docs: networking: convert strparser.txt to ReST

- add SPDX header;
- adjust title markup;
- mark code blocks and literals as such;
- mark tables as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 240 insertions, 0 deletions

diff --git a/Documentation/networking/strparser.rst b/Documentation/networking/strparser.rst
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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)