  * But, this can also be called on packets in the established flow when
  * the fast version below fails.
  */
 void tcp_parse_options(const struct net *net,
                const struct sk_buff *skb,
                struct tcp_options_received *opt_rx, int estab,
                struct tcp_fastopen_cookie *foc)
 {
     const unsigned char *ptr;
     const struct tcphdr *th = tcp_hdr(skb);
     int length = (th->doff * 4) - sizeof(struct tcphdr);
 
     ptr = (const unsigned char *)(th + 1);
     opt_rx->saw_tstamp = 0;
     opt_rx->saw_unknown = 0;
 
     while (length > 0) {
         int opcode = *ptr++;
         int opsize;
 
         switch (opcode) {
         case TCPOPT_EOL:
             return;
         case TCPOPT_NOP:    /* Ref: RFC 793 section 3.1 */
             length--;
             continue;
         default:
             if (length < 2)
                 return;
             opsize = *ptr++;
             if (opsize < 2) /* "silly options" */
                 return;
             if (opsize > length)
                 return; /* don't parse partial options */
             switch (opcode) {
             case TCPOPT_MSS:
                 if (opsize == TCPOLEN_MSS && th->syn && !estab) {
                     u16 in_mss = get_unaligned_be16(ptr);
                     if (in_mss) {
                         if (opt_rx->user_mss &&
                             opt_rx->user_mss < in_mss)
                             in_mss = opt_rx->user_mss;
                         opt_rx->mss_clamp = in_mss;
                     }
                 }
                 break;
             case TCPOPT_WINDOW:
                 if (opsize == TCPOLEN_WINDOW && th->syn &&
                     !estab && READ_ONCE(net->ipv4.sysctl_tcp_window_scaling)) {
                     __u8 snd_wscale = *(__u8 *)ptr;
                     opt_rx->wscale_ok = 1;
                     if (snd_wscale > TCP_MAX_WSCALE) {
                         net_info_ratelimited("%s: Illegal window scaling value %d > %u received\n",
                                      __func__,
                                      snd_wscale,
                                      TCP_MAX_WSCALE);
                         snd_wscale = TCP_MAX_WSCALE;
                     }
                     opt_rx->snd_wscale = snd_wscale;
                 }
                 break;
             case TCPOPT_TIMESTAMP:
                 if ((opsize == TCPOLEN_TIMESTAMP) &&
                     ((estab && opt_rx->tstamp_ok) ||
                      (!estab && READ_ONCE(net->ipv4.sysctl_tcp_timestamps)))) {
                     opt_rx->saw_tstamp = 1;
                     opt_rx->rcv_tsval = get_unaligned_be32(ptr);
                     opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4);
                 }
                 break;
             case TCPOPT_SACK_PERM:
                 if (opsize == TCPOLEN_SACK_PERM && th->syn &&
                     !estab && READ_ONCE(net->ipv4.sysctl_tcp_sack)) {
                     opt_rx->sack_ok = TCP_SACK_SEEN;
                     tcp_sack_reset(opt_rx);
                 }
                 break;
 
             case TCPOPT_SACK:
                 if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
                    !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) &&
                    opt_rx->sack_ok) {
                     TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *)th;
                 }
                 break;
 #ifdef CONFIG_TCP_MD5SIG
             case TCPOPT_MD5SIG:
                 /*
                  * The MD5 Hash has already been
                  * checked (see tcp_v{4,6}_do_rcv()).
                  */
                 break;
 #endif
             case TCPOPT_FASTOPEN:
                 tcp_parse_fastopen_option(
                     opsize - TCPOLEN_FASTOPEN_BASE,
                     ptr, th->syn, foc, false);
                 break;
 
             case TCPOPT_EXP:
                 /* Fast Open option shares code 254 using a
                  * 16 bits magic number.
                  */
                 if (opsize >= TCPOLEN_EXP_FASTOPEN_BASE &&
                     get_unaligned_be16(ptr) ==
                     TCPOPT_FASTOPEN_MAGIC) {
                     tcp_parse_fastopen_option(opsize -
                         TCPOLEN_EXP_FASTOPEN_BASE,
                         ptr + 2, th->syn, foc, true);
                     break;
                 }
 
                 if (smc_parse_options(th, opt_rx, ptr, opsize))
                     break;
 
                 opt_rx->saw_unknown = 1;
                 break;
 
             default:
                 opt_rx->saw_unknown = 1;
             }
             ptr += opsize-2;
             length -= opsize;
         }
     }
 }
 EXPORT_SYMBOL(tcp_parse_options);
 
 static bool tcp_parse_aligned_timestamp(struct tcp_sock *tp, const struct tcphdr *th)
 {
     const __be32 *ptr = (const __be32 *)(th + 1);
 
     if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
               | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
         tp->rx_opt.saw_tstamp = 1;
         ++ptr;
         tp->rx_opt.rcv_tsval = ntohl(*ptr);
         ++ptr;
         if (*ptr)
             tp->rx_opt.rcv_tsecr = ntohl(*ptr) - tp->tsoffset;
         else
             tp->rx_opt.rcv_tsecr = 0;
         return true;
     }
     return false;
 }
 
 /* Fast parse options. This hopes to only see timestamps.
  * If it is wrong it falls back on tcp_parse_options().
  */
 static bool tcp_fast_parse_options(const struct net *net,
                    const struct sk_buff *skb,
                    const struct tcphdr *th, struct tcp_sock *tp)
 {
     /* In the spirit of fast parsing, compare doff directly to constant
      * values.  Because equality is used, short doff can be ignored here.
      */
     if (th->doff == (sizeof(*th) / 4)) {
         tp->rx_opt.saw_tstamp = 0;
         return false;
     } else if (tp->rx_opt.tstamp_ok &&
            th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
         if (tcp_parse_aligned_timestamp(tp, th))
             return true;
     }
 
     tcp_parse_options(net, skb, &tp->rx_opt, 1, NULL);
     if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
         tp->rx_opt.rcv_tsecr -= tp->tsoffset;
 
     return true;
 }
 
 #ifdef CONFIG_TCP_MD5SIG
 /*
  * Parse MD5 Signature option
  */
 const u8 *tcp_parse_md5sig_option(const struct tcphdr *th)
 {
     int length = (th->doff << 2) - sizeof(*th);
     const u8 *ptr = (const u8 *)(th + 1);
 
     /* If not enough data remaining, we can short cut */
     while (length >= TCPOLEN_MD5SIG) {
         int opcode = *ptr++;
         int opsize;
 
         switch (opcode) {
         case TCPOPT_EOL:
             return NULL;
         case TCPOPT_NOP:
             length--;
             continue;
         default:
             opsize = *ptr++;
             if (opsize < 2 || opsize > length)
                 return NULL;
             if (opcode == TCPOPT_MD5SIG)
                 return opsize == TCPOLEN_MD5SIG ? ptr : NULL;
         }
         ptr += opsize - 2;
         length -= opsize;
     }
     return NULL;
 }
 EXPORT_SYMBOL(tcp_parse_md5sig_option);
 #endif
 
 /* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM
  *
  * It is not fatal. If this ACK does _not_ change critical state (seqs, window)
  * it can pass through stack. So, the following predicate verifies that
  * this segment is not used for anything but congestion avoidance or
  * fast retransmit. Moreover, we even are able to eliminate most of such
  * second order effects, if we apply some small "replay" window (~RTO)
  * to timestamp space.
  *
  * All these measures still do not guarantee that we reject wrapped ACKs
  * on networks with high bandwidth, when sequence space is recycled fastly,
  * but it guarantees that such events will be very rare and do not affect
  * connection seriously. This doesn't look nice, but alas, PAWS is really
  * buggy extension.
  *
  * [ Later note. Even worse! It is buggy for segments _with_ data. RFC
  * states that events when retransmit arrives after original data are rare.
  * It is a blatant lie. VJ forgot about fast retransmit! 8)8) It is
  * the biggest problem on large power networks even with minor reordering.
  * OK, let's give it small replay window. If peer clock is even 1hz, it is safe
  * up to bandwidth of 18Gigabit/sec. 8) ]
  */
 
 static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     const struct tcphdr *th = tcp_hdr(skb);
     u32 seq = TCP_SKB_CB(skb)->seq;
     u32 ack = TCP_SKB_CB(skb)->ack_seq;
 
     return (/* 1. Pure ACK with correct sequence number. */
         (th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&
 
         /* 2. ... and duplicate ACK. */
         ack == tp->snd_una &&
 
         /* 3. ... and does not update window. */
         !tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&
 
         /* 4. ... and sits in replay window. */
         (s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
 }
 
 static inline bool tcp_paws_discard(const struct sock *sk,
                    const struct sk_buff *skb)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
 
     return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
            !tcp_disordered_ack(sk, skb);
 }
 
 /* Check segment sequence number for validity.
  *
  * Segment controls are considered valid, if the segment
  * fits to the window after truncation to the window. Acceptability
  * of data (and SYN, FIN, of course) is checked separately.
  * See tcp_data_queue(), for example.
  *
  * Also, controls (RST is main one) are accepted using RCV.WUP instead
  * of RCV.NXT. Peer still did not advance his SND.UNA when we
  * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
  * (borrowed from freebsd)
  */
 
 static inline bool tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
 {
     return  !before(end_seq, tp->rcv_wup) &&
         !after(seq, tp->rcv_nxt + tcp_receive_window(tp));
 }
 
 /* When we get a reset we do this. */
 void tcp_reset(struct sock *sk, struct sk_buff *skb)
 {
     trace_tcp_receive_reset(sk);
 
     /* mptcp can't tell us to ignore reset pkts,
      * so just ignore the return value of mptcp_incoming_options().
      */
     if (sk_is_mptcp(sk))
         mptcp_incoming_options(sk, skb);
 
     /* We want the right error as BSD sees it (and indeed as we do). */
     switch (sk->sk_state) {
     case TCP_SYN_SENT:
         sk->sk_err = ECONNREFUSED;
         break;
     case TCP_CLOSE_WAIT:
         sk->sk_err = EPIPE;
         break;
     case TCP_CLOSE:
         return;
     default:
         sk->sk_err = ECONNRESET;
     }
     /* This barrier is coupled with smp_rmb() in tcp_poll() */
     smp_wmb();
 
     tcp_write_queue_purge(sk);
     tcp_done(sk);
 
     if (!sock_flag(sk, SOCK_DEAD))
         sk_error_report(sk);
 }
 
 /*
  *  Process the FIN bit. This now behaves as it is supposed to work
  *  and the FIN takes effect when it is validly part of sequence
  *  space. Not before when we get holes.
  *
  *  If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
  *  (and thence onto LAST-ACK and finally, CLOSE, we never enter
  *  TIME-WAIT)
  *
  *  If we are in FINWAIT-1, a received FIN indicates simultaneous
  *  close and we go into CLOSING (and later onto TIME-WAIT)
  *
  *  If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
  */
 void tcp_fin(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     inet_csk_schedule_ack(sk);
 
     sk->sk_shutdown |= RCV_SHUTDOWN;
     sock_set_flag(sk, SOCK_DONE);
 
     switch (sk->sk_state) {
     case TCP_SYN_RECV:
     case TCP_ESTABLISHED:
         /* Move to CLOSE_WAIT */
         tcp_set_state(sk, TCP_CLOSE_WAIT);
         inet_csk_enter_pingpong_mode(sk);
         break;
 
     case TCP_CLOSE_WAIT:
     case TCP_CLOSING:
         /* Received a retransmission of the FIN, do
          * nothing.
          */
         break;
     case TCP_LAST_ACK:
         /* RFC793: Remain in the LAST-ACK state. */
         break;
 
     case TCP_FIN_WAIT1:
         /* This case occurs when a simultaneous close
          * happens, we must ack the received FIN and
          * enter the CLOSING state.
          */
         tcp_send_ack(sk);
         tcp_set_state(sk, TCP_CLOSING);
         break;
     case TCP_FIN_WAIT2:
         /* Received a FIN -- send ACK and enter TIME_WAIT. */
         tcp_send_ack(sk);
         tcp_time_wait(sk, TCP_TIME_WAIT, 0);
         break;
     default:
         /* Only TCP_LISTEN and TCP_CLOSE are left, in these
          * cases we should never reach this piece of code.
          */
         pr_err("%s: Impossible, sk->sk_state=%d\n",
                __func__, sk->sk_state);
         break;
     }
 
