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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-only
 /*
  * 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 :   Retransmit queue handled by TCP.
  *              :   Fragmentation on mtu decrease
  *              :   Segment collapse on retransmit
  *              :   AF independence
  *
  *      Linus Torvalds  :   send_delayed_ack
  *      David S. Miller :   Charge memory using the right skb
  *                  during syn/ack processing.
  *      David S. Miller :   Output engine completely rewritten.
  *      Andrea Arcangeli:   SYNACK carry ts_recent in tsecr.
  *      Cacophonix Gaul :   draft-minshall-nagle-01
  *      J Hadi Salim    :   ECN support
  *
  */
 
 #define pr_fmt(fmt) "TCP: " fmt
 
 #include <net/tcp.h>
 #include <net/mptcp.h>
 
 #include <linux/compiler.h>
 #include <linux/gfp.h>
 #include <linux/module.h>
 #include <linux/static_key.h>
 
 #include <trace/events/tcp.h>
 
 /* Refresh clocks of a TCP socket,
  * ensuring monotically increasing values.
  */
 void tcp_mstamp_refresh(struct tcp_sock *tp)
 {
     u64 val = tcp_clock_ns();
 
     tp->tcp_clock_cache = val;
     tp->tcp_mstamp = div_u64(val, NSEC_PER_USEC);
 }
 
 static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                int push_one, gfp_t gfp);
 
 /* Account for new data that has been sent to the network. */
 static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     unsigned int prior_packets = tp->packets_out;
 
     WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(skb)->end_seq);
 
     __skb_unlink(skb, &sk->sk_write_queue);
     tcp_rbtree_insert(&sk->tcp_rtx_queue, skb);
 
     if (tp->highest_sack == NULL)
         tp->highest_sack = skb;
 
     tp->packets_out += tcp_skb_pcount(skb);
     if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
         tcp_rearm_rto(sk);
 
     NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
               tcp_skb_pcount(skb));
     tcp_check_space(sk);
 }
 
 /* SND.NXT, if window was not shrunk or the amount of shrunk was less than one
  * window scaling factor due to loss of precision.
  * If window has been shrunk, what should we make? It is not clear at all.
  * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
  * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
  * invalid. OK, let's make this for now:
  */
 static inline __u32 tcp_acceptable_seq(const struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
 
     if (!before(tcp_wnd_end(tp), tp->snd_nxt) ||
         (tp->rx_opt.wscale_ok &&
          ((tp->snd_nxt - tcp_wnd_end(tp)) < (1 << tp->rx_opt.rcv_wscale))))
         return tp->snd_nxt;
     else
         return tcp_wnd_end(tp);
 }
 
 /* Calculate mss to advertise in SYN segment.
  * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
  *
  * 1. It is independent of path mtu.
  * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
  * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
  *    attached devices, because some buggy hosts are confused by
  *    large MSS.
  * 4. We do not make 3, we advertise MSS, calculated from first
  *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
  *    This may be overridden via information stored in routing table.
  * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
  *    probably even Jumbo".
  */
 static __u16 tcp_advertise_mss(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     const struct dst_entry *dst = __sk_dst_get(sk);
     int mss = tp->advmss;
 
     if (dst) {
         unsigned int metric = dst_metric_advmss(dst);
 
         if (metric < mss) {
             mss = metric;
             tp->advmss = mss;
         }
     }
 
     return (__u16)mss;
 }
 
 /* RFC2861. Reset CWND after idle period longer RTO to "restart window".
  * This is the first part of cwnd validation mechanism.
  */
 void tcp_cwnd_restart(struct sock *sk, s32 delta)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
     u32 cwnd = tcp_snd_cwnd(tp);
 
     tcp_ca_event(sk, CA_EVENT_CWND_RESTART);
 
     tp->snd_ssthresh = tcp_current_ssthresh(sk);
     restart_cwnd = min(restart_cwnd, cwnd);
 
     while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
         cwnd >>= 1;
     tcp_snd_cwnd_set(tp, max(cwnd, restart_cwnd));
     tp->snd_cwnd_stamp = tcp_jiffies32;
     tp->snd_cwnd_used = 0;
 }
 
 /* Congestion state accounting after a packet has been sent. */
 static void tcp_event_data_sent(struct tcp_sock *tp,
                 struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     const u32 now = tcp_jiffies32;
 
     if (tcp_packets_in_flight(tp) == 0)
         tcp_ca_event(sk, CA_EVENT_TX_START);
 
     tp->lsndtime = now;
 
     /* If it is a reply for ato after last received
      * packet, enter pingpong mode.
      */
     if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
         inet_csk_enter_pingpong_mode(sk);
 }
 
 /* Account for an ACK we sent. */
 static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts,
                       u32 rcv_nxt)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (unlikely(tp->compressed_ack)) {
         NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
                   tp->compressed_ack);
         tp->compressed_ack = 0;
         if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
             __sock_put(sk);
     }
 
     if (unlikely(rcv_nxt != tp->rcv_nxt))
         return;  /* Special ACK sent by DCTCP to reflect ECN */
     tcp_dec_quickack_mode(sk, pkts);
     inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
 }
 
 /* Determine a window scaling and initial window to offer.
  * Based on the assumption that the given amount of space
  * will be offered. Store the results in the tp structure.
  * NOTE: for smooth operation initial space offering should
  * be a multiple of mss if possible. We assume here that mss >= 1.
  * This MUST be enforced by all callers.
  */
 void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
                    __u32 *rcv_wnd, __u32 *window_clamp,
                    int wscale_ok, __u8 *rcv_wscale,
                    __u32 init_rcv_wnd)
 {
     unsigned int space = (__space < 0 ? 0 : __space);
 
     /* If no clamp set the clamp to the max possible scaled window */
     if (*window_clamp == 0)
         (*window_clamp) = (U16_MAX << TCP_MAX_WSCALE);
     space = min(*window_clamp, space);
 
     /* Quantize space offering to a multiple of mss if possible. */
     if (space > mss)
         space = rounddown(space, mss);
 
     /* NOTE: offering an initial window larger than 32767
      * will break some buggy TCP stacks. If the admin tells us
      * it is likely we could be speaking with such a buggy stack
      * we will truncate our initial window offering to 32K-1
      * unless the remote has sent us a window scaling option,
      * which we interpret as a sign the remote TCP is not
      * misinterpreting the window field as a signed quantity.
      */
     if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
         (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
     else
         (*rcv_wnd) = min_t(u32, space, U16_MAX);
 
     if (init_rcv_wnd)
         *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
 
     *rcv_wscale = 0;
     if (wscale_ok) {
         /* Set window scaling on max possible window */
         space = max_t(u32, space, READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
         space = max_t(u32, space, READ_ONCE(sysctl_rmem_max));
         space = min_t(u32, space, *window_clamp);
         *rcv_wscale = clamp_t(int, ilog2(space) - 15,
                       0, TCP_MAX_WSCALE);
     }
     /* Set the clamp no higher than max representable value */
     (*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);
 }
 EXPORT_SYMBOL(tcp_select_initial_window);
 
 /* Chose a new window to advertise, update state in tcp_sock for the
  * socket, and return result with RFC1323 scaling applied.  The return
  * value can be stuffed directly into th->window for an outgoing
  * frame.
  */
 static u16 tcp_select_window(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     u32 old_win = tp->rcv_wnd;
     u32 cur_win = tcp_receive_window(tp);
     u32 new_win = __tcp_select_window(sk);
 
     /* Never shrink the offered window */
     if (new_win < cur_win) {
         /* Danger Will Robinson!
          * Don't update rcv_wup/rcv_wnd here or else
          * we will not be able to advertise a zero
          * window in time.  --DaveM
          *
          * Relax Will Robinson.
          */
         if (new_win == 0)
             NET_INC_STATS(sock_net(sk),
                       LINUX_MIB_TCPWANTZEROWINDOWADV);
         new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
     }
     tp->rcv_wnd = new_win;
     tp->rcv_wup = tp->rcv_nxt;
 
     /* Make sure we do not exceed the maximum possible
      * scaled window.
      */
     if (!tp->rx_opt.rcv_wscale &&
         READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
         new_win = min(new_win, MAX_TCP_WINDOW);
     else
         new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
 
     /* RFC1323 scaling applied */
     new_win >>= tp->rx_opt.rcv_wscale;
 
     /* If we advertise zero window, disable fast path. */
     if (new_win == 0) {
         tp->pred_flags = 0;
         if (old_win)
             NET_INC_STATS(sock_net(sk),
                       LINUX_MIB_TCPTOZEROWINDOWADV);
     } else if (old_win == 0) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFROMZEROWINDOWADV);
     }
 
     return new_win;
 }
 
 /* Packet ECN state for a SYN-ACK */
 static void tcp_ecn_send_synack(struct sock *sk, struct sk_buff *skb)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
 
     TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
     if (!(tp->ecn_flags & TCP_ECN_OK))
         TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
     else if (tcp_ca_needs_ecn(sk) ||
          tcp_bpf_ca_needs_ecn(sk))
         INET_ECN_xmit(sk);
 }
 
 /* Packet ECN state for a SYN.  */
 static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     bool bpf_needs_ecn = tcp_bpf_ca_needs_ecn(sk);
     bool use_ecn = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn) == 1 ||
         tcp_ca_needs_ecn(sk) || bpf_needs_ecn;
 
     if (!use_ecn) {
         const struct dst_entry *dst = __sk_dst_get(sk);
 
         if (dst && dst_feature(dst, RTAX_FEATURE_ECN))
             use_ecn = true;
     }
 
     tp->ecn_flags = 0;
 
     if (use_ecn) {
         TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
         tp->ecn_flags = TCP_ECN_OK;
         if (tcp_ca_needs_ecn(sk) || bpf_needs_ecn)
             INET_ECN_xmit(sk);
     }
 }
 
 static void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
 {
     if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_fallback))
         /* tp->ecn_flags are cleared at a later point in time when
          * SYN ACK is ultimatively being received.
          */
         TCP_SKB_CB(skb)->tcp_flags &= ~(TCPHDR_ECE | TCPHDR_CWR);
 }
 
 static void
 tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
 {
     if (inet_rsk(req)->ecn_ok)
         th->ece = 1;
 }
 
 /* Set up ECN state for a packet on a ESTABLISHED socket that is about to
  * be sent.
  */
 static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
              struct tcphdr *th, int tcp_header_len)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (tp->ecn_flags & TCP_ECN_OK) {
         /* Not-retransmitted data segment: set ECT and inject CWR. */
         if (skb->len != tcp_header_len &&
             !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
             INET_ECN_xmit(sk);
             if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
                 tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
                 th->cwr = 1;
                 skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
             }
         } else if (!tcp_ca_needs_ecn(sk)) {
             /* ACK or retransmitted segment: clear ECT|CE */
             INET_ECN_dontxmit(sk);
         }
         if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
             th->ece = 1;
     }
 }
 
 /* Constructs common control bits of non-data skb. If SYN/FIN is present,
  * auto increment end seqno.
  */
 static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
 {
     skb->ip_summed = CHECKSUM_PARTIAL;
 
     TCP_SKB_CB(skb)->tcp_flags = flags;
 
     tcp_skb_pcount_set(skb, 1);
 
     TCP_SKB_CB(skb)->seq = seq;
     if (flags & (TCPHDR_SYN | TCPHDR_FIN))
         seq++;
     TCP_SKB_CB(skb)->end_seq = seq;
 }
 
 static inline bool tcp_urg_mode(const struct tcp_sock *tp)
 {
     return tp->snd_una != tp->snd_up;
 }
 
 #define OPTION_SACK_ADVERTISE   BIT(0)
 #define OPTION_TS       BIT(1)
 #define OPTION_MD5      BIT(2)
 #define OPTION_WSCALE       BIT(3)
 #define OPTION_FAST_OPEN_COOKIE BIT(8)
 #define OPTION_SMC      BIT(9)
 #define OPTION_MPTCP        BIT(10)
 
 static void smc_options_write(__be32 *ptr, u16 *options)
 {
 #if IS_ENABLED(CONFIG_SMC)
     if (static_branch_unlikely(&tcp_have_smc)) {
         if (unlikely(OPTION_SMC & *options)) {
             *ptr++ = htonl((TCPOPT_NOP  << 24) |
                        (TCPOPT_NOP  << 16) |
                        (TCPOPT_EXP <<  8) |
                        (TCPOLEN_EXP_SMC_BASE));
             *ptr++ = htonl(TCPOPT_SMC_MAGIC);
         }
     }
 #endif
 }
 
 struct tcp_out_options {
     u16 options;        /* bit field of OPTION_* */
     u16 mss;        /* 0 to disable */
     u8 ws;          /* window scale, 0 to disable */
     u8 num_sack_blocks; /* number of SACK blocks to include */
     u8 hash_size;       /* bytes in hash_location */
     u8 bpf_opt_len;     /* length of BPF hdr option */
     __u8 *hash_location;    /* temporary pointer, overloaded */
     __u32 tsval, tsecr; /* need to include OPTION_TS */
     struct tcp_fastopen_cookie *fastopen_cookie;    /* Fast open cookie */
     struct mptcp_out_options mptcp;
 };
 
 static void mptcp_options_write(struct tcphdr *th, __be32 *ptr,
                 struct tcp_sock *tp,
                 struct tcp_out_options *opts)
 {
 #if IS_ENABLED(CONFIG_MPTCP)
     if (unlikely(OPTION_MPTCP & opts->options))
         mptcp_write_options(th, ptr, tp, &opts->mptcp);
 #endif
 }
 
 #ifdef CONFIG_CGROUP_BPF
 static int bpf_skops_write_hdr_opt_arg0(struct sk_buff *skb,
                     enum tcp_synack_type synack_type)
 {
     if (unlikely(!skb))
         return BPF_WRITE_HDR_TCP_CURRENT_MSS;
 
     if (unlikely(synack_type == TCP_SYNACK_COOKIE))
         return BPF_WRITE_HDR_TCP_SYNACK_COOKIE;
 
     return 0;
 }
 
 /* req, syn_skb and synack_type are used when writing synack */
 static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
                   struct request_sock *req,
                   struct sk_buff *syn_skb,
                   enum tcp_synack_type synack_type,
                   struct tcp_out_options *opts,
                   unsigned int *remaining)
 {
     struct bpf_sock_ops_kern sock_ops;
     int err;
 
     if (likely(!BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                        BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG)) ||
         !*remaining)
         return;
 
     /* *remaining has already been aligned to 4 bytes, so *remaining >= 4 */
 
     /* init sock_ops */
     memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
 
     sock_ops.op = BPF_SOCK_OPS_HDR_OPT_LEN_CB;
 
     if (req) {
         /* The listen "sk" cannot be passed here because
          * it is not locked.  It would not make too much
          * sense to do bpf_setsockopt(listen_sk) based
          * on individual connection request also.
          *
          * Thus, "req" is passed here and the cgroup-bpf-progs
          * of the listen "sk" will be run.
          *
          * "req" is also used here for fastopen even the "sk" here is
          * a fullsock "child" sk.  It is to keep the behavior
          * consistent between fastopen and non-fastopen on
          * the bpf programming side.
          */
         sock_ops.sk = (struct sock *)req;
         sock_ops.syn_skb = syn_skb;
     } else {
         sock_owned_by_me(sk);
 
         sock_ops.is_fullsock = 1;
         sock_ops.sk = sk;
     }
 
     sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
     sock_ops.remaining_opt_len = *remaining;
     /* tcp_current_mss() does not pass a skb */
     if (skb)
         bpf_skops_init_skb(&sock_ops, skb, 0);
 
     err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);
 
     if (err || sock_ops.remaining_opt_len == *remaining)
         return;
 
     opts->bpf_opt_len = *remaining - sock_ops.remaining_opt_len;
     /* round up to 4 bytes */
     opts->bpf_opt_len = (opts->bpf_opt_len + 3) & ~3;
 
