Applications use -ECONNREFUSED as returned from write() in order to
determine that a socket should be closed. However, when using connected
dgram unix sockets in a poll/write loop, a final POLLOUT event can be
missed when the remote end closes. Thus, the poll is stuck forever:
thread 1 (client) thread 2 (server)
connect() to server
write() returns -EAGAIN
unix_dgram_poll()
-> unix_recvq_full() is true
close()
->unix_release_sock()
->wake_up_interruptible_all()
unix_dgram_poll() (due to the
wake_up_interruptible_all)
-> unix_recvq_full() still is true
->free all skbs
Now thread 1 is stuck and will not receive anymore wakeups. In this
case, when thread 1 gets the -EAGAIN, it has not queued any skbs
otherwise the 'free all skbs' step would in fact cause a wakeup and
a POLLOUT return. So the race here is probably fairly rare because
it means there are no skbs that thread 1 queued and that thread 1
schedules before the 'free all skbs' step.
This issue was reported as a hang when /dev/log is closed.
The fix is to signal POLLOUT if the socket is marked as SOCK_DEAD, which
means a subsequent write() will get -ECONNREFUSED.
Reported-by: Ian Lance Taylor <i...@golang.org>
Cc: David Rientjes <rient...@google.com>
Cc: Rainer Weikusat <rweiku...@mobileactivedefense.com>
Cc: Eric Dumazet <eduma...@google.com>
Signed-off-by: Jason Baron <jba...@akamai.com>
---
v2: use check for SOCK_DEAD, since skb's can be purged in unix_sock_destructor()
---
net/unix/af_unix.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 1772a0e..d1edfa3 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -430,7 +430,12 @@ static int unix_dgram_peer_wake_me(struct sock *sk, struct
sock *other)
connected = unix_dgram_peer_wake_connect(sk, other);
- if (unix_recvq_full(other))
+ /* If other is SOCK_DEAD, we want to make sure we signal
+ * POLLOUT, such that a subsequent write() can get a
+ * -ECONNREFUSED. Otherwise, if we haven't queued any skbs
+ * to other and its full, we will hang waiting for POLLOUT.
+ */
+ if (unix_recvq_full(other) && !sock_flag(other, SOCK_DEAD))
return 1;
if (connected)
--
1.9.1