     /* It _is_ possible, that we have something out-of-order _after_ FIN.
      * Probably, we should reset in this case. For now drop them.
      */
     skb_rbtree_purge(&tp->out_of_order_queue);
     if (tcp_is_sack(tp))
         tcp_sack_reset(&tp->rx_opt);
 
     if (!sock_flag(sk, SOCK_DEAD)) {
         sk->sk_state_change(sk);
 
         /* Do not send POLL_HUP for half duplex close. */
         if (sk->sk_shutdown == SHUTDOWN_MASK ||
             sk->sk_state == TCP_CLOSE)
             sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
         else
             sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
     }
 }
 
 static inline bool tcp_sack_extend(struct tcp_sack_block *sp, u32 seq,
                   u32 end_seq)
 {
     if (!after(seq, sp->end_seq) && !after(sp->start_seq, end_seq)) {
         if (before(seq, sp->start_seq))
             sp->start_seq = seq;
         if (after(end_seq, sp->end_seq))
             sp->end_seq = end_seq;
         return true;
     }
     return false;
 }
 
 static void tcp_dsack_set(struct sock *sk, u32 seq, u32 end_seq)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tcp_is_sack(tp) && READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_dsack)) {
         int mib_idx;
 
         if (before(seq, tp->rcv_nxt))
             mib_idx = LINUX_MIB_TCPDSACKOLDSENT;
         else
             mib_idx = LINUX_MIB_TCPDSACKOFOSENT;
 
         NET_INC_STATS(sock_net(sk), mib_idx);
 
         tp->rx_opt.dsack = 1;
         tp->duplicate_sack[0].start_seq = seq;
         tp->duplicate_sack[0].end_seq = end_seq;
     }
 }
 
 static void tcp_dsack_extend(struct sock *sk, u32 seq, u32 end_seq)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (!tp->rx_opt.dsack)
         tcp_dsack_set(sk, seq, end_seq);
     else
         tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
 }
 
 static void tcp_rcv_spurious_retrans(struct sock *sk, const struct sk_buff *skb)
 {
     /* When the ACK path fails or drops most ACKs, the sender would
      * timeout and spuriously retransmit the same segment repeatedly.
      * The receiver remembers and reflects via DSACKs. Leverage the
      * DSACK state and change the txhash to re-route speculatively.
      */
     if (TCP_SKB_CB(skb)->seq == tcp_sk(sk)->duplicate_sack[0].start_seq &&
         sk_rethink_txhash(sk))
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDUPLICATEDATAREHASH);
 }
 
 static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
         before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
         tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
 
         if (tcp_is_sack(tp) && READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_dsack)) {
             u32 end_seq = TCP_SKB_CB(skb)->end_seq;
 
             tcp_rcv_spurious_retrans(sk, skb);
             if (after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))
                 end_seq = tp->rcv_nxt;
             tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, end_seq);
         }
     }
 
     tcp_send_ack(sk);
 }
 
 /* These routines update the SACK block as out-of-order packets arrive or
  * in-order packets close up the sequence space.
  */
 static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
 {
     int this_sack;
     struct tcp_sack_block *sp = &tp->selective_acks[0];
     struct tcp_sack_block *swalk = sp + 1;
 
     /* See if the recent change to the first SACK eats into
      * or hits the sequence space of other SACK blocks, if so coalesce.
      */
     for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;) {
         if (tcp_sack_extend(sp, swalk->start_seq, swalk->end_seq)) {
             int i;
 
             /* Zap SWALK, by moving every further SACK up by one slot.
              * Decrease num_sacks.
              */
             tp->rx_opt.num_sacks--;
             for (i = this_sack; i < tp->rx_opt.num_sacks; i++)
                 sp[i] = sp[i + 1];
             continue;
         }
         this_sack++;
         swalk++;
     }
 }
 
 static void tcp_sack_compress_send_ack(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (!tp->compressed_ack)
         return;
 
     if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
         __sock_put(sk);
 
     /* Since we have to send one ack finally,
      * substract one from tp->compressed_ack to keep
      * LINUX_MIB_TCPACKCOMPRESSED accurate.
      */
     NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
               tp->compressed_ack - 1);
 
     tp->compressed_ack = 0;
     tcp_send_ack(sk);
 }
 
 /* Reasonable amount of sack blocks included in TCP SACK option
  * The max is 4, but this becomes 3 if TCP timestamps are there.
  * Given that SACK packets might be lost, be conservative and use 2.
  */
 #define TCP_SACK_BLOCKS_EXPECTED 2
 
 static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct tcp_sack_block *sp = &tp->selective_acks[0];
     int cur_sacks = tp->rx_opt.num_sacks;
     int this_sack;
 
     if (!cur_sacks)
         goto new_sack;
 
     for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
         if (tcp_sack_extend(sp, seq, end_seq)) {
             if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
                 tcp_sack_compress_send_ack(sk);
             /* Rotate this_sack to the first one. */
             for (; this_sack > 0; this_sack--, sp--)
                 swap(*sp, *(sp - 1));
             if (cur_sacks > 1)
                 tcp_sack_maybe_coalesce(tp);
             return;
         }
     }
 
     if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
         tcp_sack_compress_send_ack(sk);
 
     /* Could not find an adjacent existing SACK, build a new one,
      * put it at the front, and shift everyone else down.  We
      * always know there is at least one SACK present already here.
      *
      * If the sack array is full, forget about the last one.
      */
     if (this_sack >= TCP_NUM_SACKS) {
         this_sack--;
         tp->rx_opt.num_sacks--;
         sp--;
     }
     for (; this_sack > 0; this_sack--, sp--)
         *sp = *(sp - 1);
 
 new_sack:
     /* Build the new head SACK, and we're done. */
     sp->start_seq = seq;
     sp->end_seq = end_seq;
     tp->rx_opt.num_sacks++;
 }
 
 /* RCV.NXT advances, some SACKs should be eaten. */
 
 static void tcp_sack_remove(struct tcp_sock *tp)
 {
     struct tcp_sack_block *sp = &tp->selective_acks[0];
     int num_sacks = tp->rx_opt.num_sacks;
     int this_sack;
 
     /* Empty ofo queue, hence, all the SACKs are eaten. Clear. */
     if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
         tp->rx_opt.num_sacks = 0;
         return;
     }
 
     for (this_sack = 0; this_sack < num_sacks;) {
         /* Check if the start of the sack is covered by RCV.NXT. */
         if (!before(tp->rcv_nxt, sp->start_seq)) {
             int i;
 
             /* RCV.NXT must cover all the block! */
             WARN_ON(before(tp->rcv_nxt, sp->end_seq));
 
             /* Zap this SACK, by moving forward any other SACKS. */
             for (i = this_sack+1; i < num_sacks; i++)
                 tp->selective_acks[i-1] = tp->selective_acks[i];
             num_sacks--;
             continue;
         }
         this_sack++;
         sp++;
     }
     tp->rx_opt.num_sacks = num_sacks;
 }
 
 /**
  * tcp_try_coalesce - try to merge skb to prior one
  * @sk: socket
  * @to: prior buffer
  * @from: buffer to add in queue
  * @fragstolen: pointer to boolean
  *
  * Before queueing skb @from after @to, try to merge them
  * to reduce overall memory use and queue lengths, if cost is small.
  * Packets in ofo or receive queues can stay a long time.
  * Better try to coalesce them right now to avoid future collapses.
  * Returns true if caller should free @from instead of queueing it
  */
 static bool tcp_try_coalesce(struct sock *sk,
                  struct sk_buff *to,
                  struct sk_buff *from,
                  bool *fragstolen)
 {
     int delta;
 
     *fragstolen = false;
 
     /* Its possible this segment overlaps with prior segment in queue */
     if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
         return false;
 
     if (!mptcp_skb_can_collapse(to, from))
         return false;
 
 #ifdef CONFIG_TLS_DEVICE
     if (from->decrypted != to->decrypted)
         return false;
 #endif
 
     if (!skb_try_coalesce(to, from, fragstolen, &delta))
         return false;
 
     atomic_add(delta, &sk->sk_rmem_alloc);
     sk_mem_charge(sk, delta);
     NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
     TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
     TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
     TCP_SKB_CB(to)->tcp_flags |= TCP_SKB_CB(from)->tcp_flags;
 
     if (TCP_SKB_CB(from)->has_rxtstamp) {
         TCP_SKB_CB(to)->has_rxtstamp = true;
         to->tstamp = from->tstamp;
         skb_hwtstamps(to)->hwtstamp = skb_hwtstamps(from)->hwtstamp;
     }
 
     return true;
 }
 
 static bool tcp_ooo_try_coalesce(struct sock *sk,
                  struct sk_buff *to,
                  struct sk_buff *from,
                  bool *fragstolen)
 {
     bool res = tcp_try_coalesce(sk, to, from, fragstolen);
 
     /* In case tcp_drop_reason() is called later, update to->gso_segs */
     if (res) {
         u32 gso_segs = max_t(u16, 1, skb_shinfo(to)->gso_segs) +
                    max_t(u16, 1, skb_shinfo(from)->gso_segs);
 
         skb_shinfo(to)->gso_segs = min_t(u32, gso_segs, 0xFFFF);
     }
     return res;
 }
 
 static void tcp_drop_reason(struct sock *sk, struct sk_buff *skb,
                 enum skb_drop_reason reason)
 {
     sk_drops_add(sk, skb);
     kfree_skb_reason(skb, reason);
 }
 
 /* This one checks to see if we can put data from the
  * out_of_order queue into the receive_queue.
  */
 static void tcp_ofo_queue(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     __u32 dsack_high = tp->rcv_nxt;
     bool fin, fragstolen, eaten;
     struct sk_buff *skb, *tail;
     struct rb_node *p;
 
     p = rb_first(&tp->out_of_order_queue);
     while (p) {
         skb = rb_to_skb(p);
         if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
             break;
 
         if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
             __u32 dsack = dsack_high;
             if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
                 dsack_high = TCP_SKB_CB(skb)->end_seq;
             tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
         }
         p = rb_next(p);
         rb_erase(&skb->rbnode, &tp->out_of_order_queue);
 
         if (unlikely(!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))) {
             tcp_drop_reason(sk, skb, SKB_DROP_REASON_TCP_OFO_DROP);
             continue;
         }
 
         tail = skb_peek_tail(&sk->sk_receive_queue);
         eaten = tail && tcp_try_coalesce(sk, tail, skb, &fragstolen);
         tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
         fin = TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN;
         if (!eaten)
             __skb_queue_tail(&sk->sk_receive_queue, skb);
         else
             kfree_skb_partial(skb, fragstolen);
 
         if (unlikely(fin)) {
             tcp_fin(sk);
             /* tcp_fin() purges tp->out_of_order_queue,
              * so we must end this loop right now.
              */
             break;
         }
     }
 }
 
 static bool tcp_prune_ofo_queue(struct sock *sk);
 static int tcp_prune_queue(struct sock *sk);
 
 static int tcp_try_rmem_schedule(struct sock *sk, struct sk_buff *skb,
                  unsigned int size)
 {
     if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
         !sk_rmem_schedule(sk, skb, size)) {
 
         if (tcp_prune_queue(sk) < 0)
             return -1;
 
         while (!sk_rmem_schedule(sk, skb, size)) {
             if (!tcp_prune_ofo_queue(sk))
                 return -1;
         }
     }
     return 0;
 }
 
 static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct rb_node **p, *parent;
     struct sk_buff *skb1;
     u32 seq, end_seq;
     bool fragstolen;
 
     tcp_ecn_check_ce(sk, skb);
 
     if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP);
         sk->sk_data_ready(sk);
         tcp_drop_reason(sk, skb, SKB_DROP_REASON_PROTO_MEM);
         return;
     }
 