     *remaining -= opts->bpf_opt_len;
 }
 
 static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
                     struct request_sock *req,
                     struct sk_buff *syn_skb,
                     enum tcp_synack_type synack_type,
                     struct tcp_out_options *opts)
 {
     u8 first_opt_off, nr_written, max_opt_len = opts->bpf_opt_len;
     struct bpf_sock_ops_kern sock_ops;
     int err;
 
     if (likely(!max_opt_len))
         return;
 
     memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
 
     sock_ops.op = BPF_SOCK_OPS_WRITE_HDR_OPT_CB;
 
     if (req) {
         sock_ops.sk = (struct sock *)req;
         sock_ops.syn_skb = syn_skb;
     } else {
         sock_owned_by_me(sk);
 
         sock_ops.is_fullsock = 1;
         sock_ops.sk = sk;
     }
 
     sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
     sock_ops.remaining_opt_len = max_opt_len;
     first_opt_off = tcp_hdrlen(skb) - max_opt_len;
     bpf_skops_init_skb(&sock_ops, skb, first_opt_off);
 
     err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);
 
     if (err)
         nr_written = 0;
     else
         nr_written = max_opt_len - sock_ops.remaining_opt_len;
 
     if (nr_written < max_opt_len)
         memset(skb->data + first_opt_off + nr_written, TCPOPT_NOP,
                max_opt_len - nr_written);
 }
 #else
 static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
                   struct request_sock *req,
                   struct sk_buff *syn_skb,
                   enum tcp_synack_type synack_type,
                   struct tcp_out_options *opts,
                   unsigned int *remaining)
 {
 }
 
 static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
                     struct request_sock *req,
                     struct sk_buff *syn_skb,
                     enum tcp_synack_type synack_type,
                     struct tcp_out_options *opts)
 {
 }
 #endif
 
 /* Write previously computed TCP options to the packet.
  *
  * Beware: Something in the Internet is very sensitive to the ordering of
  * TCP options, we learned this through the hard way, so be careful here.
  * Luckily we can at least blame others for their non-compliance but from
  * inter-operability perspective it seems that we're somewhat stuck with
  * the ordering which we have been using if we want to keep working with
  * those broken things (not that it currently hurts anybody as there isn't
  * particular reason why the ordering would need to be changed).
  *
  * At least SACK_PERM as the first option is known to lead to a disaster
  * (but it may well be that other scenarios fail similarly).
  */
 static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
                   struct tcp_out_options *opts)
 {
     __be32 *ptr = (__be32 *)(th + 1);
     u16 options = opts->options;    /* mungable copy */
 
     if (unlikely(OPTION_MD5 & options)) {
         *ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
                    (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
         /* overload cookie hash location */
         opts->hash_location = (__u8 *)ptr;
         ptr += 4;
     }
 
     if (unlikely(opts->mss)) {
         *ptr++ = htonl((TCPOPT_MSS << 24) |
                    (TCPOLEN_MSS << 16) |
                    opts->mss);
     }
 
     if (likely(OPTION_TS & options)) {
         if (unlikely(OPTION_SACK_ADVERTISE & options)) {
             *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
                        (TCPOLEN_SACK_PERM << 16) |
                        (TCPOPT_TIMESTAMP << 8) |
                        TCPOLEN_TIMESTAMP);
             options &= ~OPTION_SACK_ADVERTISE;
         } else {
             *ptr++ = htonl((TCPOPT_NOP << 24) |
                        (TCPOPT_NOP << 16) |
                        (TCPOPT_TIMESTAMP << 8) |
                        TCPOLEN_TIMESTAMP);
         }
         *ptr++ = htonl(opts->tsval);
         *ptr++ = htonl(opts->tsecr);
     }
 
     if (unlikely(OPTION_SACK_ADVERTISE & options)) {
         *ptr++ = htonl((TCPOPT_NOP << 24) |
                    (TCPOPT_NOP << 16) |
                    (TCPOPT_SACK_PERM << 8) |
                    TCPOLEN_SACK_PERM);
     }
 
     if (unlikely(OPTION_WSCALE & options)) {
         *ptr++ = htonl((TCPOPT_NOP << 24) |
                    (TCPOPT_WINDOW << 16) |
                    (TCPOLEN_WINDOW << 8) |
                    opts->ws);
     }
 
     if (unlikely(opts->num_sack_blocks)) {
         struct tcp_sack_block *sp = tp->rx_opt.dsack ?
             tp->duplicate_sack : tp->selective_acks;
         int this_sack;
 
         *ptr++ = htonl((TCPOPT_NOP  << 24) |
                    (TCPOPT_NOP  << 16) |
                    (TCPOPT_SACK <<  8) |
                    (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
                              TCPOLEN_SACK_PERBLOCK)));
 
         for (this_sack = 0; this_sack < opts->num_sack_blocks;
              ++this_sack) {
             *ptr++ = htonl(sp[this_sack].start_seq);
             *ptr++ = htonl(sp[this_sack].end_seq);
         }
 
         tp->rx_opt.dsack = 0;
     }
 
     if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
         struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;
         u8 *p = (u8 *)ptr;
         u32 len; /* Fast Open option length */
 
         if (foc->exp) {
             len = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
             *ptr = htonl((TCPOPT_EXP << 24) | (len << 16) |
                      TCPOPT_FASTOPEN_MAGIC);
             p += TCPOLEN_EXP_FASTOPEN_BASE;
         } else {
             len = TCPOLEN_FASTOPEN_BASE + foc->len;
             *p++ = TCPOPT_FASTOPEN;
             *p++ = len;
         }
 
         memcpy(p, foc->val, foc->len);
         if ((len & 3) == 2) {
             p[foc->len] = TCPOPT_NOP;
             p[foc->len + 1] = TCPOPT_NOP;
         }
         ptr += (len + 3) >> 2;
     }
 
     smc_options_write(ptr, &options);
 
     mptcp_options_write(th, ptr, tp, opts);
 }
 
 static void smc_set_option(const struct tcp_sock *tp,
                struct tcp_out_options *opts,
                unsigned int *remaining)
 {
 #if IS_ENABLED(CONFIG_SMC)
     if (static_branch_unlikely(&tcp_have_smc)) {
         if (tp->syn_smc) {
             if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
                 opts->options |= OPTION_SMC;
                 *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
             }
         }
     }
 #endif
 }
 
 static void smc_set_option_cond(const struct tcp_sock *tp,
                 const struct inet_request_sock *ireq,
                 struct tcp_out_options *opts,
                 unsigned int *remaining)
 {
 #if IS_ENABLED(CONFIG_SMC)
     if (static_branch_unlikely(&tcp_have_smc)) {
         if (tp->syn_smc && ireq->smc_ok) {
             if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
                 opts->options |= OPTION_SMC;
                 *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
             }
         }
     }
 #endif
 }
 
 static void mptcp_set_option_cond(const struct request_sock *req,
                   struct tcp_out_options *opts,
                   unsigned int *remaining)
 {
     if (rsk_is_mptcp(req)) {
         unsigned int size;
 
         if (mptcp_synack_options(req, &size, &opts->mptcp)) {
             if (*remaining >= size) {
                 opts->options |= OPTION_MPTCP;
                 *remaining -= size;
             }
         }
     }
 }
 
 /* Compute TCP options for SYN packets. This is not the final
  * network wire format yet.
  */
 static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                 struct tcp_out_options *opts,
                 struct tcp_md5sig_key **md5)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     unsigned int remaining = MAX_TCP_OPTION_SPACE;
     struct tcp_fastopen_request *fastopen = tp->fastopen_req;
 
     *md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
     if (static_branch_unlikely(&tcp_md5_needed) &&
         rcu_access_pointer(tp->md5sig_info)) {
         *md5 = tp->af_specific->md5_lookup(sk, sk);
         if (*md5) {
             opts->options |= OPTION_MD5;
             remaining -= TCPOLEN_MD5SIG_ALIGNED;
         }
     }
 #endif
 
     /* We always get an MSS option.  The option bytes which will be seen in
      * normal data packets should timestamps be used, must be in the MSS
      * advertised.  But we subtract them from tp->mss_cache so that
      * calculations in tcp_sendmsg are simpler etc.  So account for this
      * fact here if necessary.  If we don't do this correctly, as a
      * receiver we won't recognize data packets as being full sized when we
      * should, and thus we won't abide by the delayed ACK rules correctly.
      * SACKs don't matter, we never delay an ACK when we have any of those
      * going out.  */
     opts->mss = tcp_advertise_mss(sk);
     remaining -= TCPOLEN_MSS_ALIGNED;
 
     if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps) && !*md5)) {
         opts->options |= OPTION_TS;
         opts->tsval = tcp_skb_timestamp(skb) + tp->tsoffset;
         opts->tsecr = tp->rx_opt.ts_recent;
         remaining -= TCPOLEN_TSTAMP_ALIGNED;
     }
     if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling))) {
         opts->ws = tp->rx_opt.rcv_wscale;
         opts->options |= OPTION_WSCALE;
         remaining -= TCPOLEN_WSCALE_ALIGNED;
     }
     if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_sack))) {
         opts->options |= OPTION_SACK_ADVERTISE;
         if (unlikely(!(OPTION_TS & opts->options)))
             remaining -= TCPOLEN_SACKPERM_ALIGNED;
     }
 
     if (fastopen && fastopen->cookie.len >= 0) {
         u32 need = fastopen->cookie.len;
 
         need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
                            TCPOLEN_FASTOPEN_BASE;
         need = (need + 3) & ~3U;  /* Align to 32 bits */
         if (remaining >= need) {
             opts->options |= OPTION_FAST_OPEN_COOKIE;
             opts->fastopen_cookie = &fastopen->cookie;
             remaining -= need;
             tp->syn_fastopen = 1;
             tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
         }
     }
 
     smc_set_option(tp, opts, &remaining);
 
     if (sk_is_mptcp(sk)) {
         unsigned int size;
 
         if (mptcp_syn_options(sk, skb, &size, &opts->mptcp)) {
             opts->options |= OPTION_MPTCP;
             remaining -= size;
         }
     }
 
     bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
 
     return MAX_TCP_OPTION_SPACE - remaining;
 }
 
 /* Set up TCP options for SYN-ACKs. */
 static unsigned int tcp_synack_options(const struct sock *sk,
                        struct request_sock *req,
                        unsigned int mss, struct sk_buff *skb,
                        struct tcp_out_options *opts,
                        const struct tcp_md5sig_key *md5,
                        struct tcp_fastopen_cookie *foc,
                        enum tcp_synack_type synack_type,
                        struct sk_buff *syn_skb)
 {
     struct inet_request_sock *ireq = inet_rsk(req);
     unsigned int remaining = MAX_TCP_OPTION_SPACE;
 
 #ifdef CONFIG_TCP_MD5SIG
     if (md5) {
         opts->options |= OPTION_MD5;
         remaining -= TCPOLEN_MD5SIG_ALIGNED;
 
         /* We can't fit any SACK blocks in a packet with MD5 + TS
          * options. There was discussion about disabling SACK
          * rather than TS in order to fit in better with old,
          * buggy kernels, but that was deemed to be unnecessary.
          */
         if (synack_type != TCP_SYNACK_COOKIE)
             ireq->tstamp_ok &= !ireq->sack_ok;
     }
 #endif
 
     /* We always send an MSS option. */
     opts->mss = mss;
     remaining -= TCPOLEN_MSS_ALIGNED;
 
     if (likely(ireq->wscale_ok)) {
         opts->ws = ireq->rcv_wscale;
         opts->options |= OPTION_WSCALE;
         remaining -= TCPOLEN_WSCALE_ALIGNED;
     }
     if (likely(ireq->tstamp_ok)) {
         opts->options |= OPTION_TS;
         opts->tsval = tcp_skb_timestamp(skb) + tcp_rsk(req)->ts_off;
         opts->tsecr = req->ts_recent;
         remaining -= TCPOLEN_TSTAMP_ALIGNED;
     }
     if (likely(ireq->sack_ok)) {
         opts->options |= OPTION_SACK_ADVERTISE;
         if (unlikely(!ireq->tstamp_ok))
             remaining -= TCPOLEN_SACKPERM_ALIGNED;
     }
     if (foc != NULL && foc->len >= 0) {
         u32 need = foc->len;
 
         need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
                    TCPOLEN_FASTOPEN_BASE;
         need = (need + 3) & ~3U;  /* Align to 32 bits */
         if (remaining >= need) {
             opts->options |= OPTION_FAST_OPEN_COOKIE;
             opts->fastopen_cookie = foc;
             remaining -= need;
         }
     }
 
     mptcp_set_option_cond(req, opts, &remaining);
 
     smc_set_option_cond(tcp_sk(sk), ireq, opts, &remaining);
 
     bpf_skops_hdr_opt_len((struct sock *)sk, skb, req, syn_skb,
                   synack_type, opts, &remaining);
 
     return MAX_TCP_OPTION_SPACE - remaining;
 }
 
 /* Compute TCP options for ESTABLISHED sockets. This is not the
  * final wire format yet.
  */
 static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
                     struct tcp_out_options *opts,
                     struct tcp_md5sig_key **md5)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     unsigned int size = 0;
     unsigned int eff_sacks;
 
     opts->options = 0;
 
     *md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
     if (static_branch_unlikely(&tcp_md5_needed) &&
         rcu_access_pointer(tp->md5sig_info)) {
         *md5 = tp->af_specific->md5_lookup(sk, sk);
         if (*md5) {
             opts->options |= OPTION_MD5;
             size += TCPOLEN_MD5SIG_ALIGNED;
         }
     }
 #endif
 
     if (likely(tp->rx_opt.tstamp_ok)) {
         opts->options |= OPTION_TS;
         opts->tsval = skb ? tcp_skb_timestamp(skb) + tp->tsoffset : 0;
         opts->tsecr = tp->rx_opt.ts_recent;
         size += TCPOLEN_TSTAMP_ALIGNED;
     }
 
     /* MPTCP options have precedence over SACK for the limited TCP
      * option space because a MPTCP connection would be forced to
      * fall back to regular TCP if a required multipath option is
      * missing. SACK still gets a chance to use whatever space is
      * left.
      */
     if (sk_is_mptcp(sk)) {
         unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
         unsigned int opt_size = 0;
 
         if (mptcp_established_options(sk, skb, &opt_size, remaining,
                           &opts->mptcp)) {
             opts->options |= OPTION_MPTCP;
             size += opt_size;
         }
     }
 
     eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
     if (unlikely(eff_sacks)) {
         const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
         if (unlikely(remaining < TCPOLEN_SACK_BASE_ALIGNED +
                      TCPOLEN_SACK_PERBLOCK))
             return size;
 
         opts->num_sack_blocks =
             min_t(unsigned int, eff_sacks,
                   (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
                   TCPOLEN_SACK_PERBLOCK);
 
         size += TCPOLEN_SACK_BASE_ALIGNED +
             opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
     }
 
     if (unlikely(BPF_SOCK_OPS_TEST_FLAG(tp,
                         BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))) {
         unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
 
         bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
 
         size = MAX_TCP_OPTION_SPACE - remaining;
     }
 
     return size;
 }
 
 
 /* TCP SMALL QUEUES (TSQ)
  *
  * TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
  * to reduce RTT and bufferbloat.
  * We do this using a special skb destructor (tcp_wfree).
  *
  * Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
  * needs to be reallocated in a driver.
  * The invariant being skb->truesize subtracted from sk->sk_wmem_alloc
  *
  * Since transmit from skb destructor is forbidden, we use a tasklet
  * to process all sockets that eventually need to send more skbs.
  * We use one tasklet per cpu, with its own queue of sockets.
  */
 struct tsq_tasklet {
     struct tasklet_struct   tasklet;
     struct list_head    head; /* queue of tcp sockets */
 };
 static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);
 
 static void tcp_tsq_write(struct sock *sk)
 {
     if ((1 << sk->sk_state) &
         (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
          TCPF_CLOSE_WAIT  | TCPF_LAST_ACK)) {
         struct tcp_sock *tp = tcp_sk(sk);
 
         if (tp->lost_out > tp->retrans_out &&
             tcp_snd_cwnd(tp) > tcp_packets_in_flight(tp)) {
             tcp_mstamp_refresh(tp);
             tcp_xmit_retransmit_queue(sk);
         }
 
         tcp_write_xmit(sk, tcp_current_mss(sk), tp->nonagle,
                    0, GFP_ATOMIC);
     }
 }
 
 static void tcp_tsq_handler(struct sock *sk)
 {
     bh_lock_sock(sk);
     if (!sock_owned_by_user(sk))
         tcp_tsq_write(sk);
     else if (!test_and_set_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags))
         sock_hold(sk);
     bh_unlock_sock(sk);
 }
 /*
  * One tasklet per cpu tries to send more skbs.
  * We run in tasklet context but need to disable irqs when
  * transferring tsq->head because tcp_wfree() might
  * interrupt us (non NAPI drivers)
  */
 static void tcp_tasklet_func(struct tasklet_struct *t)
 {
     struct tsq_tasklet *tsq = from_tasklet(tsq,  t, tasklet);
     LIST_HEAD(list);
     unsigned long flags;
     struct list_head *q, *n;
     struct tcp_sock *tp;
     struct sock *sk;
 
     local_irq_save(flags);
     list_splice_init(&tsq->head, &list);
     local_irq_restore(flags);
 
     list_for_each_safe(q, n, &list) {
         tp = list_entry(q, struct tcp_sock, tsq_node);
         list_del(&tp->tsq_node);
 
         sk = (struct sock *)tp;
         smp_mb__before_atomic();
         clear_bit(TSQ_QUEUED, &sk->sk_tsq_flags);
 
         tcp_tsq_handler(sk);
         sk_free(sk);
     }
 }
 
 #define TCP_DEFERRED_ALL (TCPF_TSQ_DEFERRED |       \
               TCPF_WRITE_TIMER_DEFERRED |   \
               TCPF_DELACK_TIMER_DEFERRED |  \
               TCPF_MTU_REDUCED_DEFERRED)
 /**
  * tcp_release_cb - tcp release_sock() callback
  * @sk: socket
  *
  * called from release_sock() to perform protocol dependent
  * actions before socket release.
  */
 void tcp_release_cb(struct sock *sk)
 {
     unsigned long flags, nflags;
 
