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)
