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 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.

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.

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.

Functions
=========

     ::

	int 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. 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. The socket lock must not be held.

    ::

	void strp_data_ready(struct strparser *strp);

    The upper layer calls strp_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 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. It should parse the sk_buff as containing the headers
    for the next application layer message in the stream.

    The strparser metadata in the input skb can be accessed with
    strp_msg(skb). 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), 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.

    This callback must be set.

    ::

	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).

    The strparser metadata in the input skb can be accessed with
    strp_msg(skb). This struct contains two fields: offset and full_len.
    Offset is where the message starts in the skb, and full_len is
    the length of the message. skb->len - offset may be greater than
    full_len since strparser does not trim the skb.

    This callback must be set.

    ::

	int (*read_sock)(struct strparser *strp, read_descriptor_t *desc,
                     sk_read_actor_t recv_actor);

    The read_sock callback is used by strparser instead of
    sock->ops->read_sock, if provided.

    This callback is optional.

    ::

	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 exiting the
     loop. If the callback is not set (NULL in strp_init) a default
     function is used.

    This callback is optional.

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. 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.

Author
======

Tom Herbert (tom@quantonium.net)