[PATCH net-next] af_unix: do not report POLLOUT on listeners

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

poll(POLLOUT) on a listener should not report fd is ready for
a write().

This would break some applications using poll() and pfd.events = -1,
as they would not block in poll()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Alan Burlison <Alan.Burlison@oracle.com>
Tested-by: Eric Dumazet <edumazet@google.com>
---
 net/unix/af_unix.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 94f658235fb4..aaa0b58d6aba 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -326,9 +326,10 @@ found:
 	return s;
 }
 
-static inline int unix_writable(struct sock *sk)
+static int unix_writable(const struct sock *sk)
 {
-	return (atomic_read(&sk->sk_wmem_alloc) << 2) <= sk->sk_sndbuf;
+	return sk->sk_state != TCP_LISTEN &&
+	       (atomic_read(&sk->sk_wmem_alloc) << 2) <= sk->sk_sndbuf;
 }
 
 static void unix_write_space(struct sock *sk)