WSL2-Linux-Kernel/Documentation/networking/strparser.rst

.. 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)