[PATCH v3] can: isotp: fix poll() to not report false EPOLLOUT events

[Michal Sojka <michal.sojka@cvut.cz>]

When using select/poll/epoll() with a non-blocking ISOTP socket to
wait for when non-blocking write is possible, false EPOLLOUT event is
sometimes returned. This can happen at least after sending a message
which must be split to multiple CAN frames.

The reason is that isotp_sendmsg() returns -EAGAIN when tx.state is
not equal to ISOTP_IDLE and this behavior is not reflected in
datagram_poll(), which is used in isotp_ops.

This is fixed by introducing ISOTP-specific poll function, which
suppresses the EPOLLOUT events in that case.

Signed-off-by: Michal Sojka <michal.sojka@cvut.cz>
Reported-by: Jakub Jira <jirajak2@fel.cvut.cz>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>

---
Changelog:

v2: Added waiting for isotp-specific wait queue: poll_wait(file, &so->wait, wait).
v3: Trimmed the commit message.
Signed-off-by: Michal Sojka <michal.sojka@cvut.cz>
---
 net/can/isotp.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/net/can/isotp.c b/net/can/isotp.c
index 9bc344851704..ec163e36ac53 100644
--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -1608,6 +1608,21 @@ static int isotp_init(struct sock *sk)
 	return 0;
 }
 
+static __poll_t isotp_poll(struct file *file, struct socket *sock, poll_table *wait)
+{
+	struct sock *sk = sock->sk;
+	struct isotp_sock *so = isotp_sk(sk);
+
+	__poll_t mask = datagram_poll(file, sock, wait);
+	poll_wait(file, &so->wait, wait);
+
+	/* Check for false positives due to TX state */
+	if ((mask & EPOLLWRNORM) && (so->tx.state != ISOTP_IDLE))
+		mask &= ~(EPOLLOUT | EPOLLWRNORM);
+
+	return mask;
+}
+
 static int isotp_sock_no_ioctlcmd(struct socket *sock, unsigned int cmd,
 				  unsigned long arg)
 {
@@ -1623,7 +1638,7 @@ static const struct proto_ops isotp_ops = {
 	.socketpair = sock_no_socketpair,
 	.accept = sock_no_accept,
 	.getname = isotp_getname,
-	.poll = datagram_poll,
+	.poll = isotp_poll,
 	.ioctl = isotp_sock_no_ioctlcmd,
 	.gettstamp = sock_gettstamp,
 	.listen = sock_no_listen,
-- 
2.39.2