     /* perform an atomic operation only if at least one flag is set */
     do {
         flags = sk->sk_tsq_flags;
         if (!(flags & TCP_DEFERRED_ALL))
             return;
         nflags = flags & ~TCP_DEFERRED_ALL;
     } while (cmpxchg(&sk->sk_tsq_flags, flags, nflags) != flags);
 
     if (flags & TCPF_TSQ_DEFERRED) {
         tcp_tsq_write(sk);
         __sock_put(sk);
     }
     /* Here begins the tricky part :
      * We are called from release_sock() with :
      * 1) BH disabled
      * 2) sk_lock.slock spinlock held
      * 3) socket owned by us (sk->sk_lock.owned == 1)
      *
      * But following code is meant to be called from BH handlers,
      * so we should keep BH disabled, but early release socket ownership
      */
     sock_release_ownership(sk);
 
     if (flags & TCPF_WRITE_TIMER_DEFERRED) {
         tcp_write_timer_handler(sk);
         __sock_put(sk);
     }
     if (flags & TCPF_DELACK_TIMER_DEFERRED) {
         tcp_delack_timer_handler(sk);
         __sock_put(sk);
     }
     if (flags & TCPF_MTU_REDUCED_DEFERRED) {
         inet_csk(sk)->icsk_af_ops->mtu_reduced(sk);
         __sock_put(sk);
     }
 }
 EXPORT_SYMBOL(tcp_release_cb);
 
 void __init tcp_tasklet_init(void)
 {
     int i;
 
     for_each_possible_cpu(i) {
         struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
 
         INIT_LIST_HEAD(&tsq->head);
         tasklet_setup(&tsq->tasklet, tcp_tasklet_func);
     }
 }
 
 /*
  * Write buffer destructor automatically called from kfree_skb.
  * We can't xmit new skbs from this context, as we might already
  * hold qdisc lock.
  */
 void tcp_wfree(struct sk_buff *skb)
 {
     struct sock *sk = skb->sk;
     struct tcp_sock *tp = tcp_sk(sk);
     unsigned long flags, nval, oval;
 
     /* Keep one reference on sk_wmem_alloc.
      * Will be released by sk_free() from here or tcp_tasklet_func()
      */
     WARN_ON(refcount_sub_and_test(skb->truesize - 1, &sk->sk_wmem_alloc));
 
     /* If this softirq is serviced by ksoftirqd, we are likely under stress.
      * Wait until our queues (qdisc + devices) are drained.
      * This gives :
      * - less callbacks to tcp_write_xmit(), reducing stress (batches)
      * - chance for incoming ACK (processed by another cpu maybe)
      *   to migrate this flow (skb->ooo_okay will be eventually set)
      */
     if (refcount_read(&sk->sk_wmem_alloc) >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
         goto out;
 
     for (oval = READ_ONCE(sk->sk_tsq_flags);; oval = nval) {
         struct tsq_tasklet *tsq;
         bool empty;
 
         if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
             goto out;
 
         nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED;
         nval = cmpxchg(&sk->sk_tsq_flags, oval, nval);
         if (nval != oval)
             continue;
 
         /* queue this socket to tasklet queue */
         local_irq_save(flags);
         tsq = this_cpu_ptr(&tsq_tasklet);
         empty = list_empty(&tsq->head);
         list_add(&tp->tsq_node, &tsq->head);
         if (empty)
             tasklet_schedule(&tsq->tasklet);
         local_irq_restore(flags);
         return;
     }
 out:
     sk_free(sk);
 }
 
 /* Note: Called under soft irq.
  * We can call TCP stack right away, unless socket is owned by user.
  */
 enum hrtimer_restart tcp_pace_kick(struct hrtimer *timer)
 {
     struct tcp_sock *tp = container_of(timer, struct tcp_sock, pacing_timer);
     struct sock *sk = (struct sock *)tp;
 
     tcp_tsq_handler(sk);
     sock_put(sk);
 
     return HRTIMER_NORESTART;
 }
 
 static void tcp_update_skb_after_send(struct sock *sk, struct sk_buff *skb,
                       u64 prior_wstamp)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (sk->sk_pacing_status != SK_PACING_NONE) {
         unsigned long rate = sk->sk_pacing_rate;
 
         /* Original sch_fq does not pace first 10 MSS
          * Note that tp->data_segs_out overflows after 2^32 packets,
          * this is a minor annoyance.
          */
         if (rate != ~0UL && rate && tp->data_segs_out >= 10) {
             u64 len_ns = div64_ul((u64)skb->len * NSEC_PER_SEC, rate);
             u64 credit = tp->tcp_wstamp_ns - prior_wstamp;
 
             /* take into account OS jitter */
             len_ns -= min_t(u64, len_ns / 2, credit);
             tp->tcp_wstamp_ns += len_ns;
         }
     }
     list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
 }
 
 INDIRECT_CALLABLE_DECLARE(int ip_queue_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
 INDIRECT_CALLABLE_DECLARE(int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
 INDIRECT_CALLABLE_DECLARE(void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb));
 
 /* This routine actually transmits TCP packets queued in by
  * tcp_do_sendmsg().  This is used by both the initial
  * transmission and possible later retransmissions.
  * All SKB's seen here are completely headerless.  It is our
  * job to build the TCP header, and pass the packet down to
  * IP so it can do the same plus pass the packet off to the
  * device.
  *
  * We are working here with either a clone of the original
  * SKB, or a fresh unique copy made by the retransmit engine.
  */
 static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
                   int clone_it, gfp_t gfp_mask, u32 rcv_nxt)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     struct inet_sock *inet;
     struct tcp_sock *tp;
     struct tcp_skb_cb *tcb;
     struct tcp_out_options opts;
     unsigned int tcp_options_size, tcp_header_size;
     struct sk_buff *oskb = NULL;
     struct tcp_md5sig_key *md5;
     struct tcphdr *th;
     u64 prior_wstamp;
     int err;
 
     BUG_ON(!skb || !tcp_skb_pcount(skb));
     tp = tcp_sk(sk);
     prior_wstamp = tp->tcp_wstamp_ns;
     tp->tcp_wstamp_ns = max(tp->tcp_wstamp_ns, tp->tcp_clock_cache);
     skb_set_delivery_time(skb, tp->tcp_wstamp_ns, true);
     if (clone_it) {
         oskb = skb;
 
         tcp_skb_tsorted_save(oskb) {
             if (unlikely(skb_cloned(oskb)))
                 skb = pskb_copy(oskb, gfp_mask);
             else
                 skb = skb_clone(oskb, gfp_mask);
         } tcp_skb_tsorted_restore(oskb);
 
         if (unlikely(!skb))
             return -ENOBUFS;
         /* retransmit skbs might have a non zero value in skb->dev
          * because skb->dev is aliased with skb->rbnode.rb_left
          */
         skb->dev = NULL;
     }
 
     inet = inet_sk(sk);
     tcb = TCP_SKB_CB(skb);
     memset(&opts, 0, sizeof(opts));
 
     if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
         tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
     } else {
         tcp_options_size = tcp_established_options(sk, skb, &opts,
                                &md5);
         /* Force a PSH flag on all (GSO) packets to expedite GRO flush
          * at receiver : This slightly improve GRO performance.
          * Note that we do not force the PSH flag for non GSO packets,
          * because they might be sent under high congestion events,
          * and in this case it is better to delay the delivery of 1-MSS
          * packets and thus the corresponding ACK packet that would
          * release the following packet.
          */
         if (tcp_skb_pcount(skb) > 1)
             tcb->tcp_flags |= TCPHDR_PSH;
     }
     tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
 
     /* if no packet is in qdisc/device queue, then allow XPS to select
      * another queue. We can be called from tcp_tsq_handler()
      * which holds one reference to sk.
      *
      * TODO: Ideally, in-flight pure ACK packets should not matter here.
      * One way to get this would be to set skb->truesize = 2 on them.
      */
     skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);
 
     /* If we had to use memory reserve to allocate this skb,
      * this might cause drops if packet is looped back :
      * Other socket might not have SOCK_MEMALLOC.
      * Packets not looped back do not care about pfmemalloc.
      */
     skb->pfmemalloc = 0;
 
     skb_push(skb, tcp_header_size);
     skb_reset_transport_header(skb);
 
     skb_orphan(skb);
     skb->sk = sk;
     skb->destructor = skb_is_tcp_pure_ack(skb) ? __sock_wfree : tcp_wfree;
     refcount_add(skb->truesize, &sk->sk_wmem_alloc);
 
     skb_set_dst_pending_confirm(skb, sk->sk_dst_pending_confirm);
 
     /* Build TCP header and checksum it. */
     th = (struct tcphdr *)skb->data;
     th->source      = inet->inet_sport;
     th->dest        = inet->inet_dport;
     th->seq         = htonl(tcb->seq);
     th->ack_seq     = htonl(rcv_nxt);
     *(((__be16 *)th) + 6)   = htons(((tcp_header_size >> 2) << 12) |
                     tcb->tcp_flags);
 
     th->check       = 0;
     th->urg_ptr     = 0;
 
     /* The urg_mode check is necessary during a below snd_una win probe */
     if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
         if (before(tp->snd_up, tcb->seq + 0x10000)) {
             th->urg_ptr = htons(tp->snd_up - tcb->seq);
             th->urg = 1;
         } else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
             th->urg_ptr = htons(0xFFFF);
             th->urg = 1;
         }
     }
 
     skb_shinfo(skb)->gso_type = sk->sk_gso_type;
     if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
         th->window      = htons(tcp_select_window(sk));
         tcp_ecn_send(sk, skb, th, tcp_header_size);
     } else {
         /* RFC1323: The window in SYN & SYN/ACK segments
          * is never scaled.
          */
         th->window  = htons(min(tp->rcv_wnd, 65535U));
     }
 
     tcp_options_write(th, tp, &opts);
 
 #ifdef CONFIG_TCP_MD5SIG
     /* Calculate the MD5 hash, as we have all we need now */
     if (md5) {
         sk_gso_disable(sk);
         tp->af_specific->calc_md5_hash(opts.hash_location,
                            md5, sk, skb);
     }
 #endif
 
     /* BPF prog is the last one writing header option */
     bpf_skops_write_hdr_opt(sk, skb, NULL, NULL, 0, &opts);
 
     INDIRECT_CALL_INET(icsk->icsk_af_ops->send_check,
                tcp_v6_send_check, tcp_v4_send_check,
                sk, skb);
 
     if (likely(tcb->tcp_flags & TCPHDR_ACK))
         tcp_event_ack_sent(sk, tcp_skb_pcount(skb), rcv_nxt);
 
     if (skb->len != tcp_header_size) {
         tcp_event_data_sent(tp, sk);
         tp->data_segs_out += tcp_skb_pcount(skb);
         tp->bytes_sent += skb->len - tcp_header_size;
     }
 
     if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
         TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
                   tcp_skb_pcount(skb));
 
     tp->segs_out += tcp_skb_pcount(skb);
     skb_set_hash_from_sk(skb, sk);
     /* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
     skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
     skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);
 
     /* Leave earliest departure time in skb->tstamp (skb->skb_mstamp_ns) */
 
     /* Cleanup our debris for IP stacks */
     memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
                    sizeof(struct inet6_skb_parm)));
 
     tcp_add_tx_delay(skb, tp);
 
     err = INDIRECT_CALL_INET(icsk->icsk_af_ops->queue_xmit,
                  inet6_csk_xmit, ip_queue_xmit,
                  sk, skb, &inet->cork.fl);
 
     if (unlikely(err > 0)) {
         tcp_enter_cwr(sk);
         err = net_xmit_eval(err);
     }
     if (!err && oskb) {
         tcp_update_skb_after_send(sk, oskb, prior_wstamp);
         tcp_rate_skb_sent(sk, oskb);
     }
     return err;
 }
 
 static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
                 gfp_t gfp_mask)
 {
     return __tcp_transmit_skb(sk, skb, clone_it, gfp_mask,
                   tcp_sk(sk)->rcv_nxt);
 }
 
 /* This routine just queues the buffer for sending.
  *
  * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
  * otherwise socket can stall.
  */
 static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* Advance write_seq and place onto the write_queue. */
     WRITE_ONCE(tp->write_seq, TCP_SKB_CB(skb)->end_seq);
     __skb_header_release(skb);
     tcp_add_write_queue_tail(sk, skb);
     sk_wmem_queued_add(sk, skb->truesize);
     sk_mem_charge(sk, skb->truesize);
 }
 
 /* Initialize TSO segments for a packet. */
 static void tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
 {
     if (skb->len <= mss_now) {
         /* Avoid the costly divide in the normal
          * non-TSO case.
          */
         tcp_skb_pcount_set(skb, 1);
         TCP_SKB_CB(skb)->tcp_gso_size = 0;
     } else {
         tcp_skb_pcount_set(skb, DIV_ROUND_UP(skb->len, mss_now));
         TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
     }
 }
 
 /* Pcount in the middle of the write queue got changed, we need to do various
  * tweaks to fix counters
  */
 static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     tp->packets_out -= decr;
 
     if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
         tp->sacked_out -= decr;
     if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
         tp->retrans_out -= decr;
     if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
         tp->lost_out -= decr;
 
     /* Reno case is special. Sigh... */
     if (tcp_is_reno(tp) && decr > 0)
         tp->sacked_out -= min_t(u32, tp->sacked_out, decr);
 
     if (tp->lost_skb_hint &&
         before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
         (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
         tp->lost_cnt_hint -= decr;
 
     tcp_verify_left_out(tp);
 }
 
 static bool tcp_has_tx_tstamp(const struct sk_buff *skb)
 {
     return TCP_SKB_CB(skb)->txstamp_ack ||
         (skb_shinfo(skb)->tx_flags & SKBTX_ANY_TSTAMP);
 }
 
 static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
 {
     struct skb_shared_info *shinfo = skb_shinfo(skb);
 
     if (unlikely(tcp_has_tx_tstamp(skb)) &&
         !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
         struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
         u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;
 
         shinfo->tx_flags &= ~tsflags;
         shinfo2->tx_flags |= tsflags;
         swap(shinfo->tskey, shinfo2->tskey);
         TCP_SKB_CB(skb2)->txstamp_ack = TCP_SKB_CB(skb)->txstamp_ack;
         TCP_SKB_CB(skb)->txstamp_ack = 0;
     }
 }
 
 static void tcp_skb_fragment_eor(struct sk_buff *skb, struct sk_buff *skb2)
 {
     TCP_SKB_CB(skb2)->eor = TCP_SKB_CB(skb)->eor;
     TCP_SKB_CB(skb)->eor = 0;
 }
 
