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 .. SPDX-License-Identifier: GPL-2.0
 
 =========================
 Stream Parser (strparser)
 =========================
 
 Introduction
 ============
 
 The stream parser (strparser) is a utility that parses messages of an
 application layer protocol running over a data stream. The stream
 parser works in conjunction with an upper layer in the kernel to provide
 kernel support for application layer messages. For instance, Kernel
 Connection Multiplexor (KCM) uses the Stream Parser to parse messages
 using a BPF program.
 
 The strparser works in one of two modes: receive callback or general
 mode.
 
 In receive callback mode, the strparser is called from the data_ready
 callback of a TCP socket. Messages are parsed and delivered as they are
 received on the socket.
 
 In general mode, a sequence of skbs are fed to strparser from an
 outside source. Message are parsed and delivered as the sequence is
 processed. This modes allows strparser to be applied to arbitrary
 streams of data.
 
 Interface
 =========
 
 The API includes a context structure, a set of callbacks, utility
 functions, and a data_ready function for receive callback mode. The
 callbacks include a parse_msg function that is called to perform
 parsing (e.g.  BPF parsing in case of KCM), and a rcv_msg function
 that is called when a full message has been completed.
 
 Functions
 =========
 
      ::
 
         strp_init(struct strparser *strp, struct sock *sk,
                 const struct strp_callbacks *cb)
 
      Called to initialize a stream parser. strp is a struct of type
      strparser that is allocated by the upper layer. sk is the TCP
      socket associated with the stream parser for use with receive
      callback mode; in general mode this is set to NULL. Callbacks
      are called by the stream parser (the callbacks are listed below).
 
      ::
 
         void strp_pause(struct strparser *strp)
 
      Temporarily pause a stream parser. Message parsing is suspended
      and no new messages are delivered to the upper layer.
 
      ::
 
         void strp_unpause(struct strparser *strp)
 
      Unpause a paused stream parser.
 
      ::
 
         void strp_stop(struct strparser *strp);
 
      strp_stop is called to completely stop stream parser operations.
      This is called internally when the stream parser encounters an
      error, and it is called from the upper layer to stop parsing
      operations.
 
      ::
 
         void strp_done(struct strparser *strp);
 
      strp_done is called to release any resources held by the stream
      parser instance. This must be called after the stream processor
      has been stopped.
 
      ::
 
         int strp_process(struct strparser *strp, struct sk_buff *orig_skb,
                          unsigned int orig_offset, size_t orig_len,
                          size_t max_msg_size, long timeo)
 
     strp_process is called in general mode for a stream parser to
     parse an sk_buff. The number of bytes processed or a negative
     error number is returned. Note that strp_process does not
     consume the sk_buff. max_msg_size is maximum size the stream
     parser will parse. timeo is timeout for completing a message.
 
     ::
 
         void strp_data_ready(struct strparser *strp);
 
     The upper layer calls strp_tcp_data_ready when data is ready on
     the lower socket for strparser to process. This should be called
     from a data_ready callback that is set on the socket. Note that
     maximum messages size is the limit of the receive socket
     buffer and message timeout is the receive timeout for the socket.
 
     ::
 
         void strp_check_rcv(struct strparser *strp);
 
     strp_check_rcv is called to check for new messages on the socket.
     This is normally called at initialization of a stream parser
     instance or after strp_unpause.
 
 Callbacks
 =========
 
 There are six callbacks:
 
     ::
 
         int (*parse_msg)(struct strparser *strp, struct sk_buff *skb);
 
     parse_msg is called to determine the length of the next message
     in the stream. The upper layer must implement this function. It
     should parse the sk_buff as containing the headers for the
     next application layer message in the stream.
 
     The skb->cb in the input skb is a struct strp_msg. Only
     the offset field is relevant in parse_msg and gives the offset
     where the message starts in the skb.
 
     The return values of this function are:
 
     =========    ===========================================================
     >0           indicates length of successfully parsed message
     0            indicates more data must be received to parse the message
     -ESTRPIPE    current message should not be processed by the
                  kernel, return control of the socket to userspace which
                  can proceed to read the messages itself
     other < 0    Error in parsing, give control back to userspace
                  assuming that synchronization is lost and the stream
                  is unrecoverable (application expected to close TCP socket)
     =========    ===========================================================
 
     In the case that an error is returned (return value is less than
     zero) and the parser is in receive callback mode, then it will set
     the error on TCP socket and wake it up. If parse_msg returned
     -ESTRPIPE and the stream parser had previously read some bytes for
     the current message, then the error set on the attached socket is
     ENODATA since the stream is unrecoverable in that case.
 
     ::
 
         void (*lock)(struct strparser *strp)
 
     The lock callback is called to lock the strp structure when
     the strparser is performing an asynchronous operation (such as
     processing a timeout). In receive callback mode the default
     function is to lock_sock for the associated socket. In general
     mode the callback must be set appropriately.
 
     ::
 
         void (*unlock)(struct strparser *strp)
 
     The unlock callback is called to release the lock obtained
     by the lock callback. In receive callback mode the default
     function is release_sock for the associated socket. In general
     mode the callback must be set appropriately.
 
     ::
 
         void (*rcv_msg)(struct strparser *strp, struct sk_buff *skb);
 
     rcv_msg is called when a full message has been received and
     is queued. The callee must consume the sk_buff; it can
     call strp_pause to prevent any further messages from being
     received in rcv_msg (see strp_pause above). This callback
     must be set.
 
     The skb->cb in the input skb is a struct strp_msg. This
     struct contains two fields: offset and full_len. Offset is
     where the message starts in the skb, and full_len is the
     the length of the message. skb->len - offset may be greater
     then full_len since strparser does not trim the skb.
 
     ::
 
         int (*read_sock_done)(struct strparser *strp, int err);
 
      read_sock_done is called when the stream parser is done reading
      the TCP socket in receive callback mode. The stream parser may
      read multiple messages in a loop and this function allows cleanup
      to occur when exiting the loop. If the callback is not set (NULL
      in strp_init) a default function is used.
 
      ::
 
         void (*abort_parser)(struct strparser *strp, int err);
 
      This function is called when stream parser encounters an error
      in parsing. The default function stops the stream parser and
      sets the error in the socket if the parser is in receive callback
      mode. The default function can be changed by setting the callback
      to non-NULL in strp_init.
 
 Statistics
 ==========
 
 Various counters are kept for each stream parser instance. These are in
 the strp_stats structure. strp_aggr_stats is a convenience structure for
 accumulating statistics for multiple stream parser instances.
 save_strp_stats and aggregate_strp_stats are helper functions to save
 and aggregate statistics.
 
 Message assembly limits
 =======================
 
 The stream parser provide mechanisms to limit the resources consumed by
 message assembly.
 
 A timer is set when assembly starts for a new message. In receive
 callback mode the message timeout is taken from rcvtime for the
 associated TCP socket. In general mode, the timeout is passed as an
 argument in strp_process. If the timer fires before assembly completes
 the stream parser is aborted and the ETIMEDOUT error is set on the TCP
 socket if in receive callback mode.
 
 In receive callback mode, message length is limited to the receive
 buffer size of the associated TCP socket. If the length returned by
 parse_msg is greater than the socket buffer size then the stream parser
 is aborted with EMSGSIZE error set on the TCP socket. Note that this
 makes the maximum size of receive skbuffs for a socket with a stream
 parser to be 2*sk_rcvbuf of the TCP socket.
 
 In general mode the message length limit is passed in as an argument
 to strp_process.
 
 Author
 ======
 
 Tom Herbert (tom@quantonium.net)

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