[3.16.y-ckt stable] Patch "af_unix: Revert 'lock_interruptible' in stream receive code" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    af_unix: Revert 'lock_interruptible' in stream receive code

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt22.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 9d819664f0a9f2439da623760bfc1b6ea079ac26 Mon Sep 17 00:00:00 2001
From: Rainer Weikusat <rweikusat at mobileactivedefense.com>
Date: Wed, 16 Dec 2015 20:09:25 +0000
Subject: af_unix: Revert 'lock_interruptible' in stream receive code

commit 3822b5c2fc62e3de8a0f33806ff279fb7df92432 upstream.

With b3ca9b02b00704053a38bfe4c31dbbb9c13595d0, the AF_UNIX SOCK_STREAM
receive code was changed from using mutex_lock(&u->readlock) to
mutex_lock_interruptible(&u->readlock) to prevent signals from being
delayed for an indefinite time if a thread sleeping on the mutex
happened to be selected for handling the signal. But this was never a
problem with the stream receive code (as opposed to its datagram
counterpart) as that never went to sleep waiting for new messages with the
mutex held and thus, wouldn't cause secondary readers to block on the
mutex waiting for the sleeping primary reader. As the interruptible
locking makes the code more complicated in exchange for no benefit,
change it back to using mutex_lock.

Signed-off-by: Rainer Weikusat <rweikusat at mobileactivedefense.com>
Acked-by: Hannes Frederic Sowa <hannes at stressinduktion.org>
Signed-off-by: David S. Miller <davem at davemloft.net>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/unix/af_unix.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 9acfd79f0d13..7229794c1419 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2096,14 +2096,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 		memset(&tmp_scm, 0, sizeof(tmp_scm));
 	}

-	err = mutex_lock_interruptible(&u->readlock);
-	if (unlikely(err)) {
-		/* recvmsg() in non blocking mode is supposed to return -EAGAIN
-		 * sk_rcvtimeo is not honored by mutex_lock_interruptible()
-		 */
-		err = noblock ? -EAGAIN : -ERESTARTSYS;
-		goto out;
-	}
+	mutex_lock(&u->readlock);

 	if (flags & MSG_PEEK)
 		skip = sk_peek_offset(sk, flags);
@@ -2144,12 +2137,12 @@ again:

 			timeo = unix_stream_data_wait(sk, timeo, last);

-			if (signal_pending(current)
-			    ||  mutex_lock_interruptible(&u->readlock)) {
+			if (signal_pending(current)) {
 				err = sock_intr_errno(timeo);
 				goto out;
 			}

+			mutex_lock(&u->readlock);
 			continue;
  unlock:
 			unix_state_unlock(sk);