 /* Insert buff after skb on the write or rtx queue of sk.  */
 static void tcp_insert_write_queue_after(struct sk_buff *skb,
                      struct sk_buff *buff,
                      struct sock *sk,
                      enum tcp_queue tcp_queue)
 {
     if (tcp_queue == TCP_FRAG_IN_WRITE_QUEUE)
         __skb_queue_after(&sk->sk_write_queue, skb, buff);
     else
         tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
 }
 
 /* Function to create two new TCP segments.  Shrinks the given segment
  * to the specified size and appends a new segment with the rest of the
  * packet to the list.  This won't be called frequently, I hope.
  * Remember, these are still headerless SKBs at this point.
  */
 int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
          struct sk_buff *skb, u32 len,
          unsigned int mss_now, gfp_t gfp)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *buff;
     int nsize, old_factor;
     long limit;
     int nlen;
     u8 flags;
 
     if (WARN_ON(len > skb->len))
         return -EINVAL;
 
     nsize = skb_headlen(skb) - len;
     if (nsize < 0)
         nsize = 0;
 
     /* tcp_sendmsg() can overshoot sk_wmem_queued by one full size skb.
      * We need some allowance to not penalize applications setting small
      * SO_SNDBUF values.
      * Also allow first and last skb in retransmit queue to be split.
      */
     limit = sk->sk_sndbuf + 2 * SKB_TRUESIZE(GSO_LEGACY_MAX_SIZE);
     if (unlikely((sk->sk_wmem_queued >> 1) > limit &&
              tcp_queue != TCP_FRAG_IN_WRITE_QUEUE &&
              skb != tcp_rtx_queue_head(sk) &&
              skb != tcp_rtx_queue_tail(sk))) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
         return -ENOMEM;
     }
 
     if (skb_unclone_keeptruesize(skb, gfp))
         return -ENOMEM;
 
     /* Get a new skb... force flag on. */
     buff = tcp_stream_alloc_skb(sk, nsize, gfp, true);
     if (!buff)
         return -ENOMEM; /* We'll just try again later. */
     skb_copy_decrypted(buff, skb);
     mptcp_skb_ext_copy(buff, skb);
 
     sk_wmem_queued_add(sk, buff->truesize);
     sk_mem_charge(sk, buff->truesize);
     nlen = skb->len - len - nsize;
     buff->truesize += nlen;
     skb->truesize -= nlen;
 
     /* Correct the sequence numbers. */
     TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
     TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
     TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
 
     /* PSH and FIN should only be set in the second packet. */
     flags = TCP_SKB_CB(skb)->tcp_flags;
     TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
     TCP_SKB_CB(buff)->tcp_flags = flags;
     TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
     tcp_skb_fragment_eor(skb, buff);
 
     skb_split(skb, buff, len);
 
     skb_set_delivery_time(buff, skb->tstamp, true);
     tcp_fragment_tstamp(skb, buff);
 
     old_factor = tcp_skb_pcount(skb);
 
     /* Fix up tso_factor for both original and new SKB.  */
     tcp_set_skb_tso_segs(skb, mss_now);
     tcp_set_skb_tso_segs(buff, mss_now);
 
     /* Update delivered info for the new segment */
     TCP_SKB_CB(buff)->tx = TCP_SKB_CB(skb)->tx;
 
     /* If this packet has been sent out already, we must
      * adjust the various packet counters.
      */
     if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
         int diff = old_factor - tcp_skb_pcount(skb) -
             tcp_skb_pcount(buff);
 
         if (diff)
             tcp_adjust_pcount(sk, skb, diff);
     }
 
     /* Link BUFF into the send queue. */
     __skb_header_release(buff);
     tcp_insert_write_queue_after(skb, buff, sk, tcp_queue);
     if (tcp_queue == TCP_FRAG_IN_RTX_QUEUE)
         list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);
 
     return 0;
 }
 
 /* This is similar to __pskb_pull_tail(). The difference is that pulled
  * data is not copied, but immediately discarded.
  */
 static int __pskb_trim_head(struct sk_buff *skb, int len)
 {
     struct skb_shared_info *shinfo;
     int i, k, eat;
 
     eat = min_t(int, len, skb_headlen(skb));
     if (eat) {
         __skb_pull(skb, eat);
         len -= eat;
         if (!len)
             return 0;
     }
     eat = len;
     k = 0;
     shinfo = skb_shinfo(skb);
     for (i = 0; i < shinfo->nr_frags; i++) {
         int size = skb_frag_size(&shinfo->frags[i]);
 
         if (size <= eat) {
             skb_frag_unref(skb, i);
             eat -= size;
         } else {
             shinfo->frags[k] = shinfo->frags[i];
             if (eat) {
                 skb_frag_off_add(&shinfo->frags[k], eat);
                 skb_frag_size_sub(&shinfo->frags[k], eat);
                 eat = 0;
             }
             k++;
         }
     }
     shinfo->nr_frags = k;
 
     skb->data_len -= len;
     skb->len = skb->data_len;
     return len;
 }
 
 /* Remove acked data from a packet in the transmit queue. */
 int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 {
     u32 delta_truesize;
 
     if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
         return -ENOMEM;
 
     delta_truesize = __pskb_trim_head(skb, len);
 
     TCP_SKB_CB(skb)->seq += len;
 
     if (delta_truesize) {
         skb->truesize      -= delta_truesize;
         sk_wmem_queued_add(sk, -delta_truesize);
         if (!skb_zcopy_pure(skb))
             sk_mem_uncharge(sk, delta_truesize);
     }
 
     /* Any change of skb->len requires recalculation of tso factor. */
     if (tcp_skb_pcount(skb) > 1)
         tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));
 
     return 0;
 }
 
 /* Calculate MSS not accounting any TCP options.  */
 static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     const struct inet_connection_sock *icsk = inet_csk(sk);
     int mss_now;
 
     /* Calculate base mss without TCP options:
        It is MMS_S - sizeof(tcphdr) of rfc1122
      */
     mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);
 
     /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
     if (icsk->icsk_af_ops->net_frag_header_len) {
         const struct dst_entry *dst = __sk_dst_get(sk);
 
         if (dst && dst_allfrag(dst))
             mss_now -= icsk->icsk_af_ops->net_frag_header_len;
     }
 
     /* Clamp it (mss_clamp does not include tcp options) */
     if (mss_now > tp->rx_opt.mss_clamp)
         mss_now = tp->rx_opt.mss_clamp;
 
     /* Now subtract optional transport overhead */
     mss_now -= icsk->icsk_ext_hdr_len;
 
     /* Then reserve room for full set of TCP options and 8 bytes of data */
     mss_now = max(mss_now,
               READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_snd_mss));
     return mss_now;
 }
 
 /* Calculate MSS. Not accounting for SACKs here.  */
 int tcp_mtu_to_mss(struct sock *sk, int pmtu)
 {
     /* Subtract TCP options size, not including SACKs */
     return __tcp_mtu_to_mss(sk, pmtu) -
            (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));
 }
 EXPORT_SYMBOL(tcp_mtu_to_mss);
 
 /* Inverse of above */
 int tcp_mss_to_mtu(struct sock *sk, int mss)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     const struct inet_connection_sock *icsk = inet_csk(sk);
     int mtu;
 
     mtu = mss +
           tp->tcp_header_len +
           icsk->icsk_ext_hdr_len +
           icsk->icsk_af_ops->net_header_len;
 
     /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
     if (icsk->icsk_af_ops->net_frag_header_len) {
         const struct dst_entry *dst = __sk_dst_get(sk);
 
         if (dst && dst_allfrag(dst))
             mtu += icsk->icsk_af_ops->net_frag_header_len;
     }
     return mtu;
 }
 EXPORT_SYMBOL(tcp_mss_to_mtu);
 
 /* MTU probing init per socket */
 void tcp_mtup_init(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct net *net = sock_net(sk);
 
     icsk->icsk_mtup.enabled = READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing) > 1;
     icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
                    icsk->icsk_af_ops->net_header_len;
     icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, READ_ONCE(net->ipv4.sysctl_tcp_base_mss));
     icsk->icsk_mtup.probe_size = 0;
     if (icsk->icsk_mtup.enabled)
         icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
 }
 EXPORT_SYMBOL(tcp_mtup_init);
 
 /* This function synchronize snd mss to current pmtu/exthdr set.
 
    tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
    for TCP options, but includes only bare TCP header.
 
    tp->rx_opt.mss_clamp is mss negotiated at connection setup.
    It is minimum of user_mss and mss received with SYN.
    It also does not include TCP options.
 
    inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.
 
    tp->mss_cache is current effective sending mss, including
    all tcp options except for SACKs. It is evaluated,
    taking into account current pmtu, but never exceeds
    tp->rx_opt.mss_clamp.
 
    NOTE1. rfc1122 clearly states that advertised MSS
    DOES NOT include either tcp or ip options.
 
    NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
    are READ ONLY outside this function.     --ANK (980731)
  */
 unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct inet_connection_sock *icsk = inet_csk(sk);
     int mss_now;
 
     if (icsk->icsk_mtup.search_high > pmtu)
         icsk->icsk_mtup.search_high = pmtu;
 
     mss_now = tcp_mtu_to_mss(sk, pmtu);
     mss_now = tcp_bound_to_half_wnd(tp, mss_now);
 
     /* And store cached results */
     icsk->icsk_pmtu_cookie = pmtu;
     if (icsk->icsk_mtup.enabled)
         mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
     tp->mss_cache = mss_now;
 
     return mss_now;
 }
 EXPORT_SYMBOL(tcp_sync_mss);
 
 /* Compute the current effective MSS, taking SACKs and IP options,
  * and even PMTU discovery events into account.
  */
 unsigned int tcp_current_mss(struct sock *sk)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     const struct dst_entry *dst = __sk_dst_get(sk);
     u32 mss_now;
     unsigned int header_len;
     struct tcp_out_options opts;
     struct tcp_md5sig_key *md5;
 
     mss_now = tp->mss_cache;
 
     if (dst) {
         u32 mtu = dst_mtu(dst);
         if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
             mss_now = tcp_sync_mss(sk, mtu);
     }
 
     header_len = tcp_established_options(sk, NULL, &opts, &md5) +
              sizeof(struct tcphdr);
     /* The mss_cache is sized based on tp->tcp_header_len, which assumes
      * some common options. If this is an odd packet (because we have SACK
      * blocks etc) then our calculated header_len will be different, and
      * we have to adjust mss_now correspondingly */
     if (header_len != tp->tcp_header_len) {
         int delta = (int) header_len - tp->tcp_header_len;
         mss_now -= delta;
     }
 
     return mss_now;
 }
 
 /* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
  * As additional protections, we do not touch cwnd in retransmission phases,
  * and if application hit its sndbuf limit recently.
  */
 static void tcp_cwnd_application_limited(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
         sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
         /* Limited by application or receiver window. */
         u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
         u32 win_used = max(tp->snd_cwnd_used, init_win);
         if (win_used < tcp_snd_cwnd(tp)) {
             tp->snd_ssthresh = tcp_current_ssthresh(sk);
             tcp_snd_cwnd_set(tp, (tcp_snd_cwnd(tp) + win_used) >> 1);
         }
         tp->snd_cwnd_used = 0;
     }
     tp->snd_cwnd_stamp = tcp_jiffies32;
 }
 
 static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
 {
     const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* Track the maximum number of outstanding packets in each
      * window, and remember whether we were cwnd-limited then.
      */
     if (!before(tp->snd_una, tp->max_packets_seq) ||
         tp->packets_out > tp->max_packets_out ||
         is_cwnd_limited) {
         tp->max_packets_out = tp->packets_out;
         tp->max_packets_seq = tp->snd_nxt;
         tp->is_cwnd_limited = is_cwnd_limited;
     }
 
     if (tcp_is_cwnd_limited(sk)) {
         /* Network is feed fully. */
         tp->snd_cwnd_used = 0;
         tp->snd_cwnd_stamp = tcp_jiffies32;
     } else {
         /* Network starves. */
         if (tp->packets_out > tp->snd_cwnd_used)
             tp->snd_cwnd_used = tp->packets_out;
 
         if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_slow_start_after_idle) &&
             (s32)(tcp_jiffies32 - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto &&
             !ca_ops->cong_control)
             tcp_cwnd_application_limited(sk);
 
         /* The following conditions together indicate the starvation
          * is caused by insufficient sender buffer:
          * 1) just sent some data (see tcp_write_xmit)
          * 2) not cwnd limited (this else condition)
          * 3) no more data to send (tcp_write_queue_empty())
          * 4) application is hitting buffer limit (SOCK_NOSPACE)
          */
         if (tcp_write_queue_empty(sk) && sk->sk_socket &&
             test_bit(SOCK_NOSPACE, &sk->sk_socket->flags) &&
             (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))
             tcp_chrono_start(sk, TCP_CHRONO_SNDBUF_LIMITED);
     }
 }
 
 /* Minshall's variant of the Nagle send check. */
 static bool tcp_minshall_check(const struct tcp_sock *tp)
 {
     return after(tp->snd_sml, tp->snd_una) &&
         !after(tp->snd_sml, tp->snd_nxt);
 }
 
 /* Update snd_sml if this skb is under mss
  * Note that a TSO packet might end with a sub-mss segment
  * The test is really :
  * if ((skb->len % mss) != 0)
  *        tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
  * But we can avoid doing the divide again given we already have
  *  skb_pcount = skb->len / mss_now
  */
 static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now,
                 const struct sk_buff *skb)
 {
     if (skb->len < tcp_skb_pcount(skb) * mss_now)
         tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
 }
 
 /* Return false, if packet can be sent now without violation Nagle's rules:
  * 1. It is full sized. (provided by caller in %partial bool)
  * 2. Or it contains FIN. (already checked by caller)
  * 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
  * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
  *    With Minshall's modification: all sent small packets are ACKed.
  */
 static bool tcp_nagle_check(bool partial, const struct tcp_sock *tp,
                 int nonagle)
 {
     return partial &&
         ((nonagle & TCP_NAGLE_CORK) ||
          (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
 }
 
 /* Return how many segs we'd like on a TSO packet,
  * depending on current pacing rate, and how close the peer is.
  *
  * Rationale is:
  * - For close peers, we rather send bigger packets to reduce
  *   cpu costs, because occasional losses will be repaired fast.
  * - For long distance/rtt flows, we would like to get ACK clocking
  *   with 1 ACK per ms.
  *
  * Use min_rtt to help adapt TSO burst size, with smaller min_rtt resulting
  * in bigger TSO bursts. We we cut the RTT-based allowance in half
  * for every 2^9 usec (aka 512 us) of RTT, so that the RTT-based allowance
  * is below 1500 bytes after 6 * ~500 usec = 3ms.
  */
 static u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
                 int min_tso_segs)
 {
     unsigned long bytes;
     u32 r;
 
     bytes = sk->sk_pacing_rate >> READ_ONCE(sk->sk_pacing_shift);
 
     r = tcp_min_rtt(tcp_sk(sk)) >> READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_rtt_log);
     if (r < BITS_PER_TYPE(sk->sk_gso_max_size))
         bytes += sk->sk_gso_max_size >> r;
 
     bytes = min_t(unsigned long, bytes, sk->sk_gso_max_size);
 
     return max_t(u32, bytes / mss_now, min_tso_segs);
 }
 
 /* Return the number of segments we want in the skb we are transmitting.
  * See if congestion control module wants to decide; otherwise, autosize.
  */
 static u32 tcp_tso_segs(struct sock *sk, unsigned int mss_now)
 {
     const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
     u32 min_tso, tso_segs;
 
     min_tso = ca_ops->min_tso_segs ?
             ca_ops->min_tso_segs(sk) :
             READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_tso_segs);
 
     tso_segs = tcp_tso_autosize(sk, mss_now, min_tso);
     return min_t(u32, tso_segs, sk->sk_gso_max_segs);
 }
 