     /* Disable header prediction. */
     tp->pred_flags = 0;
     inet_csk_schedule_ack(sk);
 
     tp->rcv_ooopack += max_t(u16, 1, skb_shinfo(skb)->gso_segs);
     NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
     seq = TCP_SKB_CB(skb)->seq;
     end_seq = TCP_SKB_CB(skb)->end_seq;
 
     p = &tp->out_of_order_queue.rb_node;
     if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
         /* Initial out of order segment, build 1 SACK. */
         if (tcp_is_sack(tp)) {
             tp->rx_opt.num_sacks = 1;
             tp->selective_acks[0].start_seq = seq;
             tp->selective_acks[0].end_seq = end_seq;
         }
         rb_link_node(&skb->rbnode, NULL, p);
         rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);
         tp->ooo_last_skb = skb;
         goto end;
     }
 
     /* In the typical case, we are adding an skb to the end of the list.
      * Use of ooo_last_skb avoids the O(Log(N)) rbtree lookup.
      */
     if (tcp_ooo_try_coalesce(sk, tp->ooo_last_skb,
                  skb, &fragstolen)) {
 coalesce_done:
         /* For non sack flows, do not grow window to force DUPACK
          * and trigger fast retransmit.
          */
         if (tcp_is_sack(tp))
             tcp_grow_window(sk, skb, true);
         kfree_skb_partial(skb, fragstolen);
         skb = NULL;
         goto add_sack;
     }
     /* Can avoid an rbtree lookup if we are adding skb after ooo_last_skb */
     if (!before(seq, TCP_SKB_CB(tp->ooo_last_skb)->end_seq)) {
         parent = &tp->ooo_last_skb->rbnode;
         p = &parent->rb_right;
         goto insert;
     }
 
     /* Find place to insert this segment. Handle overlaps on the way. */
     parent = NULL;
     while (*p) {
         parent = *p;
         skb1 = rb_to_skb(parent);
         if (before(seq, TCP_SKB_CB(skb1)->seq)) {
             p = &parent->rb_left;
             continue;
         }
         if (before(seq, TCP_SKB_CB(skb1)->end_seq)) {
             if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
                 /* All the bits are present. Drop. */
                 NET_INC_STATS(sock_net(sk),
                           LINUX_MIB_TCPOFOMERGE);
                 tcp_drop_reason(sk, skb,
                         SKB_DROP_REASON_TCP_OFOMERGE);
                 skb = NULL;
                 tcp_dsack_set(sk, seq, end_seq);
                 goto add_sack;
             }
             if (after(seq, TCP_SKB_CB(skb1)->seq)) {
                 /* Partial overlap. */
                 tcp_dsack_set(sk, seq, TCP_SKB_CB(skb1)->end_seq);
             } else {
                 /* skb's seq == skb1's seq and skb covers skb1.
                  * Replace skb1 with skb.
                  */
                 rb_replace_node(&skb1->rbnode, &skb->rbnode,
                         &tp->out_of_order_queue);
                 tcp_dsack_extend(sk,
                          TCP_SKB_CB(skb1)->seq,
                          TCP_SKB_CB(skb1)->end_seq);
                 NET_INC_STATS(sock_net(sk),
                           LINUX_MIB_TCPOFOMERGE);
                 tcp_drop_reason(sk, skb1,
                         SKB_DROP_REASON_TCP_OFOMERGE);
                 goto merge_right;
             }
         } else if (tcp_ooo_try_coalesce(sk, skb1,
                         skb, &fragstolen)) {
             goto coalesce_done;
         }
         p = &parent->rb_right;
     }
 insert:
     /* Insert segment into RB tree. */
     rb_link_node(&skb->rbnode, parent, p);
     rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);
 
 merge_right:
     /* Remove other segments covered by skb. */
     while ((skb1 = skb_rb_next(skb)) != NULL) {
         if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
             break;
         if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
             tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                      end_seq);
             break;
         }
         rb_erase(&skb1->rbnode, &tp->out_of_order_queue);
         tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                  TCP_SKB_CB(skb1)->end_seq);
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
         tcp_drop_reason(sk, skb1, SKB_DROP_REASON_TCP_OFOMERGE);
     }
     /* If there is no skb after us, we are the last_skb ! */
     if (!skb1)
         tp->ooo_last_skb = skb;
 
 add_sack:
     if (tcp_is_sack(tp))
         tcp_sack_new_ofo_skb(sk, seq, end_seq);
 end:
     if (skb) {
         /* For non sack flows, do not grow window to force DUPACK
          * and trigger fast retransmit.
          */
         if (tcp_is_sack(tp))
             tcp_grow_window(sk, skb, false);
         skb_condense(skb);
         skb_set_owner_r(skb, sk);
     }
 }
 
 static int __must_check tcp_queue_rcv(struct sock *sk, struct sk_buff *skb,
                       bool *fragstolen)
 {
     int eaten;
     struct sk_buff *tail = skb_peek_tail(&sk->sk_receive_queue);
 
     eaten = (tail &&
          tcp_try_coalesce(sk, tail,
                   skb, fragstolen)) ? 1 : 0;
     tcp_rcv_nxt_update(tcp_sk(sk), TCP_SKB_CB(skb)->end_seq);
     if (!eaten) {
         __skb_queue_tail(&sk->sk_receive_queue, skb);
         skb_set_owner_r(skb, sk);
     }
     return eaten;
 }
 
 int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
 {
     struct sk_buff *skb;
     int err = -ENOMEM;
     int data_len = 0;
     bool fragstolen;
 
     if (size == 0)
         return 0;
 
     if (size > PAGE_SIZE) {
         int npages = min_t(size_t, size >> PAGE_SHIFT, MAX_SKB_FRAGS);
 
         data_len = npages << PAGE_SHIFT;
         size = data_len + (size & ~PAGE_MASK);
     }
     skb = alloc_skb_with_frags(size - data_len, data_len,
                    PAGE_ALLOC_COSTLY_ORDER,
                    &err, sk->sk_allocation);
     if (!skb)
         goto err;
 
     skb_put(skb, size - data_len);
     skb->data_len = data_len;
     skb->len = size;
 
     if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
         goto err_free;
     }
 
     err = skb_copy_datagram_from_iter(skb, 0, &msg->msg_iter, size);
     if (err)
         goto err_free;
 
     TCP_SKB_CB(skb)->seq = tcp_sk(sk)->rcv_nxt;
     TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + size;
     TCP_SKB_CB(skb)->ack_seq = tcp_sk(sk)->snd_una - 1;
 
     if (tcp_queue_rcv(sk, skb, &fragstolen)) {
         WARN_ON_ONCE(fragstolen); /* should not happen */
         __kfree_skb(skb);
     }
     return size;
 
 err_free:
     kfree_skb(skb);
 err:
     return err;
 
 }
 
 void tcp_data_ready(struct sock *sk)
 {
     if (tcp_epollin_ready(sk, sk->sk_rcvlowat) || sock_flag(sk, SOCK_DONE))
         sk->sk_data_ready(sk);
 }
 
 static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     enum skb_drop_reason reason;
     bool fragstolen;
     int eaten;
 
     /* If a subflow has been reset, the packet should not continue
      * to be processed, drop the packet.
      */
     if (sk_is_mptcp(sk) && !mptcp_incoming_options(sk, skb)) {
         __kfree_skb(skb);
         return;
     }
 
     if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
         __kfree_skb(skb);
         return;
     }
     skb_dst_drop(skb);
     __skb_pull(skb, tcp_hdr(skb)->doff * 4);
 
     reason = SKB_DROP_REASON_NOT_SPECIFIED;
     tp->rx_opt.dsack = 0;
 
     /*  Queue data for delivery to the user.
      *  Packets in sequence go to the receive queue.
      *  Out of sequence packets to the out_of_order_queue.
      */
     if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
         if (tcp_receive_window(tp) == 0) {
             reason = SKB_DROP_REASON_TCP_ZEROWINDOW;
             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
             goto out_of_window;
         }
 
         /* Ok. In sequence. In window. */
 queue_and_out:
         if (skb_queue_len(&sk->sk_receive_queue) == 0)
             sk_forced_mem_schedule(sk, skb->truesize);
         else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
             reason = SKB_DROP_REASON_PROTO_MEM;
             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
             sk->sk_data_ready(sk);
             goto drop;
         }
 
         eaten = tcp_queue_rcv(sk, skb, &fragstolen);
         if (skb->len)
             tcp_event_data_recv(sk, skb);
         if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
             tcp_fin(sk);
 
         if (!RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
             tcp_ofo_queue(sk);
 
             /* RFC5681. 4.2. SHOULD send immediate ACK, when
              * gap in queue is filled.
              */
             if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
                 inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
         }
 
         if (tp->rx_opt.num_sacks)
             tcp_sack_remove(tp);
 
         tcp_fast_path_check(sk);
 
         if (eaten > 0)
             kfree_skb_partial(skb, fragstolen);
         if (!sock_flag(sk, SOCK_DEAD))
             tcp_data_ready(sk);
         return;
     }
 
     if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
         tcp_rcv_spurious_retrans(sk, skb);
         /* A retransmit, 2nd most common case.  Force an immediate ack. */
         reason = SKB_DROP_REASON_TCP_OLD_DATA;
         NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
         tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
 
 out_of_window:
         tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
         inet_csk_schedule_ack(sk);
 drop:
         tcp_drop_reason(sk, skb, reason);
         return;
     }
 
     /* Out of window. F.e. zero window probe. */
     if (!before(TCP_SKB_CB(skb)->seq,
             tp->rcv_nxt + tcp_receive_window(tp))) {
         reason = SKB_DROP_REASON_TCP_OVERWINDOW;
         goto out_of_window;
     }
 
     if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
         /* Partial packet, seq < rcv_next < end_seq */
         tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);
 
         /* If window is closed, drop tail of packet. But after
          * remembering D-SACK for its head made in previous line.
          */
         if (!tcp_receive_window(tp)) {
             reason = SKB_DROP_REASON_TCP_ZEROWINDOW;
             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
             goto out_of_window;
         }
         goto queue_and_out;
     }
 
     tcp_data_queue_ofo(sk, skb);
 }
 
 static struct sk_buff *tcp_skb_next(struct sk_buff *skb, struct sk_buff_head *list)
 {
     if (list)
         return !skb_queue_is_last(list, skb) ? skb->next : NULL;
 
     return skb_rb_next(skb);
 }
 
 static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb,
                     struct sk_buff_head *list,
                     struct rb_root *root)
 {
     struct sk_buff *next = tcp_skb_next(skb, list);
 
     if (list)
         __skb_unlink(skb, list);
     else
         rb_erase(&skb->rbnode, root);
 
     __kfree_skb(skb);
     NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);
 
     return next;
 }
 
 /* Insert skb into rb tree, ordered by TCP_SKB_CB(skb)->seq */
 void tcp_rbtree_insert(struct rb_root *root, struct sk_buff *skb)
 {
     struct rb_node **p = &root->rb_node;
     struct rb_node *parent = NULL;
     struct sk_buff *skb1;
 
     while (*p) {
         parent = *p;
         skb1 = rb_to_skb(parent);
         if (before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb1)->seq))
             p = &parent->rb_left;
         else
             p = &parent->rb_right;
     }
     rb_link_node(&skb->rbnode, parent, p);
     rb_insert_color(&skb->rbnode, root);
 }
 
 /* Collapse contiguous sequence of skbs head..tail with
  * sequence numbers start..end.
  *
  * If tail is NULL, this means until the end of the queue.
  *
  * Segments with FIN/SYN are not collapsed (only because this
  * simplifies code)
  */
 static void
 tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
          struct sk_buff *head, struct sk_buff *tail, u32 start, u32 end)
 {
     struct sk_buff *skb = head, *n;
     struct sk_buff_head tmp;
     bool end_of_skbs;
 
