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.
This can be tested with a program available at
https://github.com/wentasah/isotp-poll-test/blob/fd095b2242c49dc5d3e36faf5ac9f4f47fd002c7/isotp-poll-test.c
which can trigger the problem on a vcan interface. When running the
program as:
./isotp-poll-test -s 123 -d 321 -o
it starts sending ISOTP messages that include increasing ASCII
numbers. poll() is used to wait before the next transmission.
With current mainline Linux, once the message length is greater than 7
bytes, write() returns -EAGAIN and the program terminates. This should
not happen, because the previous poll() reported that the write()
would not block.
After applying this patch, the above command doesn't fail - if one
runs some ISOTP reader such as:
isotprecv -l -s 321 -d 123 vcan0
This test program can also show another problem. When running:
./isotp-poll-test -s 321 -d 123 -i -a
and then in another terminal:
./isotp-poll-test -s 123 -d 321 -o
The first program receives the messages and uses the counter values to
check for lost messages. After a random number of iterations a lost
message is always detected. I believe that ISOTP should be reliable
protocol, at least on vcan, shouldn't it?
Anyway, this patch doesn't try to address this problem.
Signed-off-by: Michal Sojka <michal.sojka@xxxxxxx>
Reported-by: Jakub Jira <jirajak2@xxxxxxxxxxx>
---
Changelog:
v2: Added waiting for isotp-specific wait queue: poll_wait(file, &so->wait, wait).
---
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
