.. SPDX-License-Identifier: GPL-2.0 =========================Stream Parser (strparser)========================= Introduction============ The stream parser (strparser) is a utility that parses messages of anapplication layer protocol running over a data stream. The streamparser works in conjunction with an upper layer in the kernel to providekernel support for application layer messages. For instance, KernelConnection Multiplexor (KCM) uses the Stream Parser to parse messagesusing a BPF program. The strparser works in one of two modes: receive callback or generalmode. In receive callback mode, the strparser is called from the data_readycallback of a TCP socket. Messages are parsed and delivered as they arereceived on the socket. In general mode, a sequence of skbs are fed to strparser from anoutside source. Message are parsed and delivered as the sequence isprocessed. This modes allows strparser to be applied to arbitrarystreams of data. Interface========= The API includes a context structure, a set of callbacks, utilityfunctions, and a data_ready function for receive callback mode. Thecallbacks include a parse_msg function that is called to performparsing (e.g.  BPF parsing in case of KCM), and a rcv_msg functionthat 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 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_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 seven 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 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) 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 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.     :: 	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.    :: 	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 inthe strp_stats structure. strp_aggr_stats is a convenience structure foraccumulating statistics for multiple stream parser instances.save_strp_stats and aggregate_strp_stats are helper functions to saveand aggregate statistics. Message assembly limits======================= The stream parser provide mechanisms to limit the resources consumed bymessage assembly. A timer is set when assembly starts for a new message. In receivecallback mode the message timeout is taken from rcvtime for theassociated TCP socket. In general mode, the timeout is passed as anargument in strp_process. If the timer fires before assembly completesthe stream parser is aborted and the ETIMEDOUT error is set on the TCPsocket if in receive callback mode. In receive callback mode, message length is limited to the receivebuffer size of the associated TCP socket. If the length returned byparse_msg is greater than the socket buffer size then the stream parseris aborted with EMSGSIZE error set on the TCP socket. Note that thismakes the maximum size of receive skbuffs for a socket with a streamparser to be 2*sk_rcvbuf of the TCP socket. In general mode the message length limit is passed in as an argumentto strp_process. Author====== Tom Herbert (tom@quantonium.net)