     /* First, check that queue is collapsible and find
      * the point where collapsing can be useful.
      */
 restart:
     for (end_of_skbs = true; skb != NULL && skb != tail; skb = n) {
         n = tcp_skb_next(skb, list);
 
         /* No new bits? It is possible on ofo queue. */
         if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
             skb = tcp_collapse_one(sk, skb, list, root);
             if (!skb)
                 break;
             goto restart;
         }
 
         /* The first skb to collapse is:
          * - not SYN/FIN and
          * - bloated or contains data before "start" or
          *   overlaps to the next one and mptcp allow collapsing.
          */
         if (!(TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)) &&
             (tcp_win_from_space(sk, skb->truesize) > skb->len ||
              before(TCP_SKB_CB(skb)->seq, start))) {
             end_of_skbs = false;
             break;
         }
 
         if (n && n != tail && mptcp_skb_can_collapse(skb, n) &&
             TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(n)->seq) {
             end_of_skbs = false;
             break;
         }
 
         /* Decided to skip this, advance start seq. */
         start = TCP_SKB_CB(skb)->end_seq;
     }
     if (end_of_skbs ||
         (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
         return;
 
     __skb_queue_head_init(&tmp);
 
     while (before(start, end)) {
         int copy = min_t(int, SKB_MAX_ORDER(0, 0), end - start);
         struct sk_buff *nskb;
 
         nskb = alloc_skb(copy, GFP_ATOMIC);
         if (!nskb)
             break;
 
         memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
 #ifdef CONFIG_TLS_DEVICE
         nskb->decrypted = skb->decrypted;
 #endif
         TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
         if (list)
             __skb_queue_before(list, skb, nskb);
         else
             __skb_queue_tail(&tmp, nskb); /* defer rbtree insertion */
         skb_set_owner_r(nskb, sk);
         mptcp_skb_ext_move(nskb, skb);
 
         /* Copy data, releasing collapsed skbs. */
         while (copy > 0) {
             int offset = start - TCP_SKB_CB(skb)->seq;
             int size = TCP_SKB_CB(skb)->end_seq - start;
 
             BUG_ON(offset < 0);
             if (size > 0) {
                 size = min(copy, size);
                 if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
                     BUG();
                 TCP_SKB_CB(nskb)->end_seq += size;
                 copy -= size;
                 start += size;
             }
             if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
                 skb = tcp_collapse_one(sk, skb, list, root);
                 if (!skb ||
                     skb == tail ||
                     !mptcp_skb_can_collapse(nskb, skb) ||
                     (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
                     goto end;
 #ifdef CONFIG_TLS_DEVICE
                 if (skb->decrypted != nskb->decrypted)
                     goto end;
 #endif
             }
         }
     }
 end:
     skb_queue_walk_safe(&tmp, skb, n)
         tcp_rbtree_insert(root, skb);
 }
 
 /* Collapse ofo queue. Algorithm: select contiguous sequence of skbs
  * and tcp_collapse() them until all the queue is collapsed.
  */
 static void tcp_collapse_ofo_queue(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 range_truesize, sum_tiny = 0;
     struct sk_buff *skb, *head;
     u32 start, end;
 
     skb = skb_rb_first(&tp->out_of_order_queue);
 new_range:
     if (!skb) {
         tp->ooo_last_skb = skb_rb_last(&tp->out_of_order_queue);
         return;
     }
     start = TCP_SKB_CB(skb)->seq;
     end = TCP_SKB_CB(skb)->end_seq;
     range_truesize = skb->truesize;
 
     for (head = skb;;) {
         skb = skb_rb_next(skb);
 
         /* Range is terminated when we see a gap or when
          * we are at the queue end.
          */
         if (!skb ||
             after(TCP_SKB_CB(skb)->seq, end) ||
             before(TCP_SKB_CB(skb)->end_seq, start)) {
             /* Do not attempt collapsing tiny skbs */
             if (range_truesize != head->truesize ||
                 end - start >= SKB_WITH_OVERHEAD(PAGE_SIZE)) {
                 tcp_collapse(sk, NULL, &tp->out_of_order_queue,
                          head, skb, start, end);
             } else {
                 sum_tiny += range_truesize;
                 if (sum_tiny > sk->sk_rcvbuf >> 3)
                     return;
             }
             goto new_range;
         }
 
         range_truesize += skb->truesize;
         if (unlikely(before(TCP_SKB_CB(skb)->seq, start)))
             start = TCP_SKB_CB(skb)->seq;
         if (after(TCP_SKB_CB(skb)->end_seq, end))
             end = TCP_SKB_CB(skb)->end_seq;
     }
 }
 
 /*
  * Clean the out-of-order queue to make room.
  * We drop high sequences packets to :
  * 1) Let a chance for holes to be filled.
  * 2) not add too big latencies if thousands of packets sit there.
  *    (But if application shrinks SO_RCVBUF, we could still end up
  *     freeing whole queue here)
  * 3) Drop at least 12.5 % of sk_rcvbuf to avoid malicious attacks.
  *
  * Return true if queue has shrunk.
  */
 static bool tcp_prune_ofo_queue(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct rb_node *node, *prev;
     int goal;
 
     if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
         return false;
 
     NET_INC_STATS(sock_net(sk), LINUX_MIB_OFOPRUNED);
     goal = sk->sk_rcvbuf >> 3;
     node = &tp->ooo_last_skb->rbnode;
     do {
         prev = rb_prev(node);
         rb_erase(node, &tp->out_of_order_queue);
         goal -= rb_to_skb(node)->truesize;
         tcp_drop_reason(sk, rb_to_skb(node),
                 SKB_DROP_REASON_TCP_OFO_QUEUE_PRUNE);
         if (!prev || goal <= 0) {
             if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf &&
                 !tcp_under_memory_pressure(sk))
                 break;
             goal = sk->sk_rcvbuf >> 3;
         }
         node = prev;
     } while (node);
     tp->ooo_last_skb = rb_to_skb(prev);
 
     /* Reset SACK state.  A conforming SACK implementation will
      * do the same at a timeout based retransmit.  When a connection
      * is in a sad state like this, we care only about integrity
      * of the connection not performance.
      */
     if (tp->rx_opt.sack_ok)
         tcp_sack_reset(&tp->rx_opt);
     return true;
 }
 
 /* Reduce allocated memory if we can, trying to get
  * the socket within its memory limits again.
  *
  * Return less than zero if we should start dropping frames
  * until the socket owning process reads some of the data
  * to stabilize the situation.
  */
 static int tcp_prune_queue(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     NET_INC_STATS(sock_net(sk), LINUX_MIB_PRUNECALLED);
 
     if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
         tcp_clamp_window(sk);
     else if (tcp_under_memory_pressure(sk))
         tcp_adjust_rcv_ssthresh(sk);
 
     if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
         return 0;
 
     tcp_collapse_ofo_queue(sk);
     if (!skb_queue_empty(&sk->sk_receive_queue))
         tcp_collapse(sk, &sk->sk_receive_queue, NULL,
                  skb_peek(&sk->sk_receive_queue),
                  NULL,
                  tp->copied_seq, tp->rcv_nxt);
 
     if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
         return 0;
 
     /* Collapsing did not help, destructive actions follow.
      * This must not ever occur. */
 
     tcp_prune_ofo_queue(sk);
 
     if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
         return 0;
 
     /* If we are really being abused, tell the caller to silently
      * drop receive data on the floor.  It will get retransmitted
      * and hopefully then we'll have sufficient space.
      */
     NET_INC_STATS(sock_net(sk), LINUX_MIB_RCVPRUNED);
 
     /* Massive buffer overcommit. */
     tp->pred_flags = 0;
     return -1;
 }
 
 static bool tcp_should_expand_sndbuf(struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
 
     /* If the user specified a specific send buffer setting, do
      * not modify it.
      */
     if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
         return false;
 
     /* If we are under global TCP memory pressure, do not expand.  */
     if (tcp_under_memory_pressure(sk)) {
         int unused_mem = sk_unused_reserved_mem(sk);
 
         /* Adjust sndbuf according to reserved mem. But make sure
          * it never goes below SOCK_MIN_SNDBUF.
          * See sk_stream_moderate_sndbuf() for more details.
          */
         if (unused_mem > SOCK_MIN_SNDBUF)
             WRITE_ONCE(sk->sk_sndbuf, unused_mem);
 
         return false;
     }
 
     /* If we are under soft global TCP memory pressure, do not expand.  */
     if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
         return false;
 
     /* If we filled the congestion window, do not expand.  */
     if (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp))
         return false;
 
     return true;
 }
 
 static void tcp_new_space(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tcp_should_expand_sndbuf(sk)) {
         tcp_sndbuf_expand(sk);
         tp->snd_cwnd_stamp = tcp_jiffies32;
     }
 
     INDIRECT_CALL_1(sk->sk_write_space, sk_stream_write_space, sk);
 }
 
 /* Caller made space either from:
  * 1) Freeing skbs in rtx queues (after tp->snd_una has advanced)
  * 2) Sent skbs from output queue (and thus advancing tp->snd_nxt)
  *
  * We might be able to generate EPOLLOUT to the application if:
  * 1) Space consumed in output/rtx queues is below sk->sk_sndbuf/2
  * 2) notsent amount (tp->write_seq - tp->snd_nxt) became
  *    small enough that tcp_stream_memory_free() decides it
  *    is time to generate EPOLLOUT.
  */
 void tcp_check_space(struct sock *sk)
 {
     /* pairs with tcp_poll() */
     smp_mb();
     if (sk->sk_socket &&
         test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
         tcp_new_space(sk);
         if (!test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
             tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
     }
 }
 
 static inline void tcp_data_snd_check(struct sock *sk)
 {
     tcp_push_pending_frames(sk);
     tcp_check_space(sk);
 }
 
 /*
  * Check if sending an ack is needed.
  */
 static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     unsigned long rtt, delay;
 
         /* More than one full frame received... */
     if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
          /* ... and right edge of window advances far enough.
           * (tcp_recvmsg() will send ACK otherwise).
           * If application uses SO_RCVLOWAT, we want send ack now if
           * we have not received enough bytes to satisfy the condition.
           */
         (tp->rcv_nxt - tp->copied_seq < sk->sk_rcvlowat ||
          __tcp_select_window(sk) >= tp->rcv_wnd)) ||
         /* We ACK each frame or... */
         tcp_in_quickack_mode(sk) ||
         /* Protocol state mandates a one-time immediate ACK */
         inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOW) {
 send_now:
         tcp_send_ack(sk);
         return;
     }
 
     if (!ofo_possible || RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
         tcp_send_delayed_ack(sk);
         return;
     }
 
     if (!tcp_is_sack(tp) ||
         tp->compressed_ack >= READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr))
         goto send_now;
 
     if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
         tp->compressed_ack_rcv_nxt = tp->rcv_nxt;
         tp->dup_ack_counter = 0;
     }
     if (tp->dup_ack_counter < TCP_FASTRETRANS_THRESH) {
         tp->dup_ack_counter++;
         goto send_now;
     }
     tp->compressed_ack++;
     if (hrtimer_is_queued(&tp->compressed_ack_timer))
         return;
 
     /* compress ack timer : 5 % of rtt, but no more than tcp_comp_sack_delay_ns */
 
     rtt = tp->rcv_rtt_est.rtt_us;
     if (tp->srtt_us && tp->srtt_us < rtt)
         rtt = tp->srtt_us;
 
     delay = min_t(unsigned long,
               READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns),
               rtt * (NSEC_PER_USEC >> 3)/20);
     sock_hold(sk);
     hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
                    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns),
                    HRTIMER_MODE_REL_PINNED_SOFT);
 }
 
 static inline void tcp_ack_snd_check(struct sock *sk)
 {
     if (!inet_csk_ack_scheduled(sk)) {
         /* We sent a data segment already. */
         return;
     }
     __tcp_ack_snd_check(sk, 1);
 }
 
