af_unix: Don't set err in unix_stream_read_generic unless there was an error

commit 1b92ee3d upstream.

The present unix_stream_read_generic contains various code sequences of
the form

err = -EDISASTER;
if (<test>)
	goto out;

This has the unfortunate side effect of possibly causing the error code
to bleed through to the final

out:
	return copied ? : err;

and then to be wrongly returned if no data was copied because the caller
didn't supply a data buffer, as demonstrated by the program available at

http://pad.lv/1540731

Change it such that err is only set if an error condition was detected.

Fixes: 3822b5c2 ("af_unix: Revert 'lock_interruptible' in stream receive code")
Reported-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Signed-off-by: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/unix/af_unix.c

@@ -2059,13 +2059,15 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 	int err = 0;
 	long timeo;
 
+	if (unlikely(sk->sk_state != TCP_ESTABLISHED)) {
 		err = -EINVAL;
-	if (sk->sk_state != TCP_ESTABLISHED)
 		goto out;
+	}
 
+	if (unlikely(flags & MSG_OOB)) {
 		err = -EOPNOTSUPP;
-	if (flags&MSG_OOB)
 		goto out;
+	}
 
 	target = sock_rcvlowat(sk, flags&MSG_WAITALL, size);
 	timeo = sock_rcvtimeo(sk, noblock);
@@ -2107,9 +2109,11 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 				goto unlock;
 
 			unix_state_unlock(sk);
+			if (!timeo) {
 				err = -EAGAIN;
-			if (!timeo)
 				break;
+			}
+
 			mutex_unlock(&u->readlock);
 
 			timeo = unix_stream_data_wait(sk, timeo);