 /* Returns the portion of skb which can be sent right away */
 static unsigned int tcp_mss_split_point(const struct sock *sk,
                     const struct sk_buff *skb,
                     unsigned int mss_now,
                     unsigned int max_segs,
                     int nonagle)
 {
     const struct tcp_sock *tp = tcp_sk(sk);
     u32 partial, needed, window, max_len;
 
     window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
     max_len = mss_now * max_segs;
 
     if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
         return max_len;
 
     needed = min(skb->len, window);
 
     if (max_len <= needed)
         return max_len;
 
     partial = needed % mss_now;
     /* If last segment is not a full MSS, check if Nagle rules allow us
      * to include this last segment in this skb.
      * Otherwise, we'll split the skb at last MSS boundary
      */
     if (tcp_nagle_check(partial != 0, tp, nonagle))
         return needed - partial;
 
     return needed;
 }
 
 /* Can at least one segment of SKB be sent right now, according to the
  * congestion window rules?  If so, return how many segments are allowed.
  */
 static inline unsigned int tcp_cwnd_test(const struct tcp_sock *tp,
                      const struct sk_buff *skb)
 {
     u32 in_flight, cwnd, halfcwnd;
 
     /* Don't be strict about the congestion window for the final FIN.  */
     if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
         tcp_skb_pcount(skb) == 1)
         return 1;
 
     in_flight = tcp_packets_in_flight(tp);
     cwnd = tcp_snd_cwnd(tp);
     if (in_flight >= cwnd)
         return 0;
 
     /* For better scheduling, ensure we have at least
      * 2 GSO packets in flight.
      */
     halfcwnd = max(cwnd >> 1, 1U);
     return min(halfcwnd, cwnd - in_flight);
 }
 
 /* Initialize TSO state of a skb.
  * This must be invoked the first time we consider transmitting
  * SKB onto the wire.
  */
 static int tcp_init_tso_segs(struct sk_buff *skb, unsigned int mss_now)
 {
     int tso_segs = tcp_skb_pcount(skb);
 
     if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
         tcp_set_skb_tso_segs(skb, mss_now);
         tso_segs = tcp_skb_pcount(skb);
     }
     return tso_segs;
 }
 
 
 /* Return true if the Nagle test allows this packet to be
  * sent now.
  */
 static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
                   unsigned int cur_mss, int nonagle)
 {
     /* Nagle rule does not apply to frames, which sit in the middle of the
      * write_queue (they have no chances to get new data).
      *
      * This is implemented in the callers, where they modify the 'nonagle'
      * argument based upon the location of SKB in the send queue.
      */
     if (nonagle & TCP_NAGLE_PUSH)
         return true;
 
     /* Don't use the nagle rule for urgent data (or for the final FIN). */
     if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
         return true;
 
     if (!tcp_nagle_check(skb->len < cur_mss, tp, nonagle))
         return true;
 
     return false;
 }
 
 /* Does at least the first segment of SKB fit into the send window? */
 static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
                  const struct sk_buff *skb,
                  unsigned int cur_mss)
 {
     u32 end_seq = TCP_SKB_CB(skb)->end_seq;
 
     if (skb->len > cur_mss)
         end_seq = TCP_SKB_CB(skb)->seq + cur_mss;
 
     return !after(end_seq, tcp_wnd_end(tp));
 }
 
 /* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
  * which is put after SKB on the list.  It is very much like
  * tcp_fragment() except that it may make several kinds of assumptions
  * in order to speed up the splitting operation.  In particular, we
  * know that all the data is in scatter-gather pages, and that the
  * packet has never been sent out before (and thus is not cloned).
  */
 static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
             unsigned int mss_now, gfp_t gfp)
 {
     int nlen = skb->len - len;
     struct sk_buff *buff;
     u8 flags;
 
     /* All of a TSO frame must be composed of paged data.  */
     if (skb->len != skb->data_len)
         return tcp_fragment(sk, TCP_FRAG_IN_WRITE_QUEUE,
                     skb, len, mss_now, gfp);
 
     buff = tcp_stream_alloc_skb(sk, 0, gfp, true);
     if (unlikely(!buff))
         return -ENOMEM;
     skb_copy_decrypted(buff, skb);
     mptcp_skb_ext_copy(buff, skb);
 
     sk_wmem_queued_add(sk, buff->truesize);
     sk_mem_charge(sk, buff->truesize);
     buff->truesize += nlen;
     skb->truesize -= nlen;
 
     /* Correct the sequence numbers. */
     TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
     TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
     TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
 
     /* PSH and FIN should only be set in the second packet. */
     flags = TCP_SKB_CB(skb)->tcp_flags;
     TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
     TCP_SKB_CB(buff)->tcp_flags = flags;
 
     tcp_skb_fragment_eor(skb, buff);
 
     skb_split(skb, buff, len);
     tcp_fragment_tstamp(skb, buff);
 
     /* Fix up tso_factor for both original and new SKB.  */
     tcp_set_skb_tso_segs(skb, mss_now);
     tcp_set_skb_tso_segs(buff, mss_now);
 
     /* Link BUFF into the send queue. */
     __skb_header_release(buff);
     tcp_insert_write_queue_after(skb, buff, sk, TCP_FRAG_IN_WRITE_QUEUE);
 
     return 0;
 }
 
 /* Try to defer sending, if possible, in order to minimize the amount
  * of TSO splitting we do.  View it as a kind of TSO Nagle test.
  *
  * This algorithm is from John Heffner.
  */
 static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb,
                  bool *is_cwnd_limited,
                  bool *is_rwnd_limited,
                  u32 max_segs)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     u32 send_win, cong_win, limit, in_flight;
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *head;
     int win_divisor;
     s64 delta;
 
     if (icsk->icsk_ca_state >= TCP_CA_Recovery)
         goto send_now;
 
     /* Avoid bursty behavior by allowing defer
      * only if the last write was recent (1 ms).
      * Note that tp->tcp_wstamp_ns can be in the future if we have
      * packets waiting in a qdisc or device for EDT delivery.
      */
     delta = tp->tcp_clock_cache - tp->tcp_wstamp_ns - NSEC_PER_MSEC;
     if (delta > 0)
         goto send_now;
 
     in_flight = tcp_packets_in_flight(tp);
 
     BUG_ON(tcp_skb_pcount(skb) <= 1);
     BUG_ON(tcp_snd_cwnd(tp) <= in_flight);
 
     send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
 
     /* From in_flight test above, we know that cwnd > in_flight.  */
     cong_win = (tcp_snd_cwnd(tp) - in_flight) * tp->mss_cache;
 
     limit = min(send_win, cong_win);
 
     /* If a full-sized TSO skb can be sent, do it. */
     if (limit >= max_segs * tp->mss_cache)
         goto send_now;
 
     /* Middle in queue won't get any more data, full sendable already? */
     if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
         goto send_now;
 
     win_divisor = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_win_divisor);
     if (win_divisor) {
         u32 chunk = min(tp->snd_wnd, tcp_snd_cwnd(tp) * tp->mss_cache);
 
         /* If at least some fraction of a window is available,
          * just use it.
          */
         chunk /= win_divisor;
         if (limit >= chunk)
             goto send_now;
     } else {
         /* Different approach, try not to defer past a single
          * ACK.  Receiver should ACK every other full sized
          * frame, so if we have space for more than 3 frames
          * then send now.
          */
         if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
             goto send_now;
     }
 
     /* TODO : use tsorted_sent_queue ? */
     head = tcp_rtx_queue_head(sk);
     if (!head)
         goto send_now;
     delta = tp->tcp_clock_cache - head->tstamp;
     /* If next ACK is likely to come too late (half srtt), do not defer */
     if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0)
         goto send_now;
 
     /* Ok, it looks like it is advisable to defer.
      * Three cases are tracked :
      * 1) We are cwnd-limited
      * 2) We are rwnd-limited
      * 3) We are application limited.
      */
     if (cong_win < send_win) {
         if (cong_win <= skb->len) {
             *is_cwnd_limited = true;
             return true;
         }
     } else {
         if (send_win <= skb->len) {
             *is_rwnd_limited = true;
             return true;
         }
     }
 
     /* If this packet won't get more data, do not wait. */
     if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) ||
         TCP_SKB_CB(skb)->eor)
         goto send_now;
 
     return true;
 
 send_now:
     return false;
 }
 
 static inline void tcp_mtu_check_reprobe(struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct net *net = sock_net(sk);
     u32 interval;
     s32 delta;
 
     interval = READ_ONCE(net->ipv4.sysctl_tcp_probe_interval);
     delta = tcp_jiffies32 - icsk->icsk_mtup.probe_timestamp;
     if (unlikely(delta >= interval * HZ)) {
         int mss = tcp_current_mss(sk);
 
         /* Update current search range */
         icsk->icsk_mtup.probe_size = 0;
         icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp +
             sizeof(struct tcphdr) +
             icsk->icsk_af_ops->net_header_len;
         icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
 
         /* Update probe time stamp */
         icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
     }
 }
 
 static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
 {
     struct sk_buff *skb, *next;
 
     skb = tcp_send_head(sk);
     tcp_for_write_queue_from_safe(skb, next, sk) {
         if (len <= skb->len)
             break;
 
         if (unlikely(TCP_SKB_CB(skb)->eor) ||
             tcp_has_tx_tstamp(skb) ||
             !skb_pure_zcopy_same(skb, next))
             return false;
 
         len -= skb->len;
     }
 
     return true;
 }
 
 /* Create a new MTU probe if we are ready.
  * MTU probe is regularly attempting to increase the path MTU by
  * deliberately sending larger packets.  This discovers routing
  * changes resulting in larger path MTUs.
  *
  * Returns 0 if we should wait to probe (no cwnd available),
  *         1 if a probe was sent,
  *         -1 otherwise
  */
 static int tcp_mtu_probe(struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb, *nskb, *next;
     struct net *net = sock_net(sk);
     int probe_size;
     int size_needed;
     int copy, len;
     int mss_now;
     int interval;
 
     /* Not currently probing/verifying,
      * not in recovery,
      * have enough cwnd, and
      * not SACKing (the variable headers throw things off)
      */
     if (likely(!icsk->icsk_mtup.enabled ||
            icsk->icsk_mtup.probe_size ||
            inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
            tcp_snd_cwnd(tp) < 11 ||
            tp->rx_opt.num_sacks || tp->rx_opt.dsack))
         return -1;
 
     /* Use binary search for probe_size between tcp_mss_base,
      * and current mss_clamp. if (search_high - search_low)
      * smaller than a threshold, backoff from probing.
      */
     mss_now = tcp_current_mss(sk);
     probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
                     icsk->icsk_mtup.search_low) >> 1);
     size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
     interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
     /* When misfortune happens, we are reprobing actively,
      * and then reprobe timer has expired. We stick with current
      * probing process by not resetting search range to its orignal.
      */
     if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
         interval < READ_ONCE(net->ipv4.sysctl_tcp_probe_threshold)) {
         /* Check whether enough time has elaplased for
          * another round of probing.
          */
         tcp_mtu_check_reprobe(sk);
         return -1;
     }
 
     /* Have enough data in the send queue to probe? */
     if (tp->write_seq - tp->snd_nxt < size_needed)
         return -1;
 
     if (tp->snd_wnd < size_needed)
         return -1;
     if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
         return 0;
 
     /* Do we need to wait to drain cwnd? With none in flight, don't stall */
     if (tcp_packets_in_flight(tp) + 2 > tcp_snd_cwnd(tp)) {
         if (!tcp_packets_in_flight(tp))
             return -1;
         else
             return 0;
     }
 
     if (!tcp_can_coalesce_send_queue_head(sk, probe_size))
         return -1;
 
     /* We're allowed to probe.  Build it now. */
     nskb = tcp_stream_alloc_skb(sk, probe_size, GFP_ATOMIC, false);
     if (!nskb)
         return -1;
     sk_wmem_queued_add(sk, nskb->truesize);
     sk_mem_charge(sk, nskb->truesize);
 
     skb = tcp_send_head(sk);
     skb_copy_decrypted(nskb, skb);
     mptcp_skb_ext_copy(nskb, skb);
 
     TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
     TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
     TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;
 
     tcp_insert_write_queue_before(nskb, skb, sk);
     tcp_highest_sack_replace(sk, skb, nskb);
 
     len = 0;
     tcp_for_write_queue_from_safe(skb, next, sk) {
         copy = min_t(int, skb->len, probe_size - len);
         skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
 
         if (skb->len <= copy) {
             /* We've eaten all the data from this skb.
              * Throw it away. */
             TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
             /* If this is the last SKB we copy and eor is set
              * we need to propagate it to the new skb.
              */
             TCP_SKB_CB(nskb)->eor = TCP_SKB_CB(skb)->eor;
             tcp_skb_collapse_tstamp(nskb, skb);
             tcp_unlink_write_queue(skb, sk);
             tcp_wmem_free_skb(sk, skb);
         } else {
             TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
                            ~(TCPHDR_FIN|TCPHDR_PSH);
             if (!skb_shinfo(skb)->nr_frags) {
                 skb_pull(skb, copy);
             } else {
                 __pskb_trim_head(skb, copy);
                 tcp_set_skb_tso_segs(skb, mss_now);
             }
             TCP_SKB_CB(skb)->seq += copy;
         }
 
         len += copy;
 
         if (len >= probe_size)
             break;
     }
     tcp_init_tso_segs(nskb, nskb->len);
 
     /* We're ready to send.  If this fails, the probe will
      * be resegmented into mss-sized pieces by tcp_write_xmit().
      */
     if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
         /* Decrement cwnd here because we are sending
          * effectively two packets. */
         tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1);
         tcp_event_new_data_sent(sk, nskb);
 
         icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
         tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
         tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;
 
         return 1;
     }
 
     return -1;
 }
 
 static bool tcp_pacing_check(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     if (!tcp_needs_internal_pacing(sk))
         return false;
 
     if (tp->tcp_wstamp_ns <= tp->tcp_clock_cache)
         return false;
 
     if (!hrtimer_is_queued(&tp->pacing_timer)) {
         hrtimer_start(&tp->pacing_timer,
                   ns_to_ktime(tp->tcp_wstamp_ns),
                   HRTIMER_MODE_ABS_PINNED_SOFT);
         sock_hold(sk);
     }
     return true;
 }
 
 /* TCP Small Queues :
  * Control number of packets in qdisc/devices to two packets / or ~1 ms.
  * (These limits are doubled for retransmits)
  * This allows for :
  *  - better RTT estimation and ACK scheduling
  *  - faster recovery
  *  - high rates
  * Alas, some drivers / subsystems require a fair amount
  * of queued bytes to ensure line rate.
  * One example is wifi aggregation (802.11 AMPDU)
  */
 static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
                   unsigned int factor)
 {
     unsigned long limit;
 
     limit = max_t(unsigned long,
               2 * skb->truesize,
               sk->sk_pacing_rate >> READ_ONCE(sk->sk_pacing_shift));
     if (sk->sk_pacing_status == SK_PACING_NONE)
         limit = min_t(unsigned long, limit,
                   READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes));
     limit <<= factor;
 
     if (static_branch_unlikely(&tcp_tx_delay_enabled) &&
         tcp_sk(sk)->tcp_tx_delay) {
         u64 extra_bytes = (u64)sk->sk_pacing_rate * tcp_sk(sk)->tcp_tx_delay;
 
         /* TSQ is based on skb truesize sum (sk_wmem_alloc), so we
          * approximate our needs assuming an ~100% skb->truesize overhead.
          * USEC_PER_SEC is approximated by 2^20.
          * do_div(extra_bytes, USEC_PER_SEC/2) is replaced by a right shift.
          */
         extra_bytes >>= (20 - 1);
         limit += extra_bytes;
     }
     if (refcount_read(&sk->sk_wmem_alloc) > limit) {
         /* Always send skb if rtx queue is empty.
          * No need to wait for TX completion to call us back,
          * after softirq/tasklet schedule.
          * This helps when TX completions are delayed too much.
          */
         if (tcp_rtx_queue_empty(sk))
             return false;
 
         set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
         /* It is possible TX completion already happened
          * before we set TSQ_THROTTLED, so we must
          * test again the condition.
          */
         smp_mb__after_atomic();
         if (refcount_read(&sk->sk_wmem_alloc) > limit)
             return true;
     }
     return false;
 }
 
 static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
 {
     const u32 now = tcp_jiffies32;
     enum tcp_chrono old = tp->chrono_type;
 
     if (old > TCP_CHRONO_UNSPEC)
         tp->chrono_stat[old - 1] += now - tp->chrono_start;
     tp->chrono_start = now;
     tp->chrono_type = new;
 }
 
 void tcp_chrono_start(struct sock *sk, const enum tcp_chrono type)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* If there are multiple conditions worthy of tracking in a
      * chronograph then the highest priority enum takes precedence
      * over the other conditions. So that if something "more interesting"
      * starts happening, stop the previous chrono and start a new one.
      */
     if (type > tp->chrono_type)
         tcp_chrono_set(tp, type);
 }
 
 void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
 {
     struct tcp_sock *tp = tcp_sk(sk);
 
 
     /* There are multiple conditions worthy of tracking in a
      * chronograph, so that the highest priority enum takes
      * precedence over the other conditions (see tcp_chrono_start).
      * If a condition stops, we only stop chrono tracking if
      * it's the "most interesting" or current chrono we are
      * tracking and starts busy chrono if we have pending data.
      */
     if (tcp_rtx_and_write_queues_empty(sk))
         tcp_chrono_set(tp, TCP_CHRONO_UNSPEC);
     else if (type == tp->chrono_type)
         tcp_chrono_set(tp, TCP_CHRONO_BUSY);
 }
 
 /* This routine writes packets to the network.  It advances the
  * send_head.  This happens as incoming acks open up the remote
  * window for us.
  *
  * LARGESEND note: !tcp_urg_mode is overkill, only frames between
  * snd_up-64k-mss .. snd_up cannot be large. However, taking into
  * account rare use of URG, this is not a big flaw.
  *
  * Send at most one packet when push_one > 0. Temporarily ignore
  * cwnd limit to force at most one packet out when push_one == 2.
 
