strparser: Stream parser for messages

This patch introduces a utility for parsing application layer protocol
messages in a TCP stream. This is a generalization of the mechanism
implemented of Kernel Connection Multiplexor.

The API includes a context structure, a set of callbacks, utility
functions, and a data ready function.

A stream parser instance is defined by a strparse structure that
is bound to a TCP socket. The function to initialize the structure
is:

int strp_init(struct strparser *strp, struct sock *csk,
              struct strp_callbacks *cb);

csk is the TCP socket being bound to and cb are the parser callbacks.

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:

void strp_tcp_data_ready(struct strparser *strp);

A parser is bound to a TCP socket by setting data_ready function to
strp_tcp_data_ready so that all receive indications on the socket
go through the parser. This is assumes that sk_user_data is set to
the strparser structure.

There are four callbacks.
 - parse_msg is called to parse the message (returns length or error).
 - rcv_msg is called when a complete message has been received
 - read_sock_done is called when data_ready function exits
 - abort_parser is called to abort the parser

The input to parse_msg is an skbuff which contains next message under
construction. The backend processing of parse_msg will parse the
application layer protocol headers to determine the length of
the message in the stream. The possible return values 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 is parsing, give control back to userspace
      assuming that synchronzation is lost and the stream
      is unrecoverable (application expected to close TCP socket)

In the case of error return (< 0) strparse will stop the parser
and report and error to userspace. The application must deal
with the error. To handle the error the strparser is unbound
from the TCP socket. If the error indicates that the stream
TCP socket is at recoverable point (ESTRPIPE) then the application
can read the TCP socket to process the stream. Once the application
has dealt with the exceptions in the stream, it may again bind the
socket to a strparser to continue data operations.

Note that ENODATA may be returned to the application. In this case
parse_msg returned -ESTRPIPE, however strparser was unable to maintain
synchronization of the stream (i.e. some of the message in question
was already read by the parser).

strp_pause and strp_unpause are used to provide flow control. For
instance, if rcv_msg is called but the upper layer can't immediately
consume the message it can hold the message and pause strparser.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 497 insertions, 0 deletions