 /*
  *  This routine is only called when we have urgent data
  *  signaled. Its the 'slow' part of tcp_urg. It could be
  *  moved inline now as tcp_urg is only called from one
  *  place. We handle URGent data wrong. We have to - as
  *  BSD still doesn't use the correction from RFC961.
  *  For 1003.1g we should support a new option TCP_STDURG to permit
  *  either form (or just set the sysctl tcp_stdurg).
  */
 
 static void tcp_check_urg(struct sock *sk, const struct tcphdr *th)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 ptr = ntohs(th->urg_ptr);
 
     if (ptr && !READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_stdurg))
         ptr--;
     ptr += ntohl(th->seq);
 
     /* Ignore urgent data that we've already seen and read. */
     if (after(tp->copied_seq, ptr))
         return;
 
     /* Do not replay urg ptr.
      *
      * NOTE: interesting situation not covered by specs.
      * Misbehaving sender may send urg ptr, pointing to segment,
      * which we already have in ofo queue. We are not able to fetch
      * such data and will stay in TCP_URG_NOTYET until will be eaten
      * by recvmsg(). Seems, we are not obliged to handle such wicked
      * situations. But it is worth to think about possibility of some
      * DoSes using some hypothetical application level deadlock.
      */
     if (before(ptr, tp->rcv_nxt))
         return;
 
     /* Do we already have a newer (or duplicate) urgent pointer? */
     if (tp->urg_data && !after(ptr, tp->urg_seq))
         return;
 
     /* Tell the world about our new urgent pointer. */
     sk_send_sigurg(sk);
 
     /* We may be adding urgent data when the last byte read was
      * urgent. To do this requires some care. We cannot just ignore
      * tp->copied_seq since we would read the last urgent byte again
      * as data, nor can we alter copied_seq until this data arrives
      * or we break the semantics of SIOCATMARK (and thus sockatmark())
      *
      * NOTE. Double Dutch. Rendering to plain English: author of comment
      * above did something sort of  send("A", MSG_OOB); send("B", MSG_OOB);
      * and expect that both A and B disappear from stream. This is _wrong_.
      * Though this happens in BSD with high probability, this is occasional.
      * Any application relying on this is buggy. Note also, that fix "works"
      * only in this artificial test. Insert some normal data between A and B and we will
      * decline of BSD again. Verdict: it is better to remove to trap
      * buggy users.
      */
     if (tp->urg_seq == tp->copied_seq && tp->urg_data &&
         !sock_flag(sk, SOCK_URGINLINE) && tp->copied_seq != tp->rcv_nxt) {
         struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
         tp->copied_seq++;
         if (skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq)) {
             __skb_unlink(skb, &sk->sk_receive_queue);
             __kfree_skb(skb);
         }
     }
 
     WRITE_ONCE(tp->urg_data, TCP_URG_NOTYET);
     WRITE_ONCE(tp->urg_seq, ptr);
 
     /* Disable header prediction. */
     tp->pred_flags = 0;
 }
 
 /* This is the 'fast' part of urgent handling. */
 static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *th)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* Check if we get a new urgent pointer - normally not. */
     if (unlikely(th->urg))
         tcp_check_urg(sk, th);
 
     /* Do we wait for any urgent data? - normally not... */
     if (unlikely(tp->urg_data == TCP_URG_NOTYET)) {
         u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff * 4) -
               th->syn;
 
         /* Is the urgent pointer pointing into this packet? */
         if (ptr < skb->len) {
             u8 tmp;
             if (skb_copy_bits(skb, ptr, &tmp, 1))
                 BUG();
             WRITE_ONCE(tp->urg_data, TCP_URG_VALID | tmp);
             if (!sock_flag(sk, SOCK_DEAD))
                 sk->sk_data_ready(sk);
         }
     }
 }
 
 /* Accept RST for rcv_nxt - 1 after a FIN.
  * When tcp connections are abruptly terminated from Mac OSX (via ^C), a
  * FIN is sent followed by a RST packet. The RST is sent with the same
  * sequence number as the FIN, and thus according to RFC 5961 a challenge
  * ACK should be sent. However, Mac OSX rate limits replies to challenge
  * ACKs on the closed socket. In addition middleboxes can drop either the
  * challenge ACK or a subsequent RST.
  */
 static bool tcp_reset_check(const struct sock *sk, const struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     return unlikely(TCP_SKB_CB(skb)->seq == (tp->rcv_nxt - 1) &&
             (1 << sk->sk_state) & (TCPF_CLOSE_WAIT | TCPF_LAST_ACK |
                            TCPF_CLOSING));
 }
 
 /* Does PAWS and seqno based validation of an incoming segment, flags will
  * play significant role here.
  */
 static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
                   const struct tcphdr *th, int syn_inerr)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     SKB_DR(reason);
 
     /* RFC1323: H1. Apply PAWS check first. */
     if (tcp_fast_parse_options(sock_net(sk), skb, th, tp) &&
         tp->rx_opt.saw_tstamp &&
         tcp_paws_discard(sk, skb)) {
         if (!th->rst) {
             NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
             if (!tcp_oow_rate_limited(sock_net(sk), skb,
                           LINUX_MIB_TCPACKSKIPPEDPAWS,
                           &tp->last_oow_ack_time))
                 tcp_send_dupack(sk, skb);
             SKB_DR_SET(reason, TCP_RFC7323_PAWS);
             goto discard;
         }
         /* Reset is accepted even if it did not pass PAWS. */
     }
 
     /* Step 1: check sequence number */
     if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
         /* RFC793, page 37: "In all states except SYN-SENT, all reset
          * (RST) segments are validated by checking their SEQ-fields."
          * And page 69: "If an incoming segment is not acceptable,
          * an acknowledgment should be sent in reply (unless the RST
          * bit is set, if so drop the segment and return)".
          */
         if (!th->rst) {
             if (th->syn)
                 goto syn_challenge;
             if (!tcp_oow_rate_limited(sock_net(sk), skb,
                           LINUX_MIB_TCPACKSKIPPEDSEQ,
                           &tp->last_oow_ack_time))
                 tcp_send_dupack(sk, skb);
         } else if (tcp_reset_check(sk, skb)) {
             goto reset;
         }
         SKB_DR_SET(reason, TCP_INVALID_SEQUENCE);
         goto discard;
     }
 
     /* Step 2: check RST bit */
     if (th->rst) {
         /* RFC 5961 3.2 (extend to match against (RCV.NXT - 1) after a
          * FIN and SACK too if available):
          * If seq num matches RCV.NXT or (RCV.NXT - 1) after a FIN, or
          * the right-most SACK block,
          * then
          *     RESET the connection
          * else
          *     Send a challenge ACK
          */
         if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt ||
             tcp_reset_check(sk, skb))
             goto reset;
 
         if (tcp_is_sack(tp) && tp->rx_opt.num_sacks > 0) {
             struct tcp_sack_block *sp = &tp->selective_acks[0];
             int max_sack = sp[0].end_seq;
             int this_sack;
 
             for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;
                  ++this_sack) {
                 max_sack = after(sp[this_sack].end_seq,
                          max_sack) ?
                     sp[this_sack].end_seq : max_sack;
             }
 
             if (TCP_SKB_CB(skb)->seq == max_sack)
                 goto reset;
         }
 
         /* Disable TFO if RST is out-of-order
          * and no data has been received
          * for current active TFO socket
          */
         if (tp->syn_fastopen && !tp->data_segs_in &&
             sk->sk_state == TCP_ESTABLISHED)
             tcp_fastopen_active_disable(sk);
         tcp_send_challenge_ack(sk);
         SKB_DR_SET(reason, TCP_RESET);
         goto discard;
     }
 
     /* step 3: check security and precedence [ignored] */
 
     /* step 4: Check for a SYN
      * RFC 5961 4.2 : Send a challenge ack
      */
     if (th->syn) {
 syn_challenge:
         if (syn_inerr)
             TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
         tcp_send_challenge_ack(sk);
         SKB_DR_SET(reason, TCP_INVALID_SYN);
         goto discard;
     }
 
     bpf_skops_parse_hdr(sk, skb);
 
     return true;
 
 discard:
     tcp_drop_reason(sk, skb, reason);
     return false;
 
 reset:
     tcp_reset(sk, skb);
     __kfree_skb(skb);
     return false;
 }
 
 /*
  *  TCP receive function for the ESTABLISHED state.
  *
  *  It is split into a fast path and a slow path. The fast path is
  *  disabled when:
  *  - A zero window was announced from us - zero window probing
  *        is only handled properly in the slow path.
  *  - Out of order segments arrived.
  *  - Urgent data is expected.
  *  - There is no buffer space left
  *  - Unexpected TCP flags/window values/header lengths are received
  *    (detected by checking the TCP header against pred_flags)
  *  - Data is sent in both directions. Fast path only supports pure senders
  *    or pure receivers (this means either the sequence number or the ack
  *    value must stay constant)
  *  - Unexpected TCP option.
  *
  *  When these conditions are not satisfied it drops into a standard
  *  receive procedure patterned after RFC793 to handle all cases.
  *  The first three cases are guaranteed by proper pred_flags setting,
  *  the rest is checked inline. Fast processing is turned on in
  *  tcp_data_queue when everything is OK.
  */
 void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 {
     enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
     const struct tcphdr *th = (const struct tcphdr *)skb->data;
     struct tcp_sock *tp = tcp_sk(sk);
     unsigned int len = skb->len;
 
     /* TCP congestion window tracking */
     trace_tcp_probe(sk, skb);
 
     tcp_mstamp_refresh(tp);
     if (unlikely(!rcu_access_pointer(sk->sk_rx_dst)))
         inet_csk(sk)->icsk_af_ops->sk_rx_dst_set(sk, skb);
     /*
      *  Header prediction.
      *  The code loosely follows the one in the famous
      *  "30 instruction TCP receive" Van Jacobson mail.
      *
      *  Van's trick is to deposit buffers into socket queue
      *  on a device interrupt, to call tcp_recv function
      *  on the receive process context and checksum and copy
      *  the buffer to user space. smart...
      *
      *  Our current scheme is not silly either but we take the
      *  extra cost of the net_bh soft interrupt processing...
      *  We do checksum and copy also but from device to kernel.
      */
 
     tp->rx_opt.saw_tstamp = 0;
 
     /*  pred_flags is 0xS?10 << 16 + snd_wnd
      *  if header_prediction is to be made
      *  'S' will always be tp->tcp_header_len >> 2
      *  '?' will be 0 for the fast path, otherwise pred_flags is 0 to
      *  turn it off (when there are holes in the receive
      *   space for instance)
      *  PSH flag is ignored.
      */
 
     if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
         TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
         !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
         int tcp_header_len = tp->tcp_header_len;
 
         /* Timestamp header prediction: tcp_header_len
          * is automatically equal to th->doff*4 due to pred_flags
          * match.
          */
 
         /* Check timestamp */
         if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
             /* No? Slow path! */
             if (!tcp_parse_aligned_timestamp(tp, th))
                 goto slow_path;
 
             /* If PAWS failed, check it more carefully in slow path */
             if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
                 goto slow_path;
 
             /* DO NOT update ts_recent here, if checksum fails
              * and timestamp was corrupted part, it will result
              * in a hung connection since we will drop all
              * future packets due to the PAWS test.
              */
         }
 
         if (len <= tcp_header_len) {
             /* Bulk data transfer: sender */
             if (len == tcp_header_len) {
                 /* Predicted packet is in window by definition.
                  * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                  * Hence, check seq<=rcv_wup reduces to:
                  */
                 if (tcp_header_len ==
                     (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                     tp->rcv_nxt == tp->rcv_wup)
                     tcp_store_ts_recent(tp);
 
                 /* We know that such packets are checksummed
                  * on entry.
                  */
                 tcp_ack(sk, skb, 0);
                 __kfree_skb(skb);
                 tcp_data_snd_check(sk);
                 /* When receiving pure ack in fast path, update
                  * last ts ecr directly instead of calling
                  * tcp_rcv_rtt_measure_ts()
                  */
                 tp->rcv_rtt_last_tsecr = tp->rx_opt.rcv_tsecr;
                 return;
             } else { /* Header too small */
                 reason = SKB_DROP_REASON_PKT_TOO_SMALL;
                 TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
                 goto discard;
             }
         } else {
             int eaten = 0;
             bool fragstolen = false;
 
             if (tcp_checksum_complete(skb))
                 goto csum_error;
 
             if ((int)skb->truesize > sk->sk_forward_alloc)
                 goto step5;
 
