can: isotp: isotp_ops: fix poll() to not report false EPOLLOUT events

When using select()/poll()/epoll() with a non-blocking ISOTP socket to
wait for when non-blocking write is possible, a 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.

v2: https://lore.kernel.org/all/20230302092812.320643-1-michal.sojka@cvut.cz
v1: https://lore.kernel.org/all/20230224010659.48420-1-michal.sojka@cvut.cz
    https://lore.kernel.org/all/b53a04a2-ba1f-3858-84c1-d3eb3301ae15@hartkopp.net

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>
Fixes: e057dd3fc20f ("can: add ISO 15765-2:2016 transport protocol")
Link: https://lore.kernel.org/all/20230331125511.372783-1-michal.sojka@cvut.cz
Cc: stable@vger.kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

diff --git a/net/can/isotp.c b/net/can/isotp.c
index 47c2ebad10edc2f15de187b7e02cc8f52f141c5b..281b7766c54ec8075cf223fd15825512975ebe37 100644 (file)
--- 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,