208 lines
8.1 KiB
Plaintext
208 lines
8.1 KiB
Plaintext
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Stream Parser (strparser)
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Introduction
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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 data stream. 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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The strparser works in one of two modes: receive callback or general
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mode.
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In receive callback mode, the strparser is called from the data_ready
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callback of a TCP socket. Messages are parsed and delivered as they are
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received on the socket.
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In general mode, a sequence of skbs are fed to strparser from an
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outside source. Message are parsed and delivered as the sequence is
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processed. This modes allows strparser to be applied to arbitrary
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streams of data.
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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 for receive callback mode. The
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callbacks include a parse_msg function that is called to perform
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parsing (e.g. BPF parsing in case of KCM), and a rcv_msg function
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that is called when a full message has been completed.
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Functions
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=========
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strp_init(struct strparser *strp, struct sock *sk,
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const struct strp_callbacks *cb)
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Called to initialize a stream parser. strp is a struct of type
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strparser that is allocated by the upper layer. sk is the TCP
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socket associated with the stream parser for use with receive
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callback mode; in general mode this is set to NULL. Callbacks
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are called by the stream parser (the callbacks are listed below).
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void strp_pause(struct strparser *strp)
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Temporarily pause a stream parser. Message parsing is suspended
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and no new messages are delivered to the upper layer.
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void strp_unpause(struct strparser *strp)
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Unpause a paused stream parser.
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void strp_stop(struct strparser *strp);
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strp_stop is called to completely stop stream parser operations.
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This is called internally when the stream parser encounters an
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error, and it is called from the upper layer to stop parsing
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operations.
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void strp_done(struct strparser *strp);
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strp_done is called to release any resources held by the stream
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parser instance. This must be called after the stream processor
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has been stopped.
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int strp_process(struct strparser *strp, struct sk_buff *orig_skb,
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unsigned int orig_offset, size_t orig_len,
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size_t max_msg_size, long timeo)
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strp_process is called in general mode for a stream parser to
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parse an sk_buff. The number of bytes processed or a negative
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error number is returned. Note that strp_process does not
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consume the sk_buff. max_msg_size is maximum size the stream
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parser will parse. timeo is timeout for completing a message.
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void strp_data_ready(struct strparser *strp);
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The upper layer calls strp_tcp_data_ready when data is ready on
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the lower socket for strparser to process. This should be called
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from a data_ready callback that is set on the socket. Note that
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maximum messages size is the limit of the receive socket
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buffer and message timeout is the receive timeout for the socket.
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void strp_check_rcv(struct strparser *strp);
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strp_check_rcv is called to check for new messages on the socket.
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This is normally called at initialization of a stream parser
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instance or after strp_unpause.
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Callbacks
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=========
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There are six 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 message in the stream.
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The skb->cb in the input skb is a struct strp_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 in 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) and the parser is in receive callback mode, then it will set
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the error on TCP socket and wake it up. If parse_msg returned
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-ESTRPIPE and the stream parser had previously read some bytes for
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the current message, then the error set on the attached socket is
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ENODATA since the stream is unrecoverable in that case.
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void (*lock)(struct strparser *strp)
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The lock callback is called to lock the strp structure when
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the strparser is performing an asynchronous operation (such as
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processing a timeout). In receive callback mode the default
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function is to lock_sock for the associated socket. In general
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mode the callback must be set appropriately.
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void (*unlock)(struct strparser *strp)
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The unlock callback is called to release the lock obtained
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by the lock callback. In receive callback mode the default
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function is release_sock for the associated socket. In general
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mode the callback must be set appropriately.
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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 above). This callback
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must be set.
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The skb->cb in the input skb is a struct strp_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 in receive callback mode. The stream parser may
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read multiple messages in a loop and this function allows cleanup
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to occur when exiting the loop. If the callback is not set (NULL
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in strp_init) a 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 and
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sets the error in the socket if the parser is in receive callback
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mode. The default function can be changed by setting the callback
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to non-NULL in strp_init.
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Statistics
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==========
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Various counters are kept for each stream parser instance. These are in
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the strp_stats structure. strp_aggr_stats is a convenience structure for
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accumulating statistics for multiple stream parser instances.
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save_strp_stats and aggregate_strp_stats are helper functions to save
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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. In receive
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callback mode the message timeout is taken from rcvtime for the
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associated TCP socket. In general mode, the timeout is passed as an
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argument in strp_process. If the timer fires before assembly completes
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the stream parser is aborted and the ETIMEDOUT error is set on the TCP
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socket if in receive callback mode.
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In receive callback mode, message length is limited to the receive
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buffer size of the associated TCP socket. If the length returned by
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parse_msg is greater than the socket buffer size then the stream parser
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is aborted with EMSGSIZE error set on the TCP socket. Note that this
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makes the maximum size of receive skbuffs for a socket with a stream
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parser to be 2*sk_rcvbuf of the TCP socket.
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In general mode the message length limit is passed in as an argument
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to strp_process.
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Author
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======
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Tom Herbert (tom@quantonium.net)
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