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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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