  * Returns true, if no segments are in flight and we have queued segments,
  * but cannot send anything now because of SWS or another problem.
  */
 static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                int push_one, gfp_t gfp)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb;
     unsigned int tso_segs, sent_pkts;
     int cwnd_quota;
     int result;
     bool is_cwnd_limited = false, is_rwnd_limited = false;
     u32 max_segs;
 
     sent_pkts = 0;
 
     tcp_mstamp_refresh(tp);
     if (!push_one) {
         /* Do MTU probing. */
         result = tcp_mtu_probe(sk);
         if (!result) {
             return false;
         } else if (result > 0) {
             sent_pkts = 1;
         }
     }
 
     max_segs = tcp_tso_segs(sk, mss_now);
     while ((skb = tcp_send_head(sk))) {
         unsigned int limit;
 
         if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
             /* "skb_mstamp_ns" is used as a start point for the retransmit timer */
             tp->tcp_wstamp_ns = tp->tcp_clock_cache;
             skb_set_delivery_time(skb, tp->tcp_wstamp_ns, true);
             list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
             tcp_init_tso_segs(skb, mss_now);
             goto repair; /* Skip network transmission */
         }
 
         if (tcp_pacing_check(sk))
             break;
 
         tso_segs = tcp_init_tso_segs(skb, mss_now);
         BUG_ON(!tso_segs);
 
         cwnd_quota = tcp_cwnd_test(tp, skb);
         if (!cwnd_quota) {
             if (push_one == 2)
                 /* Force out a loss probe pkt. */
                 cwnd_quota = 1;
             else
                 break;
         }
 
         if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
             is_rwnd_limited = true;
             break;
         }
 
         if (tso_segs == 1) {
             if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
                              (tcp_skb_is_last(sk, skb) ?
                               nonagle : TCP_NAGLE_PUSH))))
                 break;
         } else {
             if (!push_one &&
                 tcp_tso_should_defer(sk, skb, &is_cwnd_limited,
                          &is_rwnd_limited, max_segs))
                 break;
         }
 
         limit = mss_now;
         if (tso_segs > 1 && !tcp_urg_mode(tp))
             limit = tcp_mss_split_point(sk, skb, mss_now,
                             min_t(unsigned int,
                               cwnd_quota,
                               max_segs),
                             nonagle);
 
         if (skb->len > limit &&
             unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
             break;
 
         if (tcp_small_queue_check(sk, skb, 0))
             break;
 
         /* Argh, we hit an empty skb(), presumably a thread
          * is sleeping in sendmsg()/sk_stream_wait_memory().
          * We do not want to send a pure-ack packet and have
          * a strange looking rtx queue with empty packet(s).
          */
         if (TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq)
             break;
 
         if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
             break;
 
 repair:
         /* Advance the send_head.  This one is sent out.
          * This call will increment packets_out.
          */
         tcp_event_new_data_sent(sk, skb);
 
         tcp_minshall_update(tp, mss_now, skb);
         sent_pkts += tcp_skb_pcount(skb);
 
         if (push_one)
             break;
     }
 
     if (is_rwnd_limited)
         tcp_chrono_start(sk, TCP_CHRONO_RWND_LIMITED);
     else
         tcp_chrono_stop(sk, TCP_CHRONO_RWND_LIMITED);
 
     is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp));
     if (likely(sent_pkts || is_cwnd_limited))
         tcp_cwnd_validate(sk, is_cwnd_limited);
 
     if (likely(sent_pkts)) {
         if (tcp_in_cwnd_reduction(sk))
             tp->prr_out += sent_pkts;
 
         /* Send one loss probe per tail loss episode. */
         if (push_one != 2)
             tcp_schedule_loss_probe(sk, false);
         return false;
     }
     return !tp->packets_out && !tcp_write_queue_empty(sk);
 }
 
 bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     u32 timeout, rto_delta_us;
     int early_retrans;
 
     /* Don't do any loss probe on a Fast Open connection before 3WHS
      * finishes.
      */
     if (rcu_access_pointer(tp->fastopen_rsk))
         return false;
 
     early_retrans = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_early_retrans);
     /* Schedule a loss probe in 2*RTT for SACK capable connections
      * not in loss recovery, that are either limited by cwnd or application.
      */
     if ((early_retrans != 3 && early_retrans != 4) ||
         !tp->packets_out || !tcp_is_sack(tp) ||
         (icsk->icsk_ca_state != TCP_CA_Open &&
          icsk->icsk_ca_state != TCP_CA_CWR))
         return false;
 
     /* Probe timeout is 2*rtt. Add minimum RTO to account
      * for delayed ack when there's one outstanding packet. If no RTT
      * sample is available then probe after TCP_TIMEOUT_INIT.
      */
     if (tp->srtt_us) {
         timeout = usecs_to_jiffies(tp->srtt_us >> 2);
         if (tp->packets_out == 1)
             timeout += TCP_RTO_MIN;
         else
             timeout += TCP_TIMEOUT_MIN;
     } else {
         timeout = TCP_TIMEOUT_INIT;
     }
 
     /* If the RTO formula yields an earlier time, then use that time. */
     rto_delta_us = advancing_rto ?
             jiffies_to_usecs(inet_csk(sk)->icsk_rto) :
             tcp_rto_delta_us(sk);  /* How far in future is RTO? */
     if (rto_delta_us > 0)
         timeout = min_t(u32, timeout, usecs_to_jiffies(rto_delta_us));
 
     tcp_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout, TCP_RTO_MAX);
     return true;
 }
 
 /* Thanks to skb fast clones, we can detect if a prior transmit of
  * a packet is still in a qdisc or driver queue.
  * In this case, there is very little point doing a retransmit !
  */
 static bool skb_still_in_host_queue(struct sock *sk,
                     const struct sk_buff *skb)
 {
     if (unlikely(skb_fclone_busy(sk, skb))) {
         set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
         smp_mb__after_atomic();
         if (skb_fclone_busy(sk, skb)) {
             NET_INC_STATS(sock_net(sk),
                       LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
             return true;
         }
     }
     return false;
 }
 
 /* When probe timeout (PTO) fires, try send a new segment if possible, else
  * retransmit the last segment.
  */
 void tcp_send_loss_probe(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb;
     int pcount;
     int mss = tcp_current_mss(sk);
 
     /* At most one outstanding TLP */
     if (tp->tlp_high_seq)
         goto rearm_timer;
 
     tp->tlp_retrans = 0;
     skb = tcp_send_head(sk);
     if (skb && tcp_snd_wnd_test(tp, skb, mss)) {
         pcount = tp->packets_out;
         tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
         if (tp->packets_out > pcount)
             goto probe_sent;
         goto rearm_timer;
     }
     skb = skb_rb_last(&sk->tcp_rtx_queue);
     if (unlikely(!skb)) {
         WARN_ONCE(tp->packets_out,
               "invalid inflight: %u state %u cwnd %u mss %d\n",
               tp->packets_out, sk->sk_state, tcp_snd_cwnd(tp), mss);
         inet_csk(sk)->icsk_pending = 0;
         return;
     }
 
     if (skb_still_in_host_queue(sk, skb))
         goto rearm_timer;
 
     pcount = tcp_skb_pcount(skb);
     if (WARN_ON(!pcount))
         goto rearm_timer;
 
     if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
         if (unlikely(tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
                       (pcount - 1) * mss, mss,
                       GFP_ATOMIC)))
             goto rearm_timer;
         skb = skb_rb_next(skb);
     }
 
     if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
         goto rearm_timer;
 
     if (__tcp_retransmit_skb(sk, skb, 1))
         goto rearm_timer;
 
     tp->tlp_retrans = 1;
 
 probe_sent:
     /* Record snd_nxt for loss detection. */
     tp->tlp_high_seq = tp->snd_nxt;
 
     NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSPROBES);
     /* Reset s.t. tcp_rearm_rto will restart timer from now */
     inet_csk(sk)->icsk_pending = 0;
 rearm_timer:
     tcp_rearm_rto(sk);
 }
 
 /* Push out any pending frames which were held back due to
  * TCP_CORK or attempt at coalescing tiny packets.
  * The socket must be locked by the caller.
  */
 void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
                    int nonagle)
 {
     /* If we are closed, the bytes will have to remain here.
      * In time closedown will finish, we empty the write queue and
      * all will be happy.
      */
     if (unlikely(sk->sk_state == TCP_CLOSE))
         return;
 
     if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
                sk_gfp_mask(sk, GFP_ATOMIC)))
         tcp_check_probe_timer(sk);
 }
 
 /* Send _single_ skb sitting at the send head. This function requires
  * true push pending frames to setup probe timer etc.
  */
 void tcp_push_one(struct sock *sk, unsigned int mss_now)
 {
     struct sk_buff *skb = tcp_send_head(sk);
 
     BUG_ON(!skb || skb->len < mss_now);
 
     tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
 }
 
 /* This function returns the amount that we can raise the
  * usable window based on the following constraints
  *
  * 1. The window can never be shrunk once it is offered (RFC 793)
  * 2. We limit memory per socket
  *
  * RFC 1122:
  * "the suggested [SWS] avoidance algorithm for the receiver is to keep
  *  RECV.NEXT + RCV.WIN fixed until:
  *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
  *
  * i.e. don't raise the right edge of the window until you can raise
  * it at least MSS bytes.
  *
  * Unfortunately, the recommended algorithm breaks header prediction,
  * since header prediction assumes th->window stays fixed.
  *
  * Strictly speaking, keeping th->window fixed violates the receiver
  * side SWS prevention criteria. The problem is that under this rule
  * a stream of single byte packets will cause the right side of the
  * window to always advance by a single byte.
  *
  * Of course, if the sender implements sender side SWS prevention
  * then this will not be a problem.
  *
  * BSD seems to make the following compromise:
  *
  *  If the free space is less than the 1/4 of the maximum
  *  space available and the free space is less than 1/2 mss,
  *  then set the window to 0.
  *  [ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
  *  Otherwise, just prevent the window from shrinking
  *  and from being larger than the largest representable value.
  *
  * This prevents incremental opening of the window in the regime
  * where TCP is limited by the speed of the reader side taking
  * data out of the TCP receive queue. It does nothing about
  * those cases where the window is constrained on the sender side
  * because the pipeline is full.
  *
  * BSD also seems to "accidentally" limit itself to windows that are a
  * multiple of MSS, at least until the free space gets quite small.
  * This would appear to be a side effect of the mbuf implementation.
  * Combining these two algorithms results in the observed behavior
  * of having a fixed window size at almost all times.
  *
  * Below we obtain similar behavior by forcing the offered window to
  * a multiple of the mss when it is feasible to do so.
  *
  * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
  * Regular options like TIMESTAMP are taken into account.
  */
 u32 __tcp_select_window(struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     /* MSS for the peer's data.  Previous versions used mss_clamp
      * here.  I don't know if the value based on our guesses
      * of peer's MSS is better for the performance.  It's more correct
      * but may be worse for the performance because of rcv_mss
      * fluctuations.  --SAW  1998/11/1
      */
     int mss = icsk->icsk_ack.rcv_mss;
     int free_space = tcp_space(sk);
     int allowed_space = tcp_full_space(sk);
     int full_space, window;
 
     if (sk_is_mptcp(sk))
         mptcp_space(sk, &free_space, &allowed_space);
 
     full_space = min_t(int, tp->window_clamp, allowed_space);
 
     if (unlikely(mss > full_space)) {
         mss = full_space;
         if (mss <= 0)
             return 0;
     }
     if (free_space < (full_space >> 1)) {
         icsk->icsk_ack.quick = 0;
 
         if (tcp_under_memory_pressure(sk))
             tcp_adjust_rcv_ssthresh(sk);
 
         /* free_space might become our new window, make sure we don't
          * increase it due to wscale.
          */
         free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
 
         /* if free space is less than mss estimate, or is below 1/16th
          * of the maximum allowed, try to move to zero-window, else
          * tcp_clamp_window() will grow rcv buf up to tcp_rmem[2], and
          * new incoming data is dropped due to memory limits.
          * With large window, mss test triggers way too late in order
          * to announce zero window in time before rmem limit kicks in.
          */
         if (free_space < (allowed_space >> 4) || free_space < mss)
             return 0;
     }
 
     if (free_space > tp->rcv_ssthresh)
         free_space = tp->rcv_ssthresh;
 
     /* Don't do rounding if we are using window scaling, since the
      * scaled window will not line up with the MSS boundary anyway.
      */
     if (tp->rx_opt.rcv_wscale) {
         window = free_space;
 
         /* Advertise enough space so that it won't get scaled away.
          * Import case: prevent zero window announcement if
          * 1<<rcv_wscale > mss.
          */
         window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
     } else {
         window = tp->rcv_wnd;
         /* Get the largest window that is a nice multiple of mss.
          * Window clamp already applied above.
          * If our current window offering is within 1 mss of the
          * free space we just keep it. This prevents the divide
          * and multiply from happening most of the time.
          * We also don't do any window rounding when the free space
          * is too small.
          */
         if (window <= free_space - mss || window > free_space)
             window = rounddown(free_space, mss);
         else if (mss == full_space &&
              free_space > window + (full_space >> 1))
             window = free_space;
     }
 
     return window;
 }
 
 void tcp_skb_collapse_tstamp(struct sk_buff *skb,
                  const struct sk_buff *next_skb)
 {
     if (unlikely(tcp_has_tx_tstamp(next_skb))) {
         const struct skb_shared_info *next_shinfo =
             skb_shinfo(next_skb);
         struct skb_shared_info *shinfo = skb_shinfo(skb);
 
         shinfo->tx_flags |= next_shinfo->tx_flags & SKBTX_ANY_TSTAMP;
         shinfo->tskey = next_shinfo->tskey;
         TCP_SKB_CB(skb)->txstamp_ack |=
             TCP_SKB_CB(next_skb)->txstamp_ack;
     }
 }
 
 /* Collapses two adjacent SKB's during retransmission. */
 static bool tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *next_skb = skb_rb_next(skb);
     int next_skb_size;
 
     next_skb_size = next_skb->len;
 
     BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);
 
     if (next_skb_size && !tcp_skb_shift(skb, next_skb, 1, next_skb_size))
         return false;
 
     tcp_highest_sack_replace(sk, next_skb, skb);
 
     /* Update sequence range on original skb. */
     TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
 
     /* Merge over control information. This moves PSH/FIN etc. over */
     TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;
 
     /* All done, get rid of second SKB and account for it so
      * packet counting does not break.
      */
     TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;
     TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
 