             /* Predicted packet is in window by definition.
              * seq == rcv_nxt and rcv_wup <= rcv_nxt.
              * Hence, check seq<=rcv_wup reduces to:
              */
             if (tcp_header_len ==
                 (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                 tp->rcv_nxt == tp->rcv_wup)
                 tcp_store_ts_recent(tp);
 
             tcp_rcv_rtt_measure_ts(sk, skb);
 
             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPHITS);
 
             /* Bulk data transfer: receiver */
             skb_dst_drop(skb);
             __skb_pull(skb, tcp_header_len);
             eaten = tcp_queue_rcv(sk, skb, &fragstolen);
 
             tcp_event_data_recv(sk, skb);
 
             if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
                 /* Well, only one small jumplet in fast path... */
                 tcp_ack(sk, skb, FLAG_DATA);
                 tcp_data_snd_check(sk);
                 if (!inet_csk_ack_scheduled(sk))
                     goto no_ack;
             } else {
                 tcp_update_wl(tp, TCP_SKB_CB(skb)->seq);
             }
 
             __tcp_ack_snd_check(sk, 0);
 no_ack:
             if (eaten)
                 kfree_skb_partial(skb, fragstolen);
             tcp_data_ready(sk);
             return;
         }
     }
 
 slow_path:
     if (len < (th->doff << 2) || tcp_checksum_complete(skb))
         goto csum_error;
 
     if (!th->ack && !th->rst && !th->syn) {
         reason = SKB_DROP_REASON_TCP_FLAGS;
         goto discard;
     }
 
     /*
      *  Standard slow path.
      */
 
     if (!tcp_validate_incoming(sk, skb, th, 1))
         return;
 
 step5:
     reason = tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT);
     if ((int)reason < 0) {
         reason = -reason;
         goto discard;
     }
     tcp_rcv_rtt_measure_ts(sk, skb);
 
     /* Process urgent data. */
     tcp_urg(sk, skb, th);
 
     /* step 7: process the segment text */
     tcp_data_queue(sk, skb);
 
     tcp_data_snd_check(sk);
     tcp_ack_snd_check(sk);
     return;
 
 csum_error:
     reason = SKB_DROP_REASON_TCP_CSUM;
     trace_tcp_bad_csum(skb);
     TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
     TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
 
 discard:
     tcp_drop_reason(sk, skb, reason);
 }
 EXPORT_SYMBOL(tcp_rcv_established);
 
 void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
 
     tcp_mtup_init(sk);
     icsk->icsk_af_ops->rebuild_header(sk);
     tcp_init_metrics(sk);
 
     /* Initialize the congestion window to start the transfer.
      * Cut cwnd down to 1 per RFC5681 if SYN or SYN-ACK has been
      * retransmitted. In light of RFC6298 more aggressive 1sec
      * initRTO, we only reset cwnd when more than 1 SYN/SYN-ACK
      * retransmission has occurred.
      */
     if (tp->total_retrans > 1 && tp->undo_marker)
         tcp_snd_cwnd_set(tp, 1);
     else
         tcp_snd_cwnd_set(tp, tcp_init_cwnd(tp, __sk_dst_get(sk)));
     tp->snd_cwnd_stamp = tcp_jiffies32;
 
     bpf_skops_established(sk, bpf_op, skb);
     /* Initialize congestion control unless BPF initialized it already: */
     if (!icsk->icsk_ca_initialized)
         tcp_init_congestion_control(sk);
     tcp_init_buffer_space(sk);
 }
 
 void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_connection_sock *icsk = inet_csk(sk);
 
     tcp_set_state(sk, TCP_ESTABLISHED);
     icsk->icsk_ack.lrcvtime = tcp_jiffies32;
 
     if (skb) {
         icsk->icsk_af_ops->sk_rx_dst_set(sk, skb);
         security_inet_conn_established(sk, skb);
         sk_mark_napi_id(sk, skb);
     }
 
     tcp_init_transfer(sk, BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB, skb);
 
     /* Prevent spurious tcp_cwnd_restart() on first data
      * packet.
      */
     tp->lsndtime = tcp_jiffies32;
 
     if (sock_flag(sk, SOCK_KEEPOPEN))
         inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));
 
     if (!tp->rx_opt.snd_wscale)
         __tcp_fast_path_on(tp, tp->snd_wnd);
     else
         tp->pred_flags = 0;
 }
 
 static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                     struct tcp_fastopen_cookie *cookie)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *data = tp->syn_data ? tcp_rtx_queue_head(sk) : NULL;
     u16 mss = tp->rx_opt.mss_clamp, try_exp = 0;
     bool syn_drop = false;
 
     if (mss == tp->rx_opt.user_mss) {
         struct tcp_options_received opt;
 
         /* Get original SYNACK MSS value if user MSS sets mss_clamp */
         tcp_clear_options(&opt);
         opt.user_mss = opt.mss_clamp = 0;
         tcp_parse_options(sock_net(sk), synack, &opt, 0, NULL);
         mss = opt.mss_clamp;
     }
 
     if (!tp->syn_fastopen) {
         /* Ignore an unsolicited cookie */
         cookie->len = -1;
     } else if (tp->total_retrans) {
         /* SYN timed out and the SYN-ACK neither has a cookie nor
          * acknowledges data. Presumably the remote received only
          * the retransmitted (regular) SYNs: either the original
          * SYN-data or the corresponding SYN-ACK was dropped.
          */
         syn_drop = (cookie->len < 0 && data);
     } else if (cookie->len < 0 && !tp->syn_data) {
         /* We requested a cookie but didn't get it. If we did not use
          * the (old) exp opt format then try so next time (try_exp=1).
          * Otherwise we go back to use the RFC7413 opt (try_exp=2).
          */
         try_exp = tp->syn_fastopen_exp ? 2 : 1;
     }
 
     tcp_fastopen_cache_set(sk, mss, cookie, syn_drop, try_exp);
 
     if (data) { /* Retransmit unacked data in SYN */
         if (tp->total_retrans)
             tp->fastopen_client_fail = TFO_SYN_RETRANSMITTED;
         else
             tp->fastopen_client_fail = TFO_DATA_NOT_ACKED;
         skb_rbtree_walk_from(data)
              tcp_mark_skb_lost(sk, data);
         tcp_xmit_retransmit_queue(sk);
         NET_INC_STATS(sock_net(sk),
                 LINUX_MIB_TCPFASTOPENACTIVEFAIL);
         return true;
     }
     tp->syn_data_acked = tp->syn_data;
     if (tp->syn_data_acked) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
         /* SYN-data is counted as two separate packets in tcp_ack() */
         if (tp->delivered > 1)
             --tp->delivered;
     }
 
     tcp_fastopen_add_skb(sk, synack);
 
     return false;
 }
 
 static void smc_check_reset_syn(struct tcp_sock *tp)
 {
 #if IS_ENABLED(CONFIG_SMC)
     if (static_branch_unlikely(&tcp_have_smc)) {
         if (tp->syn_smc && !tp->rx_opt.smc_ok)
             tp->syn_smc = 0;
     }
 #endif
 }
 
 static void tcp_try_undo_spurious_syn(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 syn_stamp;
 
     /* undo_marker is set when SYN or SYNACK times out. The timeout is
      * spurious if the ACK's timestamp option echo value matches the
      * original SYN timestamp.
      */
     syn_stamp = tp->retrans_stamp;
     if (tp->undo_marker && syn_stamp && tp->rx_opt.saw_tstamp &&
         syn_stamp == tp->rx_opt.rcv_tsecr)
         tp->undo_marker = 0;
 }
 
 static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                      const struct tcphdr *th)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct tcp_fastopen_cookie foc = { .len = -1 };
     int saved_clamp = tp->rx_opt.mss_clamp;
     bool fastopen_fail;
     SKB_DR(reason);
 
     tcp_parse_options(sock_net(sk), skb, &tp->rx_opt, 0, &foc);
     if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
         tp->rx_opt.rcv_tsecr -= tp->tsoffset;
 
     if (th->ack) {
         /* rfc793:
          * "If the state is SYN-SENT then
          *    first check the ACK bit
          *      If the ACK bit is set
          *    If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
          *        a reset (unless the RST bit is set, if so drop
          *        the segment and return)"
          */
         if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
             after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
             /* Previous FIN/ACK or RST/ACK might be ignored. */
             if (icsk->icsk_retransmits == 0)
                 inet_csk_reset_xmit_timer(sk,
                         ICSK_TIME_RETRANS,
                         TCP_TIMEOUT_MIN, TCP_RTO_MAX);
             goto reset_and_undo;
         }
 
         if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
             !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
                  tcp_time_stamp(tp))) {
             NET_INC_STATS(sock_net(sk),
                     LINUX_MIB_PAWSACTIVEREJECTED);
             goto reset_and_undo;
         }
 
         /* Now ACK is acceptable.
          *
          * "If the RST bit is set
          *    If the ACK was acceptable then signal the user "error:
          *    connection reset", drop the segment, enter CLOSED state,
          *    delete TCB, and return."
          */
 
         if (th->rst) {
             tcp_reset(sk, skb);
 consume:
             __kfree_skb(skb);
             return 0;
         }
 
         /* rfc793:
          *   "fifth, if neither of the SYN or RST bits is set then
          *    drop the segment and return."
          *
          *    See note below!
          *                                        --ANK(990513)
          */
         if (!th->syn) {
             SKB_DR_SET(reason, TCP_FLAGS);
             goto discard_and_undo;
         }
         /* rfc793:
          *   "If the SYN bit is on ...
          *    are acceptable then ...
          *    (our SYN has been ACKed), change the connection
          *    state to ESTABLISHED..."
          */
 
         tcp_ecn_rcv_synack(tp, th);
 
         tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
         tcp_try_undo_spurious_syn(sk);
         tcp_ack(sk, skb, FLAG_SLOWPATH);
 
         /* Ok.. it's good. Set up sequence numbers and
          * move to established.
          */
         WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1);
         tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
 
         /* RFC1323: The window in SYN & SYN/ACK segments is
          * never scaled.
          */
         tp->snd_wnd = ntohs(th->window);
 
         if (!tp->rx_opt.wscale_ok) {
             tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
             tp->window_clamp = min(tp->window_clamp, 65535U);
         }
 
         if (tp->rx_opt.saw_tstamp) {
             tp->rx_opt.tstamp_ok       = 1;
             tp->tcp_header_len =
                 sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
             tp->advmss      -= TCPOLEN_TSTAMP_ALIGNED;
             tcp_store_ts_recent(tp);
         } else {
             tp->tcp_header_len = sizeof(struct tcphdr);
         }
 
         tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
         tcp_initialize_rcv_mss(sk);
 
         /* Remember, tcp_poll() does not lock socket!
          * Change state from SYN-SENT only after copied_seq
          * is initialized. */
         WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
 
         smc_check_reset_syn(tp);
 
         smp_mb();
 
         tcp_finish_connect(sk, skb);
 
         fastopen_fail = (tp->syn_fastopen || tp->syn_data) &&
                 tcp_rcv_fastopen_synack(sk, skb, &foc);
 
         if (!sock_flag(sk, SOCK_DEAD)) {
             sk->sk_state_change(sk);
             sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
         }
         if (fastopen_fail)
             return -1;
         if (sk->sk_write_pending ||
             icsk->icsk_accept_queue.rskq_defer_accept ||
             inet_csk_in_pingpong_mode(sk)) {
             /* Save one ACK. Data will be ready after
              * several ticks, if write_pending is set.
              *
              * It may be deleted, but with this feature tcpdumps
              * look so _wonderfully_ clever, that I was not able
              * to stand against the temptation 8)     --ANK
              */
             inet_csk_schedule_ack(sk);
             tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
             inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                           TCP_DELACK_MAX, TCP_RTO_MAX);
             goto consume;
         }
         tcp_send_ack(sk);
         return -1;
     }
 