diff --git a/net/strparser/Kconfig b/net/strparser/Kconfig
new file mode 100644
index 0000000..6cff3f6
--- /dev/null
+++ b/net/strparser/Kconfig
@@ -0,0 +1,4 @@
+
+config STREAM_PARSER
+	tristate
+	default n
diff --git a/net/strparser/Makefile b/net/strparser/Makefile
new file mode 100644
index 0000000..858a126
--- /dev/null
+++ b/net/strparser/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_STREAM_PARSER) += strparser.o
diff --git a/net/strparser/strparser.c b/net/strparser/strparser.c
new file mode 100644
index 0000000..fd688c0
--- /dev/null
+++ b/net/strparser/strparser.c
@@ -0,0 +1,492 @@
+/*
+ * Stream Parser
+ *
+ * Copyright (c) 2016 Tom Herbert <tom@herbertland.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2
+ * as published by the Free Software Foundation.
+ */
+
+#include <linux/bpf.h>
+#include <linux/errno.h>
+#include <linux/errqueue.h>
+#include <linux/file.h>
+#include <linux/in.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/net.h>
+#include <linux/netdevice.h>
+#include <linux/poll.h>
+#include <linux/rculist.h>
+#include <linux/skbuff.h>
+#include <linux/socket.h>
+#include <linux/uaccess.h>
+#include <linux/workqueue.h>
+#include <net/strparser.h>
+#include <net/netns/generic.h>
+#include <net/sock.h>
+#include <net/tcp.h>
+
+static struct workqueue_struct *strp_wq;
+
+struct _strp_rx_msg {
+	/* Internal cb structure. struct strp_rx_msg must be first for passing
+	 * to upper layer.
+	 */
+	struct strp_rx_msg strp;
+	int accum_len;
+	int early_eaten;
+};
+
+static inline struct _strp_rx_msg *_strp_rx_msg(struct sk_buff *skb)
+{
+	return (struct _strp_rx_msg *)((void *)skb->cb +
+		offsetof(struct qdisc_skb_cb, data));
+}
+
+/* Lower lock held */
+static void strp_abort_rx_strp(struct strparser *strp, int err)
+{
+	struct sock *csk = strp->sk;
+
+	/* Unrecoverable error in receive */
+
+	del_timer(&strp->rx_msg_timer);
+
+	if (strp->rx_stopped)
+		return;
+
+	strp->rx_stopped = 1;
+
+	/* Report an error on the lower socket */
+	csk->sk_err = err;
+	csk->sk_error_report(csk);
+}
+
+static void strp_start_rx_timer(struct strparser *strp)
+{
+	if (strp->sk->sk_rcvtimeo)
+		mod_timer(&strp->rx_msg_timer, strp->sk->sk_rcvtimeo);
+}
+
+/* Lower lock held */
+static void strp_parser_err(struct strparser *strp, int err,
+			    read_descriptor_t *desc)
+{
+	desc->error = err;
+	kfree_skb(strp->rx_skb_head);
+	strp->rx_skb_head = NULL;
+	strp->cb.abort_parser(strp, err);
+}
+
+/* Lower socket lock held */
+static int strp_tcp_recv(read_descriptor_t *desc, struct sk_buff *orig_skb,
+			 unsigned int orig_offset, size_t orig_len)
+{
+	struct strparser *strp = (struct strparser *)desc->arg.data;
+	struct _strp_rx_msg *rxm;
+	struct sk_buff *head, *skb;
+	size_t eaten = 0, cand_len;
+	ssize_t extra;
+	int err;
+	bool cloned_orig = false;
+
+	if (strp->rx_paused)
+		return 0;
+
+	head = strp->rx_skb_head;
+	if (head) {
+		/* Message already in progress */
+
+		rxm = _strp_rx_msg(head);
+		if (unlikely(rxm->early_eaten)) {
+			/* Already some number of bytes on the receive sock
+			 * data saved in rx_skb_head, just indicate they
+			 * are consumed.
+			 */
+			eaten = orig_len <= rxm->early_eaten ?
+				orig_len : rxm->early_eaten;
+			rxm->early_eaten -= eaten;
+
+			return eaten;
+		}
+
+		if (unlikely(orig_offset)) {
+			/* Getting data with a non-zero offset when a message is
+			 * in progress is not expected. If it does happen, we
+			 * need to clone and pull since we can't deal with
+			 * offsets in the skbs for a message expect in the head.
+			 */
+			orig_skb = skb_clone(orig_skb, GFP_ATOMIC);
+			if (!orig_skb) {
+				STRP_STATS_INCR(strp->stats.rx_mem_fail);
+				desc->error = -ENOMEM;
+				return 0;
+			}
+			if (!pskb_pull(orig_skb, orig_offset)) {
+				STRP_STATS_INCR(strp->stats.rx_mem_fail);
+				kfree_skb(orig_skb);
+				desc->error = -ENOMEM;
+				return 0;
+			}
+			cloned_orig = true;
+			orig_offset = 0;
+		}
+
+		if (!strp->rx_skb_nextp) {
+			/* We are going to append to the frags_list of head.
+			 * Need to unshare the frag_list.
+			 */
+			err = skb_unclone(head, GFP_ATOMIC);
+			if (err) {
+				STRP_STATS_INCR(strp->stats.rx_mem_fail);
+				desc->error = err;
+				return 0;
+			}
+
+			if (unlikely(skb_shinfo(head)->frag_list)) {