     /* changed transmit queue under us so clear hints */
     tcp_clear_retrans_hints_partial(tp);
     if (next_skb == tp->retransmit_skb_hint)
         tp->retransmit_skb_hint = skb;
 
     tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));
 
     tcp_skb_collapse_tstamp(skb, next_skb);
 
     tcp_rtx_queue_unlink_and_free(next_skb, sk);
     return true;
 }
 
 /* Check if coalescing SKBs is legal. */
 static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
 {
     if (tcp_skb_pcount(skb) > 1)
         return false;
     if (skb_cloned(skb))
         return false;
     /* Some heuristics for collapsing over SACK'd could be invented */
     if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
         return false;
 
     return true;
 }
 
 /* Collapse packets in the retransmit queue to make to create
  * less packets on the wire. This is only done on retransmission.
  */
 static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
                      int space)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb = to, *tmp;
     bool first = true;
 
     if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retrans_collapse))
         return;
     if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
         return;
 
     skb_rbtree_walk_from_safe(skb, tmp) {
         if (!tcp_can_collapse(sk, skb))
             break;
 
         if (!tcp_skb_can_collapse(to, skb))
             break;
 
         space -= skb->len;
 
         if (first) {
             first = false;
             continue;
         }
 
         if (space < 0)
             break;
 
         if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
             break;
 
         if (!tcp_collapse_retrans(sk, to))
             break;
     }
 }
 
 /* This retransmits one SKB.  Policy decisions and retransmit queue
  * state updates are done by the caller.  Returns non-zero if an
  * error occurred which prevented the send.
  */
 int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     unsigned int cur_mss;
     int diff, len, err;
     int avail_wnd;
 
     /* Inconclusive MTU probe */
     if (icsk->icsk_mtup.probe_size)
         icsk->icsk_mtup.probe_size = 0;
 
     if (skb_still_in_host_queue(sk, skb))
         return -EBUSY;
 
     if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
         if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
             WARN_ON_ONCE(1);
             return -EINVAL;
         }
         if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
             return -ENOMEM;
     }
 
     if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
         return -EHOSTUNREACH; /* Routing failure or similar. */
 
     cur_mss = tcp_current_mss(sk);
     avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
 
     /* If receiver has shrunk his window, and skb is out of
      * new window, do not retransmit it. The exception is the
      * case, when window is shrunk to zero. In this case
      * our retransmit of one segment serves as a zero window probe.
      */
     if (avail_wnd <= 0) {
         if (TCP_SKB_CB(skb)->seq != tp->snd_una)
             return -EAGAIN;
         avail_wnd = cur_mss;
     }
 
     len = cur_mss * segs;
     if (len > avail_wnd) {
         len = rounddown(avail_wnd, cur_mss);
         if (!len)
             len = avail_wnd;
     }
     if (skb->len > len) {
         if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
                  cur_mss, GFP_ATOMIC))
             return -ENOMEM; /* We'll try again later. */
     } else {
         if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
             return -ENOMEM;
 
         diff = tcp_skb_pcount(skb);
         tcp_set_skb_tso_segs(skb, cur_mss);
         diff -= tcp_skb_pcount(skb);
         if (diff)
             tcp_adjust_pcount(sk, skb, diff);
         avail_wnd = min_t(int, avail_wnd, cur_mss);
         if (skb->len < avail_wnd)
             tcp_retrans_try_collapse(sk, skb, avail_wnd);
     }
 
     /* RFC3168, section 6.1.1.1. ECN fallback */
     if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
         tcp_ecn_clear_syn(sk, skb);
 
     /* Update global and local TCP statistics. */
     segs = tcp_skb_pcount(skb);
     TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
     if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
         __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
     tp->total_retrans += segs;
     tp->bytes_retrans += skb->len;
 
     /* make sure skb->data is aligned on arches that require it
      * and check if ack-trimming & collapsing extended the headroom
      * beyond what csum_start can cover.
      */
     if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
              skb_headroom(skb) >= 0xFFFF)) {
         struct sk_buff *nskb;
 
         tcp_skb_tsorted_save(skb) {
             nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
             if (nskb) {
                 nskb->dev = NULL;
                 err = tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC);
             } else {
                 err = -ENOBUFS;
             }
         } tcp_skb_tsorted_restore(skb);
 
         if (!err) {
             tcp_update_skb_after_send(sk, skb, tp->tcp_wstamp_ns);
             tcp_rate_skb_sent(sk, skb);
         }
     } else {
         err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
     }
 
     /* To avoid taking spuriously low RTT samples based on a timestamp
      * for a transmit that never happened, always mark EVER_RETRANS
      */
     TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
 
     if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
         tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
                   TCP_SKB_CB(skb)->seq, segs, err);
 
     if (likely(!err)) {
         trace_tcp_retransmit_skb(sk, skb);
     } else if (err != -EBUSY) {
         NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);
     }
     return err;
 }
 
 int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     int err = __tcp_retransmit_skb(sk, skb, segs);
 
     if (err == 0) {
 #if FASTRETRANS_DEBUG > 0
         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
             net_dbg_ratelimited("retrans_out leaked\n");
         }
 #endif
         TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
         tp->retrans_out += tcp_skb_pcount(skb);
     }
 
     /* Save stamp of the first (attempted) retransmit. */
     if (!tp->retrans_stamp)
         tp->retrans_stamp = tcp_skb_timestamp(skb);
 
     if (tp->undo_retrans < 0)
         tp->undo_retrans = 0;
     tp->undo_retrans += tcp_skb_pcount(skb);
     return err;
 }
 
 /* This gets called after a retransmit timeout, and the initially
  * retransmitted data is acknowledged.  It tries to continue
  * resending the rest of the retransmit queue, until either
  * we've sent it all or the congestion window limit is reached.
  */
 void tcp_xmit_retransmit_queue(struct sock *sk)
 {
     const struct inet_connection_sock *icsk = inet_csk(sk);
     struct sk_buff *skb, *rtx_head, *hole = NULL;
     struct tcp_sock *tp = tcp_sk(sk);
     bool rearm_timer = false;
     u32 max_segs;
     int mib_idx;
 
     if (!tp->packets_out)
         return;
 
     rtx_head = tcp_rtx_queue_head(sk);
     skb = tp->retransmit_skb_hint ?: rtx_head;
     max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
     skb_rbtree_walk_from(skb) {
         __u8 sacked;
         int segs;
 
         if (tcp_pacing_check(sk))
             break;
 
         /* we could do better than to assign each time */
         if (!hole)
             tp->retransmit_skb_hint = skb;
 
         segs = tcp_snd_cwnd(tp) - tcp_packets_in_flight(tp);
         if (segs <= 0)
             break;
         sacked = TCP_SKB_CB(skb)->sacked;
         /* In case tcp_shift_skb_data() have aggregated large skbs,
          * we need to make sure not sending too bigs TSO packets
          */
         segs = min_t(int, segs, max_segs);
 
         if (tp->retrans_out >= tp->lost_out) {
             break;
         } else if (!(sacked & TCPCB_LOST)) {
             if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
                 hole = skb;
             continue;
 
         } else {
             if (icsk->icsk_ca_state != TCP_CA_Loss)
                 mib_idx = LINUX_MIB_TCPFASTRETRANS;
             else
                 mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
         }
 
         if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
             continue;
 
         if (tcp_small_queue_check(sk, skb, 1))
             break;
 
         if (tcp_retransmit_skb(sk, skb, segs))
             break;
 
         NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));
 
         if (tcp_in_cwnd_reduction(sk))
             tp->prr_out += tcp_skb_pcount(skb);
 
         if (skb == rtx_head &&
             icsk->icsk_pending != ICSK_TIME_REO_TIMEOUT)
             rearm_timer = true;
 
     }
     if (rearm_timer)
         tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                      inet_csk(sk)->icsk_rto,
                      TCP_RTO_MAX);
 }
 
 /* We allow to exceed memory limits for FIN packets to expedite
  * connection tear down and (memory) recovery.
  * Otherwise tcp_send_fin() could be tempted to either delay FIN
  * or even be forced to close flow without any FIN.
  * In general, we want to allow one skb per socket to avoid hangs
  * with edge trigger epoll()
  */
 void sk_forced_mem_schedule(struct sock *sk, int size)
 {
     int delta, amt;
 
     delta = size - sk->sk_forward_alloc;
     if (delta <= 0)
         return;
     amt = sk_mem_pages(delta);
     sk->sk_forward_alloc += amt << PAGE_SHIFT;
     sk_memory_allocated_add(sk, amt);
 
     if (mem_cgroup_sockets_enabled && sk->sk_memcg)
         mem_cgroup_charge_skmem(sk->sk_memcg, amt,
                     gfp_memcg_charge() | __GFP_NOFAIL);
 }
 
 /* Send a FIN. The caller locks the socket for us.
  * We should try to send a FIN packet really hard, but eventually give up.
  */
 void tcp_send_fin(struct sock *sk)
 {
     struct sk_buff *skb, *tskb, *tail = tcp_write_queue_tail(sk);
     struct tcp_sock *tp = tcp_sk(sk);
 
     /* Optimization, tack on the FIN if we have one skb in write queue and
      * this skb was not yet sent, or we are under memory pressure.
      * Note: in the latter case, FIN packet will be sent after a timeout,
      * as TCP stack thinks it has already been transmitted.
      */
     tskb = tail;
     if (!tskb && tcp_under_memory_pressure(sk))
         tskb = skb_rb_last(&sk->tcp_rtx_queue);
 
     if (tskb) {
         TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
         TCP_SKB_CB(tskb)->end_seq++;
         tp->write_seq++;
         if (!tail) {
             /* This means tskb was already sent.
              * Pretend we included the FIN on previous transmit.
              * We need to set tp->snd_nxt to the value it would have
              * if FIN had been sent. This is because retransmit path
              * does not change tp->snd_nxt.
              */
             WRITE_ONCE(tp->snd_nxt, tp->snd_nxt + 1);
             return;
         }
     } else {
         skb = alloc_skb_fclone(MAX_TCP_HEADER, sk->sk_allocation);
         if (unlikely(!skb))
             return;
 
         INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
         skb_reserve(skb, MAX_TCP_HEADER);
         sk_forced_mem_schedule(sk, skb->truesize);
         /* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
         tcp_init_nondata_skb(skb, tp->write_seq,
                      TCPHDR_ACK | TCPHDR_FIN);
         tcp_queue_skb(sk, skb);
     }
     __tcp_push_pending_frames(sk, tcp_current_mss(sk), TCP_NAGLE_OFF);
 }
 
 /* We get here when a process closes a file descriptor (either due to
  * an explicit close() or as a byproduct of exit()'ing) and there
  * was unread data in the receive queue.  This behavior is recommended
  * by RFC 2525, section 2.17.  -DaveM
  */
 void tcp_send_active_reset(struct sock *sk, gfp_t priority)
 {
     struct sk_buff *skb;
 
     TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
 
     /* NOTE: No TCP options attached and we never retransmit this. */
     skb = alloc_skb(MAX_TCP_HEADER, priority);
     if (!skb) {
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
         return;
     }
 
     /* Reserve space for headers and prepare control bits. */
     skb_reserve(skb, MAX_TCP_HEADER);
     tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
                  TCPHDR_ACK | TCPHDR_RST);
     tcp_mstamp_refresh(tcp_sk(sk));
     /* Send it off. */
     if (tcp_transmit_skb(sk, skb, 0, priority))
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
 
     /* skb of trace_tcp_send_reset() keeps the skb that caused RST,
      * skb here is different to the troublesome skb, so use NULL
      */
     trace_tcp_send_reset(sk, NULL);
 }
 
 /* Send a crossed SYN-ACK during socket establishment.
  * WARNING: This routine must only be called when we have already sent
  * a SYN packet that crossed the incoming SYN that caused this routine
  * to get called. If this assumption fails then the initial rcv_wnd
  * and rcv_wscale values will not be correct.
  */
 int tcp_send_synack(struct sock *sk)
 {
     struct sk_buff *skb;
 
     skb = tcp_rtx_queue_head(sk);
     if (!skb || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
         pr_err("%s: wrong queue state\n", __func__);
         return -EFAULT;
     }
     if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
         if (skb_cloned(skb)) {
             struct sk_buff *nskb;
 
             tcp_skb_tsorted_save(skb) {
                 nskb = skb_copy(skb, GFP_ATOMIC);
             } tcp_skb_tsorted_restore(skb);
             if (!nskb)
                 return -ENOMEM;
             INIT_LIST_HEAD(&nskb->tcp_tsorted_anchor);
             tcp_highest_sack_replace(sk, skb, nskb);
             tcp_rtx_queue_unlink_and_free(skb, sk);
             __skb_header_release(nskb);
             tcp_rbtree_insert(&sk->tcp_rtx_queue, nskb);
             sk_wmem_queued_add(sk, nskb->truesize);
             sk_mem_charge(sk, nskb->truesize);
             skb = nskb;
         }
 
         TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
         tcp_ecn_send_synack(sk, skb);
     }
     return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
 }
 
 /**
  * tcp_make_synack - Allocate one skb and build a SYNACK packet.
  * @sk: listener socket
  * @dst: dst entry attached to the SYNACK. It is consumed and caller
  *       should not use it again.
  * @req: request_sock pointer
  * @foc: cookie for tcp fast open
  * @synack_type: Type of synack to prepare
  * @syn_skb: SYN packet just received.  It could be NULL for rtx case.
  */
 struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
                 struct request_sock *req,
                 struct tcp_fastopen_cookie *foc,
                 enum tcp_synack_type synack_type,
                 struct sk_buff *syn_skb)
 {
     struct inet_request_sock *ireq = inet_rsk(req);
     const struct tcp_sock *tp = tcp_sk(sk);
     struct tcp_md5sig_key *md5 = NULL;
     struct tcp_out_options opts;
     struct sk_buff *skb;
     int tcp_header_size;
     struct tcphdr *th;
     int mss;
     u64 now;
 
     skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
     if (unlikely(!skb)) {
         dst_release(dst);
         return NULL;
     }
     /* Reserve space for headers. */
     skb_reserve(skb, MAX_TCP_HEADER);
 
     switch (synack_type) {
     case TCP_SYNACK_NORMAL:
         skb_set_owner_w(skb, req_to_sk(req));
         break;
     case TCP_SYNACK_COOKIE:
         /* Under synflood, we do not attach skb to a socket,
          * to avoid false sharing.
          */
         break;
     case TCP_SYNACK_FASTOPEN:
         /* sk is a const pointer, because we want to express multiple
          * cpu might call us concurrently.
          * sk->sk_wmem_alloc in an atomic, we can promote to rw.
          */
         skb_set_owner_w(skb, (struct sock *)sk);
         break;
     }
     skb_dst_set(skb, dst);
 
     mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
     memset(&opts, 0, sizeof(opts));
     now = tcp_clock_ns();
 #ifdef CONFIG_SYN_COOKIES
     if (unlikely(synack_type == TCP_SYNACK_COOKIE && ireq->tstamp_ok))
         skb_set_delivery_time(skb, cookie_init_timestamp(req, now),
                       true);
     else
 #endif
     {
         skb_set_delivery_time(skb, now, true);
         if (!tcp_rsk(req)->snt_synack) /* Timestamp first SYNACK */
             tcp_rsk(req)->snt_synack = tcp_skb_timestamp_us(skb);
     }
 
 #ifdef CONFIG_TCP_MD5SIG
     rcu_read_lock();
     md5 = tcp_rsk(req)->af_specific->req_md5_lookup(sk, req_to_sk(req));
 #endif
     skb_set_hash(skb, tcp_rsk(req)->txhash, PKT_HASH_TYPE_L4);
     /* bpf program will be interested in the tcp_flags */
     TCP_SKB_CB(skb)->tcp_flags = TCPHDR_SYN | TCPHDR_ACK;
     tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts, md5,
                          foc, synack_type,
                          syn_skb) + sizeof(*th);
 
     skb_push(skb, tcp_header_size);
     skb_reset_transport_header(skb);
 
     th = (struct tcphdr *)skb->data;
     memset(th, 0, sizeof(struct tcphdr));
     th->syn = 1;
     th->ack = 1;
     tcp_ecn_make_synack(req, th);
     th->source = htons(ireq->ir_num);
     th->dest = ireq->ir_rmt_port;
     skb->mark = ireq->ir_mark;
     skb->ip_summed = CHECKSUM_PARTIAL;
     th->seq = htonl(tcp_rsk(req)->snt_isn);
     /* XXX data is queued and acked as is. No buffer/window check */
     th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);
 
     /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
     th->window = htons(min(req->rsk_rcv_wnd, 65535U));
     tcp_options_write(th, NULL, &opts);
     th->doff = (tcp_header_size >> 2);
     __TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
 