     /* No ACK in the segment */
 
     if (th->rst) {
         /* rfc793:
          * "If the RST bit is set
          *
          *      Otherwise (no ACK) drop the segment and return."
          */
         SKB_DR_SET(reason, TCP_RESET);
         goto discard_and_undo;
     }
 
     /* PAWS check. */
     if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
         tcp_paws_reject(&tp->rx_opt, 0)) {
         SKB_DR_SET(reason, TCP_RFC7323_PAWS);
         goto discard_and_undo;
     }
     if (th->syn) {
         /* We see SYN without ACK. It is attempt of
          * simultaneous connect with crossed SYNs.
          * Particularly, it can be connect to self.
          */
         tcp_set_state(sk, TCP_SYN_RECV);
 
         if (tp->rx_opt.saw_tstamp) {
             tp->rx_opt.tstamp_ok = 1;
             tcp_store_ts_recent(tp);
             tp->tcp_header_len =
                 sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
         } else {
             tp->tcp_header_len = sizeof(struct tcphdr);
         }
 
         WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1);
         WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
         tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
 
         /* RFC1323: The window in SYN & SYN/ACK segments is
          * never scaled.
          */
         tp->snd_wnd    = ntohs(th->window);
         tp->snd_wl1    = TCP_SKB_CB(skb)->seq;
         tp->max_window = tp->snd_wnd;
 
         tcp_ecn_rcv_syn(tp, th);
 
         tcp_mtup_init(sk);
         tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
         tcp_initialize_rcv_mss(sk);
 
         tcp_send_synack(sk);
 #if 0
         /* Note, we could accept data and URG from this segment.
          * There are no obstacles to make this (except that we must
          * either change tcp_recvmsg() to prevent it from returning data
          * before 3WHS completes per RFC793, or employ TCP Fast Open).
          *
          * However, if we ignore data in ACKless segments sometimes,
          * we have no reasons to accept it sometimes.
          * Also, seems the code doing it in step6 of tcp_rcv_state_process
          * is not flawless. So, discard packet for sanity.
          * Uncomment this return to process the data.
          */
         return -1;
 #else
         goto consume;
 #endif
     }
     /* "fifth, if neither of the SYN or RST bits is set then
      * drop the segment and return."
      */
 
 discard_and_undo:
     tcp_clear_options(&tp->rx_opt);
     tp->rx_opt.mss_clamp = saved_clamp;
     tcp_drop_reason(sk, skb, reason);
     return 0;
 
 reset_and_undo:
     tcp_clear_options(&tp->rx_opt);
     tp->rx_opt.mss_clamp = saved_clamp;
     return 1;
 }
 
 static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
 {
     struct request_sock *req;
 
     /* If we are still handling the SYNACK RTO, see if timestamp ECR allows
      * undo. If peer SACKs triggered fast recovery, we can't undo here.
      */
     if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
         tcp_try_undo_loss(sk, false);
 
     /* Reset rtx states to prevent spurious retransmits_timed_out() */
     tcp_sk(sk)->retrans_stamp = 0;
     inet_csk(sk)->icsk_retransmits = 0;
 
     /* Once we leave TCP_SYN_RECV or TCP_FIN_WAIT_1,
      * we no longer need req so release it.
      */
     req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk,
                     lockdep_sock_is_held(sk));
     reqsk_fastopen_remove(sk, req, false);
 
     /* Re-arm the timer because data may have been sent out.
      * This is similar to the regular data transmission case
      * when new data has just been ack'ed.
      *
      * (TFO) - we could try to be more aggressive and
      * retransmitting any data sooner based on when they
      * are sent out.
      */
     tcp_rearm_rto(sk);
 }
 
 /*
  *  This function implements the receiving procedure of RFC 793 for
  *  all states except ESTABLISHED and TIME_WAIT.
  *  It's called from both tcp_v4_rcv and tcp_v6_rcv and should be
  *  address independent.
  */
 
 int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_connection_sock *icsk = inet_csk(sk);
     const struct tcphdr *th = tcp_hdr(skb);
     struct request_sock *req;
     int queued = 0;
     bool acceptable;
     SKB_DR(reason);
 
     switch (sk->sk_state) {
     case TCP_CLOSE:
         SKB_DR_SET(reason, TCP_CLOSE);
         goto discard;
 
     case TCP_LISTEN:
         if (th->ack)
             return 1;
 
         if (th->rst) {
             SKB_DR_SET(reason, TCP_RESET);
             goto discard;
         }
         if (th->syn) {
             if (th->fin) {
                 SKB_DR_SET(reason, TCP_FLAGS);
                 goto discard;
             }
             /* It is possible that we process SYN packets from backlog,
              * so we need to make sure to disable BH and RCU right there.
              */
             rcu_read_lock();
             local_bh_disable();
             acceptable = icsk->icsk_af_ops->conn_request(sk, skb) >= 0;
             local_bh_enable();
             rcu_read_unlock();
 
             if (!acceptable)
                 return 1;
             consume_skb(skb);
             return 0;
         }
         SKB_DR_SET(reason, TCP_FLAGS);
         goto discard;
 
     case TCP_SYN_SENT:
         tp->rx_opt.saw_tstamp = 0;
         tcp_mstamp_refresh(tp);
         queued = tcp_rcv_synsent_state_process(sk, skb, th);
         if (queued >= 0)
             return queued;
 
         /* Do step6 onward by hand. */
         tcp_urg(sk, skb, th);
         __kfree_skb(skb);
         tcp_data_snd_check(sk);
         return 0;
     }
 
     tcp_mstamp_refresh(tp);
     tp->rx_opt.saw_tstamp = 0;
     req = rcu_dereference_protected(tp->fastopen_rsk,
                     lockdep_sock_is_held(sk));
     if (req) {
         bool req_stolen;
 
         WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
             sk->sk_state != TCP_FIN_WAIT1);
 
         if (!tcp_check_req(sk, skb, req, true, &req_stolen)) {
             SKB_DR_SET(reason, TCP_FASTOPEN);
             goto discard;
         }
     }
 
     if (!th->ack && !th->rst && !th->syn) {
         SKB_DR_SET(reason, TCP_FLAGS);
         goto discard;
     }
     if (!tcp_validate_incoming(sk, skb, th, 0))
         return 0;
 
     /* step 5: check the ACK field */
     acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
                       FLAG_UPDATE_TS_RECENT |
                       FLAG_NO_CHALLENGE_ACK) > 0;
 
     if (!acceptable) {
         if (sk->sk_state == TCP_SYN_RECV)
             return 1;   /* send one RST */
         tcp_send_challenge_ack(sk);
         SKB_DR_SET(reason, TCP_OLD_ACK);
         goto discard;
     }
     switch (sk->sk_state) {
     case TCP_SYN_RECV:
         tp->delivered++; /* SYN-ACK delivery isn't tracked in tcp_ack */
         if (!tp->srtt_us)
             tcp_synack_rtt_meas(sk, req);
 
         if (req) {
             tcp_rcv_synrecv_state_fastopen(sk);
         } else {
             tcp_try_undo_spurious_syn(sk);
             tp->retrans_stamp = 0;
             tcp_init_transfer(sk, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB,
                       skb);
             WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
         }
         smp_mb();
         tcp_set_state(sk, TCP_ESTABLISHED);
         sk->sk_state_change(sk);
 
         /* Note, that this wakeup is only for marginal crossed SYN case.
          * Passively open sockets are not waked up, because
          * sk->sk_sleep == NULL and sk->sk_socket == NULL.
          */
         if (sk->sk_socket)
             sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
 
         tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
         tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
         tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
 
         if (tp->rx_opt.tstamp_ok)
             tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
 
         if (!inet_csk(sk)->icsk_ca_ops->cong_control)
             tcp_update_pacing_rate(sk);
 
         /* Prevent spurious tcp_cwnd_restart() on first data packet */
         tp->lsndtime = tcp_jiffies32;
 
         tcp_initialize_rcv_mss(sk);
         tcp_fast_path_on(tp);
         break;
 
     case TCP_FIN_WAIT1: {
         int tmo;
 
         if (req)
             tcp_rcv_synrecv_state_fastopen(sk);
 
         if (tp->snd_una != tp->write_seq)
             break;
 
         tcp_set_state(sk, TCP_FIN_WAIT2);
         sk->sk_shutdown |= SEND_SHUTDOWN;
 
         sk_dst_confirm(sk);
 
         if (!sock_flag(sk, SOCK_DEAD)) {
             /* Wake up lingering close() */
             sk->sk_state_change(sk);
             break;
         }
 
         if (tp->linger2 < 0) {
             tcp_done(sk);
             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
             return 1;
         }
         if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
             after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
             /* Receive out of order FIN after close() */
             if (tp->syn_fastopen && th->fin)
                 tcp_fastopen_active_disable(sk);
             tcp_done(sk);
             NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
             return 1;
         }
 
         tmo = tcp_fin_time(sk);
         if (tmo > TCP_TIMEWAIT_LEN) {
             inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
         } else if (th->fin || sock_owned_by_user(sk)) {
             /* Bad case. We could lose such FIN otherwise.
              * It is not a big problem, but it looks confusing
              * and not so rare event. We still can lose it now,
              * if it spins in bh_lock_sock(), but it is really
              * marginal case.
              */
             inet_csk_reset_keepalive_timer(sk, tmo);
         } else {
             tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
             goto consume;
         }
         break;
     }
 
     case TCP_CLOSING:
         if (tp->snd_una == tp->write_seq) {
             tcp_time_wait(sk, TCP_TIME_WAIT, 0);
             goto consume;
         }
         break;
 
     case TCP_LAST_ACK:
         if (tp->snd_una == tp->write_seq) {
             tcp_update_metrics(sk);
             tcp_done(sk);
             goto consume;
         }
         break;
     }
 
     /* step 6: check the URG bit */
     tcp_urg(sk, skb, th);
 
     /* step 7: process the segment text */
     switch (sk->sk_state) {
     case TCP_CLOSE_WAIT:
     case TCP_CLOSING:
     case TCP_LAST_ACK:
         if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
             /* If a subflow has been reset, the packet should not
              * continue to be processed, drop the packet.
              */
             if (sk_is_mptcp(sk) && !mptcp_incoming_options(sk, skb))
                 goto discard;
             break;
         }
         fallthrough;
     case TCP_FIN_WAIT1:
     case TCP_FIN_WAIT2:
         /* RFC 793 says to queue data in these states,
          * RFC 1122 says we MUST send a reset.
          * BSD 4.4 also does reset.
          */
         if (sk->sk_shutdown & RCV_SHUTDOWN) {
             if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
                 after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                 tcp_reset(sk, skb);
                 return 1;
             }
         }
         fallthrough;
     case TCP_ESTABLISHED:
         tcp_data_queue(sk, skb);
         queued = 1;
         break;
     }
 
     /* tcp_data could move socket to TIME-WAIT */
     if (sk->sk_state != TCP_CLOSE) {
         tcp_data_snd_check(sk);
         tcp_ack_snd_check(sk);
     }
 
     if (!queued) {
 discard:
         tcp_drop_reason(sk, skb, reason);
     }
     return 0;
 
 consume:
     __kfree_skb(skb);
     return 0;
 }
 EXPORT_SYMBOL(tcp_rcv_state_process);
 
 static inline void pr_drop_req(struct request_sock *req, __u16 port, int family)
 {
     struct inet_request_sock *ireq = inet_rsk(req);
 
     if (family == AF_INET)
         net_dbg_ratelimited("drop open request from %pI4/%u\n",
                     &ireq->ir_rmt_addr, port);
 #if IS_ENABLED(CONFIG_IPV6)
     else if (family == AF_INET6)
         net_dbg_ratelimited("drop open request from %pI6/%u\n",
                     &ireq->ir_v6_rmt_addr, port);
 #endif
 }
 
