208 строки
8.1 KiB
Plaintext
208 строки
8.1 KiB
Plaintext
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_pause(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)
|
|
|