Stream Parser
  -------------
  
  The stream parser (strparser) is a utility that parses messages of an
  application layer protocol running over a TCP connection. 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.
  
  Interface
  ---------
  
  The API includes a context structure, a set of callbacks, utility
  functions, and a data_ready function. 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.
  
  A stream parser can be instantiated for a TCP connection. This is done
  by:
  
  strp_init(struct strparser *strp, struct sock *csk,
  	  struct strp_callbacks *cb)
  
  strp is a struct of type strparser that is allocated by the upper layer.
  csk is the TCP socket associated with the stream parser. Callbacks are
  called by the stream parser.
  
  Callbacks
  ---------
  
  There are four 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 messages in the stream.
  
      The skb->cb in the input skb is a struct strp_rx_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 is 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) the stream parser 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 (*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 below). This callback
      must be set.
  
      The skb->cb in the input skb is a struct strp_rx_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. The stream parser may read multiple messages
       in a loop and this function allows cleanup to occur when existing
       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 for the
       TCP socket and sets the error in the socket. The default function
       can be changed by setting the callback to non-NULL in strp_init.
  
  Functions
  ---------
  
  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.
  
  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 when unattaching a TCP socket.
  
  strp_done is called to unattach the stream parser from the TCP socket.
  This must be called after the stream processor has be stopped.
  
  strp_check_rcv is called to check for new messages on the socket. This
  is normally called at initialization of the a stream parser instance
  of after strp_unpause.
  
  Statistics
  ----------
  
  Various counters are kept for each stream parser for a TCP socket.
  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. The message
  timeout is taken from rcvtime for the associated TCP socket. If the
  timer fires before assembly completes the stream parser is aborted
  and the ETIMEDOUT error is set on the TCP socket.
  
  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.