+				/* We can't append to an sk_buff that already
+				 * has a frag_list. We create a new head, point
+				 * the frag_list of that to the old head, and
+				 * then are able to use the old head->next for
+				 * appending to the message.
+				 */
+				if (WARN_ON(head->next)) {
+					desc->error = -EINVAL;
+					return 0;
+				}
+
+				skb = alloc_skb(0, GFP_ATOMIC);
+				if (!skb) {
+					STRP_STATS_INCR(strp->stats.rx_mem_fail);
+					desc->error = -ENOMEM;
+					return 0;
+				}
+				skb->len = head->len;
+				skb->data_len = head->len;
+				skb->truesize = head->truesize;
+				*_strp_rx_msg(skb) = *_strp_rx_msg(head);
+				strp->rx_skb_nextp = &head->next;
+				skb_shinfo(skb)->frag_list = head;
+				strp->rx_skb_head = skb;
+				head = skb;
+			} else {
+				strp->rx_skb_nextp =
+				    &skb_shinfo(head)->frag_list;
+			}
+		}
+	}
+
+	while (eaten < orig_len) {
+		/* Always clone since we will consume something */
+		skb = skb_clone(orig_skb, GFP_ATOMIC);
+		if (!skb) {
+			STRP_STATS_INCR(strp->stats.rx_mem_fail);
+			desc->error = -ENOMEM;
+			break;
+		}
+
+		cand_len = orig_len - eaten;
+
+		head = strp->rx_skb_head;
+		if (!head) {
+			head = skb;
+			strp->rx_skb_head = head;
+			/* Will set rx_skb_nextp on next packet if needed */
+			strp->rx_skb_nextp = NULL;
+			rxm = _strp_rx_msg(head);
+			memset(rxm, 0, sizeof(*rxm));
+			rxm->strp.offset = orig_offset + eaten;
+		} else {
+			/* Unclone since we may be appending to an skb that we
+			 * already share a frag_list with.
+			 */
+			err = skb_unclone(skb, GFP_ATOMIC);
+			if (err) {
+				STRP_STATS_INCR(strp->stats.rx_mem_fail);
+				desc->error = err;
+				break;
+			}
+
+			rxm = _strp_rx_msg(head);
+			*strp->rx_skb_nextp = skb;
+			strp->rx_skb_nextp = &skb->next;
+			head->data_len += skb->len;
+			head->len += skb->len;
+			head->truesize += skb->truesize;
+		}
+
+		if (!rxm->strp.full_len) {
+			ssize_t len;
+
+			len = (*strp->cb.parse_msg)(strp, head);
+
+			if (!len) {
+				/* Need more header to determine length */
+				if (!rxm->accum_len) {
+					/* Start RX timer for new message */
+					strp_start_rx_timer(strp);
+				}
+				rxm->accum_len += cand_len;
+				eaten += cand_len;
+				STRP_STATS_INCR(strp->stats.rx_need_more_hdr);
+				WARN_ON(eaten != orig_len);
+				break;
+			} else if (len < 0) {
+				if (len == -ESTRPIPE && rxm->accum_len) {
+					len = -ENODATA;
+					strp->rx_unrecov_intr = 1;
+				} else {
+					strp->rx_interrupted = 1;
+				}
+				strp_parser_err(strp, err, desc);
+				break;
+			} else if (len > strp->sk->sk_rcvbuf) {
+				/* Message length exceeds maximum allowed */
+				STRP_STATS_INCR(strp->stats.rx_msg_too_big);
+				strp_parser_err(strp, -EMSGSIZE, desc);
+				break;
+			} else if (len <= (ssize_t)head->len -
+					  skb->len - rxm->strp.offset) {
+				/* Length must be into new skb (and also
+				 * greater than zero)
+				 */
+				STRP_STATS_INCR(strp->stats.rx_bad_hdr_len);
+				strp_parser_err(strp, -EPROTO, desc);
+				break;
+			}
+
+			rxm->strp.full_len = len;
+		}
+
+		extra = (ssize_t)(rxm->accum_len + cand_len) -
+			rxm->strp.full_len;
+
+		if (extra < 0) {
+			/* Message not complete yet. */
+			if (rxm->strp.full_len - rxm->accum_len >
+			    tcp_inq(strp->sk)) {
+				/* Don't have the whole messages in the socket
+				 * buffer. Set strp->rx_need_bytes to wait for
+				 * the rest of the message. Also, set "early
+				 * eaten" since we've already buffered the skb
+				 * but don't consume yet per tcp_read_sock.
+				 */
+
+				if (!rxm->accum_len) {
+					/* Start RX timer for new message */
+					strp_start_rx_timer(strp);
+				}
+
+				strp->rx_need_bytes = rxm->strp.full_len -
+						       rxm->accum_len;
+				rxm->accum_len += cand_len;
+				rxm->early_eaten = cand_len;
+				STRP_STATS_ADD(strp->stats.rx_bytes, cand_len);
+				desc->count = 0; /* Stop reading socket */
+				break;
+			}
+			rxm->accum_len += cand_len;
+			eaten += cand_len;
+			WARN_ON(eaten != orig_len);
+			break;
+		}
+
+		/* Positive extra indicates ore bytes than needed for the
+		 * message
+		 */
+
+		WARN_ON(extra > cand_len);
+
+		eaten += (cand_len - extra);
+
+		/* Hurray, we have a new message! */
+		del_timer(&strp->rx_msg_timer);
+		strp->rx_skb_head = NULL;
