strparser: Documentation
Signed-off-by: Tom Herbert <tom@herbertland.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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Tom Herbert
David S. Miller
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Stream Parser
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-------------
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The stream parser (strparser) is a utility that parses messages of an
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application layer protocol running over a TCP connection. The stream
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parser works in conjunction with an upper layer in the kernel to provide
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kernel support for application layer messages. For instance, Kernel
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Connection Multiplexor (KCM) uses the Stream Parser to parse messages
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using a BPF program.
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Interface
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---------
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The API includes a context structure, a set of callbacks, utility
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functions, and a data_ready function. The callbacks include
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a parse_msg function that is called to perform parsing (e.g.
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BPF parsing in case of KCM), and a rcv_msg function that is called
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when a full message has been completed.
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A stream parser can be instantiated for a TCP connection. This is done
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by:
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strp_init(struct strparser *strp, struct sock *csk,
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struct strp_callbacks *cb)
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strp is a struct of type strparser that is allocated by the upper layer.
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csk is the TCP socket associated with the stream parser. Callbacks are
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called by the stream parser.
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Callbacks
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---------
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There are four callbacks:
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int (*parse_msg)(struct strparser *strp, struct sk_buff *skb);
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parse_msg is called to determine the length of the next message
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in the stream. The upper layer must implement this function. It
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should parse the sk_buff as containing the headers for the
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next application layer messages in the stream.
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The skb->cb in the input skb is a struct strp_rx_msg. Only
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the offset field is relevant in parse_msg and gives the offset
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where the message starts in the skb.
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The return values of this function are:
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>0 : indicates length of successfully parsed message
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0 : indicates more data must be received to parse the message
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-ESTRPIPE : current message should not be processed by the
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kernel, return control of the socket to userspace which
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can proceed to read the messages itself
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other < 0 : Error is parsing, give control back to userspace
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assuming that synchronization is lost and the stream
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is unrecoverable (application expected to close TCP socket)
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In the case that an error is returned (return value is less than
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zero) the stream parser will set the error on TCP socket and wake
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it up. If parse_msg returned -ESTRPIPE and the stream parser had
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previously read some bytes for the current message, then the error
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set on the attached socket is ENODATA since the stream is
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unrecoverable in that case.
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void (*rcv_msg)(struct strparser *strp, struct sk_buff *skb);
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rcv_msg is called when a full message has been received and
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is queued. The callee must consume the sk_buff; it can
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call strp_pause to prevent any further messages from being
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received in rcv_msg (see strp_pause below). This callback
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must be set.
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The skb->cb in the input skb is a struct strp_rx_msg. This
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struct contains two fields: offset and full_len. Offset is
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where the message starts in the skb, and full_len is the
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the length of the message. skb->len - offset may be greater
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then full_len since strparser does not trim the skb.
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int (*read_sock_done)(struct strparser *strp, int err);
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read_sock_done is called when the stream parser is done reading
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the TCP socket. The stream parser may read multiple messages
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in a loop and this function allows cleanup to occur when existing
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the loop. If the callback is not set (NULL in strp_init) a
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default function is used.
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void (*abort_parser)(struct strparser *strp, int err);
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This function is called when stream parser encounters an error
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in parsing. The default function stops the stream parser for the
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TCP socket and sets the error in the socket. The default function
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can be changed by setting the callback to non-NULL in strp_init.
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Functions
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---------
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The upper layer calls strp_tcp_data_ready when data is ready on the lower
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socket for strparser to process. This should be called from a data_ready
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callback that is set on the socket.
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strp_stop is called to completely stop stream parser operations. This
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is called internally when the stream parser encounters an error, and
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it is called from the upper layer when unattaching a TCP socket.
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strp_done is called to unattach the stream parser from the TCP socket.
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This must be called after the stream processor has be stopped.
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strp_check_rcv is called to check for new messages on the socket. This
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is normally called at initialization of the a stream parser instance
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of after strp_unpause.
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Statistics
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----------
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Various counters are kept for each stream parser for a TCP socket.
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These are in the strp_stats structure. strp_aggr_stats is a convenience
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structure for accumulating statistics for multiple stream parser
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instances. save_strp_stats and aggregate_strp_stats are helper functions
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to save and aggregate statistics.
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Message assembly limits
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-----------------------
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The stream parser provide mechanisms to limit the resources consumed by
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message assembly.
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A timer is set when assembly starts for a new message. The message
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timeout is taken from rcvtime for the associated TCP socket. If the
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timer fires before assembly completes the stream parser is aborted
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and the ETIMEDOUT error is set on the TCP socket.
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Message length is limited to the receive buffer size of the associated
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TCP socket. If the length returned by parse_msg is greater than
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the socket buffer size then the stream parser is aborted with
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EMSGSIZE error set on the TCP socket. Note that this makes the
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maximum size of receive skbuffs for a socket with a stream parser
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to be 2*sk_rcvbuf of the TCP socket.
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