 #ifdef CONFIG_TCP_MD5SIG
     /* Okay, we have all we need - do the md5 hash if needed */
     if (md5)
         tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
                            md5, req_to_sk(req), skb);
     rcu_read_unlock();
 #endif
 
     bpf_skops_write_hdr_opt((struct sock *)sk, skb, req, syn_skb,
                 synack_type, &opts);
 
     skb_set_delivery_time(skb, now, true);
     tcp_add_tx_delay(skb, tp);
 
     return skb;
 }
 EXPORT_SYMBOL(tcp_make_synack);
 
 static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     const struct tcp_congestion_ops *ca;
     u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
 
     if (ca_key == TCP_CA_UNSPEC)
         return;
 
     rcu_read_lock();
     ca = tcp_ca_find_key(ca_key);
     if (likely(ca && bpf_try_module_get(ca, ca->owner))) {
         bpf_module_put(icsk->icsk_ca_ops, icsk->icsk_ca_ops->owner);
         icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
         icsk->icsk_ca_ops = ca;
     }
     rcu_read_unlock();
 }
 
 /* Do all connect socket setups that can be done AF independent. */
 static void tcp_connect_init(struct sock *sk)
 {
     const struct dst_entry *dst = __sk_dst_get(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     __u8 rcv_wscale;
     u32 rcv_wnd;
 
     /* We'll fix this up when we get a response from the other end.
      * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
      */
     tp->tcp_header_len = sizeof(struct tcphdr);
     if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps))
         tp->tcp_header_len += TCPOLEN_TSTAMP_ALIGNED;
 
 #ifdef CONFIG_TCP_MD5SIG
     if (tp->af_specific->md5_lookup(sk, sk))
         tp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
 #endif
 
     /* If user gave his TCP_MAXSEG, record it to clamp */
     if (tp->rx_opt.user_mss)
         tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
     tp->max_window = 0;
     tcp_mtup_init(sk);
     tcp_sync_mss(sk, dst_mtu(dst));
 
     tcp_ca_dst_init(sk, dst);
 
     if (!tp->window_clamp)
         tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
     tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
     tcp_initialize_rcv_mss(sk);
 
     /* limit the window selection if the user enforce a smaller rx buffer */
     if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
         (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
         tp->window_clamp = tcp_full_space(sk);
 
     rcv_wnd = tcp_rwnd_init_bpf(sk);
     if (rcv_wnd == 0)
         rcv_wnd = dst_metric(dst, RTAX_INITRWND);
 
     tcp_select_initial_window(sk, tcp_full_space(sk),
                   tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
                   &tp->rcv_wnd,
                   &tp->window_clamp,
                   READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling),
                   &rcv_wscale,
                   rcv_wnd);
 
     tp->rx_opt.rcv_wscale = rcv_wscale;
     tp->rcv_ssthresh = tp->rcv_wnd;
 
     sk->sk_err = 0;
     sock_reset_flag(sk, SOCK_DONE);
     tp->snd_wnd = 0;
     tcp_init_wl(tp, 0);
     tcp_write_queue_purge(sk);
     tp->snd_una = tp->write_seq;
     tp->snd_sml = tp->write_seq;
     tp->snd_up = tp->write_seq;
     WRITE_ONCE(tp->snd_nxt, tp->write_seq);
 
     if (likely(!tp->repair))
         tp->rcv_nxt = 0;
     else
         tp->rcv_tstamp = tcp_jiffies32;
     tp->rcv_wup = tp->rcv_nxt;
     WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
 
     inet_csk(sk)->icsk_rto = tcp_timeout_init(sk);
     inet_csk(sk)->icsk_retransmits = 0;
     tcp_clear_retrans(tp);
 }
 
 static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
 
     tcb->end_seq += skb->len;
     __skb_header_release(skb);
     sk_wmem_queued_add(sk, skb->truesize);
     sk_mem_charge(sk, skb->truesize);
     WRITE_ONCE(tp->write_seq, tcb->end_seq);
     tp->packets_out += tcp_skb_pcount(skb);
 }
 
 /* Build and send a SYN with data and (cached) Fast Open cookie. However,
  * queue a data-only packet after the regular SYN, such that regular SYNs
  * are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
  * only the SYN sequence, the data are retransmitted in the first ACK.
  * If cookie is not cached or other error occurs, falls back to send a
  * regular SYN with Fast Open cookie request option.
  */
 static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct tcp_fastopen_request *fo = tp->fastopen_req;
     int space, err = 0;
     struct sk_buff *syn_data;
 
     tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
     if (!tcp_fastopen_cookie_check(sk, &tp->rx_opt.mss_clamp, &fo->cookie))
         goto fallback;
 
     /* MSS for SYN-data is based on cached MSS and bounded by PMTU and
      * user-MSS. Reserve maximum option space for middleboxes that add
      * private TCP options. The cost is reduced data space in SYN :(
      */
     tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
     /* Sync mss_cache after updating the mss_clamp */
     tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 
     space = __tcp_mtu_to_mss(sk, icsk->icsk_pmtu_cookie) -
         MAX_TCP_OPTION_SPACE;
 
     space = min_t(size_t, space, fo->size);
 
     /* limit to order-0 allocations */
     space = min_t(size_t, space, SKB_MAX_HEAD(MAX_TCP_HEADER));
 
     syn_data = tcp_stream_alloc_skb(sk, space, sk->sk_allocation, false);
     if (!syn_data)
         goto fallback;
     memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
     if (space) {
         int copied = copy_from_iter(skb_put(syn_data, space), space,
                         &fo->data->msg_iter);
         if (unlikely(!copied)) {
             tcp_skb_tsorted_anchor_cleanup(syn_data);
             kfree_skb(syn_data);
             goto fallback;
         }
         if (copied != space) {
             skb_trim(syn_data, copied);
             space = copied;
         }
         skb_zcopy_set(syn_data, fo->uarg, NULL);
     }
     /* No more data pending in inet_wait_for_connect() */
     if (space == fo->size)
         fo->data = NULL;
     fo->copied = space;
 
     tcp_connect_queue_skb(sk, syn_data);
     if (syn_data->len)
         tcp_chrono_start(sk, TCP_CHRONO_BUSY);
 
     err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);
 
     skb_set_delivery_time(syn, syn_data->skb_mstamp_ns, true);
 
     /* Now full SYN+DATA was cloned and sent (or not),
      * remove the SYN from the original skb (syn_data)
      * we keep in write queue in case of a retransmit, as we
      * also have the SYN packet (with no data) in the same queue.
      */
     TCP_SKB_CB(syn_data)->seq++;
     TCP_SKB_CB(syn_data)->tcp_flags = TCPHDR_ACK | TCPHDR_PSH;
     if (!err) {
         tp->syn_data = (fo->copied > 0);
         tcp_rbtree_insert(&sk->tcp_rtx_queue, syn_data);
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT);
         goto done;
     }
 
     /* data was not sent, put it in write_queue */
     __skb_queue_tail(&sk->sk_write_queue, syn_data);
     tp->packets_out -= tcp_skb_pcount(syn_data);
 
 fallback:
     /* Send a regular SYN with Fast Open cookie request option */
     if (fo->cookie.len > 0)
         fo->cookie.len = 0;
     err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
     if (err)
         tp->syn_fastopen = 0;
 done:
     fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
     return err;
 }
 
 /* Build a SYN and send it off. */
 int tcp_connect(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *buff;
     int err;
 
     tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB, 0, NULL);
 
     if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
         return -EHOSTUNREACH; /* Routing failure or similar. */
 
     tcp_connect_init(sk);
 
     if (unlikely(tp->repair)) {
         tcp_finish_connect(sk, NULL);
         return 0;
     }
 
     buff = tcp_stream_alloc_skb(sk, 0, sk->sk_allocation, true);
     if (unlikely(!buff))
         return -ENOBUFS;
 
     tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
     tcp_mstamp_refresh(tp);
     tp->retrans_stamp = tcp_time_stamp(tp);
     tcp_connect_queue_skb(sk, buff);
     tcp_ecn_send_syn(sk, buff);
     tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
 
     /* Send off SYN; include data in Fast Open. */
     err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
           tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
     if (err == -ECONNREFUSED)
         return err;
 
     /* We change tp->snd_nxt after the tcp_transmit_skb() call
      * in order to make this packet get counted in tcpOutSegs.
      */
     WRITE_ONCE(tp->snd_nxt, tp->write_seq);
     tp->pushed_seq = tp->write_seq;
     buff = tcp_send_head(sk);
     if (unlikely(buff)) {
         WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(buff)->seq);
         tp->pushed_seq  = TCP_SKB_CB(buff)->seq;
     }
     TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);
 
     /* Timer for repeating the SYN until an answer. */
     inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                   inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
     return 0;
 }
 EXPORT_SYMBOL(tcp_connect);
 
 /* Send out a delayed ack, the caller does the policy checking
  * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
  * for details.
  */
 void tcp_send_delayed_ack(struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     int ato = icsk->icsk_ack.ato;
     unsigned long timeout;
 
     if (ato > TCP_DELACK_MIN) {
         const struct tcp_sock *tp = tcp_sk(sk);
         int max_ato = HZ / 2;
 
         if (inet_csk_in_pingpong_mode(sk) ||
             (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
             max_ato = TCP_DELACK_MAX;
 
         /* Slow path, intersegment interval is "high". */
 
         /* If some rtt estimate is known, use it to bound delayed ack.
          * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
          * directly.
          */
         if (tp->srtt_us) {
             int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 3),
                     TCP_DELACK_MIN);
 
             if (rtt < max_ato)
                 max_ato = rtt;
         }
 
         ato = min(ato, max_ato);
     }
 
     ato = min_t(u32, ato, inet_csk(sk)->icsk_delack_max);
 
     /* Stay within the limit we were given */
     timeout = jiffies + ato;
 
     /* Use new timeout only if there wasn't a older one earlier. */
     if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
         /* If delack timer is about to expire, send ACK now. */
         if (time_before_eq(icsk->icsk_ack.timeout, jiffies + (ato >> 2))) {
             tcp_send_ack(sk);
             return;
         }
 
         if (!time_before(timeout, icsk->icsk_ack.timeout))
             timeout = icsk->icsk_ack.timeout;
     }
     icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
     icsk->icsk_ack.timeout = timeout;
     sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
 }
 
 /* This routine sends an ack and also updates the window. */
 void __tcp_send_ack(struct sock *sk, u32 rcv_nxt)
 {
     struct sk_buff *buff;
 
     /* If we have been reset, we may not send again. */
     if (sk->sk_state == TCP_CLOSE)
         return;
 
     /* We are not putting this on the write queue, so
      * tcp_transmit_skb() will set the ownership to this
      * sock.
      */
     buff = alloc_skb(MAX_TCP_HEADER,
              sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
     if (unlikely(!buff)) {
         struct inet_connection_sock *icsk = inet_csk(sk);
         unsigned long delay;
 
         delay = TCP_DELACK_MAX << icsk->icsk_ack.retry;
         if (delay < TCP_RTO_MAX)
             icsk->icsk_ack.retry++;
         inet_csk_schedule_ack(sk);
         icsk->icsk_ack.ato = TCP_ATO_MIN;
         inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK, delay, TCP_RTO_MAX);
         return;
     }
 
     /* Reserve space for headers and prepare control bits. */
     skb_reserve(buff, MAX_TCP_HEADER);
     tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);
 
     /* We do not want pure acks influencing TCP Small Queues or fq/pacing
      * too much.
      * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
      */
     skb_set_tcp_pure_ack(buff);
 
     /* Send it off, this clears delayed acks for us. */
     __tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0, rcv_nxt);
 }
 EXPORT_SYMBOL_GPL(__tcp_send_ack);
 
 void tcp_send_ack(struct sock *sk)
 {
     __tcp_send_ack(sk, tcp_sk(sk)->rcv_nxt);
 }
 
 /* This routine sends a packet with an out of date sequence
  * number. It assumes the other end will try to ack it.
  *
  * Question: what should we make while urgent mode?
  * 4.4BSD forces sending single byte of data. We cannot send
  * out of window data, because we have SND.NXT==SND.MAX...
  *
  * Current solution: to send TWO zero-length segments in urgent mode:
  * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
  * out-of-date with SND.UNA-1 to probe window.
  */
 static int tcp_xmit_probe_skb(struct sock *sk, int urgent, int mib)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb;
 
     /* We don't queue it, tcp_transmit_skb() sets ownership. */
     skb = alloc_skb(MAX_TCP_HEADER,
             sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
     if (!skb)
         return -1;
 
     /* Reserve space for headers and set control bits. */
     skb_reserve(skb, MAX_TCP_HEADER);
     /* Use a previous sequence.  This should cause the other
      * end to send an ack.  Don't queue or clone SKB, just
      * send it.
      */
     tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPHDR_ACK);
     NET_INC_STATS(sock_net(sk), mib);
     return tcp_transmit_skb(sk, skb, 0, (__force gfp_t)0);
 }
 
 /* Called from setsockopt( ... TCP_REPAIR ) */
 void tcp_send_window_probe(struct sock *sk)
 {
     if (sk->sk_state == TCP_ESTABLISHED) {
         tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
         tcp_mstamp_refresh(tcp_sk(sk));
         tcp_xmit_probe_skb(sk, 0, LINUX_MIB_TCPWINPROBE);
     }
 }
 
 /* Initiate keepalive or window probe from timer. */
 int tcp_write_wakeup(struct sock *sk, int mib)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct sk_buff *skb;
 
     if (sk->sk_state == TCP_CLOSE)
         return -1;
 
     skb = tcp_send_head(sk);
     if (skb && before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
         int err;
         unsigned int mss = tcp_current_mss(sk);
         unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
 
         if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
             tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;
 
         /* We are probing the opening of a window
          * but the window size is != 0
          * must have been a result SWS avoidance ( sender )
          */
         if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
             skb->len > mss) {
             seg_size = min(seg_size, mss);
             TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
             if (tcp_fragment(sk, TCP_FRAG_IN_WRITE_QUEUE,
                      skb, seg_size, mss, GFP_ATOMIC))
                 return -1;
         } else if (!tcp_skb_pcount(skb))
             tcp_set_skb_tso_segs(skb, mss);
 
         TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
         err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
         if (!err)
             tcp_event_new_data_sent(sk, skb);
         return err;
     } else {
         if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
             tcp_xmit_probe_skb(sk, 1, mib);
         return tcp_xmit_probe_skb(sk, 0, mib);
     }
 }
 
 /* A window probe timeout has occurred.  If window is not closed send
  * a partial packet else a zero probe.
  */
 void tcp_send_probe0(struct sock *sk)
 {
     struct inet_connection_sock *icsk = inet_csk(sk);
     struct tcp_sock *tp = tcp_sk(sk);
     struct net *net = sock_net(sk);
     unsigned long timeout;
     int err;
 
     err = tcp_write_wakeup(sk, LINUX_MIB_TCPWINPROBE);
 
     if (tp->packets_out || tcp_write_queue_empty(sk)) {
         /* Cancel probe timer, if it is not required. */
         icsk->icsk_probes_out = 0;
         icsk->icsk_backoff = 0;
         icsk->icsk_probes_tstamp = 0;
         return;
     }
 
     icsk->icsk_probes_out++;
     if (err <= 0) {
         if (icsk->icsk_backoff < READ_ONCE(net->ipv4.sysctl_tcp_retries2))
             icsk->icsk_backoff++;
         timeout = tcp_probe0_when(sk, TCP_RTO_MAX);
     } else {
         /* If packet was not sent due to local congestion,
          * Let senders fight for local resources conservatively.
          */
         timeout = TCP_RESOURCE_PROBE_INTERVAL;
     }
 
     timeout = tcp_clamp_probe0_to_user_timeout(sk, timeout);
     tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, timeout, TCP_RTO_MAX);
 }
 
 int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
 {
     const struct tcp_request_sock_ops *af_ops = tcp_rsk(req)->af_specific;
     struct flowi fl;
     int res;
 
     /* Paired with WRITE_ONCE() in sock_setsockopt() */
     if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
         tcp_rsk(req)->txhash = net_tx_rndhash();
     res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
                   NULL);
     if (!res) {
         TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
         if (unlikely(tcp_passive_fastopen(sk)))
             tcp_sk(sk)->total_retrans++;
         trace_tcp_retransmit_synack(sk, req);
     }
     return res;
 }
 EXPORT_SYMBOL(tcp_rtx_synack);