+		STRP_STATS_INCR(strp->stats.rx_msgs);
+
+		/* Give skb to upper layer */
+		strp->cb.rcv_msg(strp, head);
+
+		if (unlikely(strp->rx_paused)) {
+			/* Upper layer paused strp */
+			break;
+		}
+	}
+
+	if (cloned_orig)
+		kfree_skb(orig_skb);
+
+	STRP_STATS_ADD(strp->stats.rx_bytes, eaten);
+
+	return eaten;
+}
+
+static int default_read_sock_done(struct strparser *strp, int err)
+{
+	return err;
+}
+
+/* Called with lock held on lower socket */
+static int strp_tcp_read_sock(struct strparser *strp)
+{
+	read_descriptor_t desc;
+
+	desc.arg.data = strp;
+	desc.error = 0;
+	desc.count = 1; /* give more than one skb per call */
+
+	/* sk should be locked here, so okay to do tcp_read_sock */
+	tcp_read_sock(strp->sk, &desc, strp_tcp_recv);
+
+	desc.error = strp->cb.read_sock_done(strp, desc.error);
+
+	return desc.error;
+}
+
+/* Lower sock lock held */
+void strp_tcp_data_ready(struct strparser *strp)
+{
+	struct sock *csk = strp->sk;
+
+	if (unlikely(strp->rx_stopped))
+		return;
+
+	/* This check is needed to synchronize with do_strp_rx_work.
+	 * do_strp_rx_work acquires a process lock (lock_sock) whereas
+	 * the lock held here is bh_lock_sock. The two locks can be
+	 * held by different threads at the same time, but bh_lock_sock
+	 * allows a thread in BH context to safely check if the process
+	 * lock is held. In this case, if the lock is held, queue work.
+	 */
+	if (sock_owned_by_user(csk)) {
+		queue_work(strp_wq, &strp->rx_work);
+		return;
+	}
+
+	if (strp->rx_paused)
+		return;
+
+	if (strp->rx_need_bytes) {
+		if (tcp_inq(csk) >= strp->rx_need_bytes)
+			strp->rx_need_bytes = 0;
+		else
+			return;
+	}
+
+	if (strp_tcp_read_sock(strp) == -ENOMEM)
+		queue_work(strp_wq, &strp->rx_work);
+}
+EXPORT_SYMBOL_GPL(strp_tcp_data_ready);
+
+static void do_strp_rx_work(struct strparser *strp)
+{
+	read_descriptor_t rd_desc;
+	struct sock *csk = strp->sk;
+
+	/* We need the read lock to synchronize with strp_tcp_data_ready. We
+	 * need the socket lock for calling tcp_read_sock.
+	 */
+	lock_sock(csk);
+
+	if (unlikely(csk->sk_user_data != strp))
+		goto out;
+
+	if (unlikely(strp->rx_stopped))
+		goto out;
+
+	if (strp->rx_paused)
+		goto out;
+
+	rd_desc.arg.data = strp;
+
+	if (strp_tcp_read_sock(strp) == -ENOMEM)
+		queue_work(strp_wq, &strp->rx_work);
+
+out:
+	release_sock(csk);
+}
+
+static void strp_rx_work(struct work_struct *w)
+{
+	do_strp_rx_work(container_of(w, struct strparser, rx_work));
+}
+
+static void strp_rx_msg_timeout(unsigned long arg)
+{
+	struct strparser *strp = (struct strparser *)arg;
+
+	/* Message assembly timed out */
+	STRP_STATS_INCR(strp->stats.rx_msg_timeouts);
+	lock_sock(strp->sk);
+	strp->cb.abort_parser(strp, ETIMEDOUT);
+	release_sock(strp->sk);
+}
+
+int strp_init(struct strparser *strp, struct sock *csk,
+	      struct strp_callbacks *cb)
+{
+	if (!cb || !cb->rcv_msg || !cb->parse_msg)
+		return -EINVAL;
+
+	memset(strp, 0, sizeof(*strp));
+
+	strp->sk = csk;
+
+	setup_timer(&strp->rx_msg_timer, strp_rx_msg_timeout,
+		    (unsigned long)strp);
+
+	INIT_WORK(&strp->rx_work, strp_rx_work);
+
+	strp->cb.rcv_msg = cb->rcv_msg;
+	strp->cb.parse_msg = cb->parse_msg;
+	strp->cb.read_sock_done = cb->read_sock_done ? : default_read_sock_done;
+	strp->cb.abort_parser = cb->abort_parser ? : strp_abort_rx_strp;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(strp_init);
+
+/* strp must already be stopped so that strp_tcp_recv will no longer be called.
+ * Note that strp_done is not called with the lower socket held.
+ */
+void strp_done(struct strparser *strp)
+{
+	WARN_ON(!strp->rx_stopped);
+
+	del_timer_sync(&strp->rx_msg_timer);
+	cancel_work_sync(&strp->rx_work);
+
+	if (strp->rx_skb_head) {
+		kfree_skb(strp->rx_skb_head);
+		strp->rx_skb_head = NULL;
+	}
+}
+EXPORT_SYMBOL_GPL(strp_done);
+
+void strp_stop(struct strparser *strp)
+{
+	strp->rx_stopped = 1;
+}
+EXPORT_SYMBOL_GPL(strp_stop);
+
+void strp_check_rcv(struct strparser *strp)
+{
+	queue_work(strp_wq, &strp->rx_work);
+}
+EXPORT_SYMBOL_GPL(strp_check_rcv);
+
+static int __init strp_mod_init(void)
+{
+	strp_wq = create_singlethread_workqueue("kstrp");
+
+	return 0;
+}
+
+static void __exit strp_mod_exit(void)
+{
+}
+module_init(strp_mod_init);
+module_exit(strp_mod_exit);
+MODULE_LICENSE("GPL");