 /* RFC3168 : 6.1.1 SYN packets must not have ECT/ECN bits set
  *
  * If we receive a SYN packet with these bits set, it means a
  * network is playing bad games with TOS bits. In order to
  * avoid possible false congestion notifications, we disable
  * TCP ECN negotiation.
  *
  * Exception: tcp_ca wants ECN. This is required for DCTCP
  * congestion control: Linux DCTCP asserts ECT on all packets,
  * including SYN, which is most optimal solution; however,
  * others, such as FreeBSD do not.
  *
  * Exception: At least one of the reserved bits of the TCP header (th->res1) is
  * set, indicating the use of a future TCP extension (such as AccECN). See
  * RFC8311 §4.3 which updates RFC3168 to allow the development of such
  * extensions.
  */
 static void tcp_ecn_create_request(struct request_sock *req,
                    const struct sk_buff *skb,
                    const struct sock *listen_sk,
                    const struct dst_entry *dst)
 {
     const struct tcphdr *th = tcp_hdr(skb);
     const struct net *net = sock_net(listen_sk);
     bool th_ecn = th->ece && th->cwr;
     bool ect, ecn_ok;
     u32 ecn_ok_dst;
 
     if (!th_ecn)
         return;
 
     ect = !INET_ECN_is_not_ect(TCP_SKB_CB(skb)->ip_dsfield);
     ecn_ok_dst = dst_feature(dst, DST_FEATURE_ECN_MASK);
     ecn_ok = READ_ONCE(net->ipv4.sysctl_tcp_ecn) || ecn_ok_dst;
 
     if (((!ect || th->res1) && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
         (ecn_ok_dst & DST_FEATURE_ECN_CA) ||
         tcp_bpf_ca_needs_ecn((struct sock *)req))
         inet_rsk(req)->ecn_ok = 1;
 }
 
 static void tcp_openreq_init(struct request_sock *req,
                  const struct tcp_options_received *rx_opt,
                  struct sk_buff *skb, const struct sock *sk)
 {
     struct inet_request_sock *ireq = inet_rsk(req);
 
     req->rsk_rcv_wnd = 0;       /* So that tcp_send_synack() knows! */
     tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
     tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
     tcp_rsk(req)->snt_synack = 0;
     tcp_rsk(req)->last_oow_ack_time = 0;
     req->mss = rx_opt->mss_clamp;
     req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
     ireq->tstamp_ok = rx_opt->tstamp_ok;
     ireq->sack_ok = rx_opt->sack_ok;
     ireq->snd_wscale = rx_opt->snd_wscale;
     ireq->wscale_ok = rx_opt->wscale_ok;
     ireq->acked = 0;
     ireq->ecn_ok = 0;
     ireq->ir_rmt_port = tcp_hdr(skb)->source;
     ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
     ireq->ir_mark = inet_request_mark(sk, skb);
 #if IS_ENABLED(CONFIG_SMC)
     ireq->smc_ok = rx_opt->smc_ok && !(tcp_sk(sk)->smc_hs_congested &&
             tcp_sk(sk)->smc_hs_congested(sk));
 #endif
 }
 
 struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
                       struct sock *sk_listener,
                       bool attach_listener)
 {
     struct request_sock *req = reqsk_alloc(ops, sk_listener,
                            attach_listener);
 
     if (req) {
         struct inet_request_sock *ireq = inet_rsk(req);
 
         ireq->ireq_opt = NULL;
 #if IS_ENABLED(CONFIG_IPV6)
         ireq->pktopts = NULL;
 #endif
         atomic64_set(&ireq->ir_cookie, 0);
         ireq->ireq_state = TCP_NEW_SYN_RECV;
         write_pnet(&ireq->ireq_net, sock_net(sk_listener));
         ireq->ireq_family = sk_listener->sk_family;
         req->timeout = TCP_TIMEOUT_INIT;
     }
 
     return req;
 }
 EXPORT_SYMBOL(inet_reqsk_alloc);
 
 /*
  * Return true if a syncookie should be sent
  */
 static bool tcp_syn_flood_action(const struct sock *sk, const char *proto)
 {
     struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
     const char *msg = "Dropping request";
     struct net *net = sock_net(sk);
     bool want_cookie = false;
     u8 syncookies;
 
     syncookies = READ_ONCE(net->ipv4.sysctl_tcp_syncookies);
 
 #ifdef CONFIG_SYN_COOKIES
     if (syncookies) {
         msg = "Sending cookies";
         want_cookie = true;
         __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDOCOOKIES);
     } else
 #endif
         __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
 
     if (!queue->synflood_warned && syncookies != 2 &&
         xchg(&queue->synflood_warned, 1) == 0)
         net_info_ratelimited("%s: Possible SYN flooding on port %d. %s.  Check SNMP counters.\n",
                      proto, sk->sk_num, msg);
 
     return want_cookie;
 }
 
 static void tcp_reqsk_record_syn(const struct sock *sk,
                  struct request_sock *req,
                  const struct sk_buff *skb)
 {
     if (tcp_sk(sk)->save_syn) {
         u32 len = skb_network_header_len(skb) + tcp_hdrlen(skb);
         struct saved_syn *saved_syn;
         u32 mac_hdrlen;
         void *base;
 
         if (tcp_sk(sk)->save_syn == 2) {  /* Save full header. */
             base = skb_mac_header(skb);
             mac_hdrlen = skb_mac_header_len(skb);
             len += mac_hdrlen;
         } else {
             base = skb_network_header(skb);
             mac_hdrlen = 0;
         }
 
         saved_syn = kmalloc(struct_size(saved_syn, data, len),
                     GFP_ATOMIC);
         if (saved_syn) {
             saved_syn->mac_hdrlen = mac_hdrlen;
             saved_syn->network_hdrlen = skb_network_header_len(skb);
             saved_syn->tcp_hdrlen = tcp_hdrlen(skb);
             memcpy(saved_syn->data, base, len);
             req->saved_syn = saved_syn;
         }
     }
 }
 
 /* If a SYN cookie is required and supported, returns a clamped MSS value to be
  * used for SYN cookie generation.
  */
 u16 tcp_get_syncookie_mss(struct request_sock_ops *rsk_ops,
               const struct tcp_request_sock_ops *af_ops,
               struct sock *sk, struct tcphdr *th)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u16 mss;
 
     if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_syncookies) != 2 &&
         !inet_csk_reqsk_queue_is_full(sk))
         return 0;
 
     if (!tcp_syn_flood_action(sk, rsk_ops->slab_name))
         return 0;
 
     if (sk_acceptq_is_full(sk)) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
         return 0;
     }
 
     mss = tcp_parse_mss_option(th, tp->rx_opt.user_mss);
     if (!mss)
         mss = af_ops->mss_clamp;
 
     return mss;
 }
 EXPORT_SYMBOL_GPL(tcp_get_syncookie_mss);
 
 int tcp_conn_request(struct request_sock_ops *rsk_ops,
              const struct tcp_request_sock_ops *af_ops,
              struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_fastopen_cookie foc = { .len = -1 };
     __u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
     struct tcp_options_received tmp_opt;
     struct tcp_sock *tp = tcp_sk(sk);
     struct net *net = sock_net(sk);
     struct sock *fastopen_sk = NULL;
     struct request_sock *req;
     bool want_cookie = false;
     struct dst_entry *dst;
     struct flowi fl;
     u8 syncookies;
 
     syncookies = READ_ONCE(net->ipv4.sysctl_tcp_syncookies);
 
     /* TW buckets are converted to open requests without
      * limitations, they conserve resources and peer is
      * evidently real one.
      */
     if ((syncookies == 2 || inet_csk_reqsk_queue_is_full(sk)) && !isn) {
         want_cookie = tcp_syn_flood_action(sk, rsk_ops->slab_name);
         if (!want_cookie)
             goto drop;
     }
 
     if (sk_acceptq_is_full(sk)) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
         goto drop;
     }
 
     req = inet_reqsk_alloc(rsk_ops, sk, !want_cookie);
     if (!req)
         goto drop;
 
     req->syncookie = want_cookie;
     tcp_rsk(req)->af_specific = af_ops;
     tcp_rsk(req)->ts_off = 0;
 #if IS_ENABLED(CONFIG_MPTCP)
     tcp_rsk(req)->is_mptcp = 0;
 #endif
 
     tcp_clear_options(&tmp_opt);
     tmp_opt.mss_clamp = af_ops->mss_clamp;
     tmp_opt.user_mss  = tp->rx_opt.user_mss;
     tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0,
               want_cookie ? NULL : &foc);
 
     if (want_cookie && !tmp_opt.saw_tstamp)
         tcp_clear_options(&tmp_opt);
 
     if (IS_ENABLED(CONFIG_SMC) && want_cookie)
         tmp_opt.smc_ok = 0;
 
     tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
     tcp_openreq_init(req, &tmp_opt, skb, sk);
     inet_rsk(req)->no_srccheck = inet_sk(sk)->transparent;
 
     /* Note: tcp_v6_init_req() might override ir_iif for link locals */
     inet_rsk(req)->ir_iif = inet_request_bound_dev_if(sk, skb);
 
     dst = af_ops->route_req(sk, skb, &fl, req);
     if (!dst)
         goto drop_and_free;
 
     if (tmp_opt.tstamp_ok)
         tcp_rsk(req)->ts_off = af_ops->init_ts_off(net, skb);
 
     if (!want_cookie && !isn) {
         int max_syn_backlog = READ_ONCE(net->ipv4.sysctl_max_syn_backlog);
 
         /* Kill the following clause, if you dislike this way. */
         if (!syncookies &&
             (max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
              (max_syn_backlog >> 2)) &&
             !tcp_peer_is_proven(req, dst)) {
             /* Without syncookies last quarter of
              * backlog is filled with destinations,
              * proven to be alive.
              * It means that we continue to communicate
              * to destinations, already remembered
              * to the moment of synflood.
              */
             pr_drop_req(req, ntohs(tcp_hdr(skb)->source),
                     rsk_ops->family);
             goto drop_and_release;
         }
 
         isn = af_ops->init_seq(skb);
     }
 
     tcp_ecn_create_request(req, skb, sk, dst);
 
     if (want_cookie) {
         isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
         if (!tmp_opt.tstamp_ok)
             inet_rsk(req)->ecn_ok = 0;
     }
 
     tcp_rsk(req)->snt_isn = isn;
     tcp_rsk(req)->txhash = net_tx_rndhash();
     tcp_rsk(req)->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
     tcp_openreq_init_rwin(req, sk, dst);
     sk_rx_queue_set(req_to_sk(req), skb);
     if (!want_cookie) {
         tcp_reqsk_record_syn(sk, req, skb);
         fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
     }
     if (fastopen_sk) {
         af_ops->send_synack(fastopen_sk, dst, &fl, req,
                     &foc, TCP_SYNACK_FASTOPEN, skb);
         /* Add the child socket directly into the accept queue */
         if (!inet_csk_reqsk_queue_add(sk, req, fastopen_sk)) {
             reqsk_fastopen_remove(fastopen_sk, req, false);
             bh_unlock_sock(fastopen_sk);
             sock_put(fastopen_sk);
             goto drop_and_free;
         }
         sk->sk_data_ready(sk);
         bh_unlock_sock(fastopen_sk);
         sock_put(fastopen_sk);
     } else {
         tcp_rsk(req)->tfo_listener = false;
         if (!want_cookie) {
             req->timeout = tcp_timeout_init((struct sock *)req);
             inet_csk_reqsk_queue_hash_add(sk, req, req->timeout);
         }
         af_ops->send_synack(sk, dst, &fl, req, &foc,
                     !want_cookie ? TCP_SYNACK_NORMAL :
                            TCP_SYNACK_COOKIE,
                     skb);
         if (want_cookie) {
             reqsk_free(req);
             return 0;
         }
     }
     reqsk_put(req);
     return 0;
 
 drop_and_release:
     dst_release(dst);
 drop_and_free:
     __reqsk_free(req);
 drop:
     tcp_listendrop(sk);
     return 0;
 }
 EXPORT_SYMBOL(tcp_conn_request);