tuntap: correctly set SOCKWQ_ASYNC_NOSPACE

When link is down, writes to the device might fail with
-EIO. Userspace needs an indication when the status is resolved.  As a
fix, tun_net_open() attempts to wake up writers - but that is only
effective if SOCKWQ_ASYNC_NOSPACE has been set in the past. This is
not the case of vhost_net which only poll for EPOLLOUT after it meets
errors during sendmsg().

This patch fixes this by making sure SOCKWQ_ASYNC_NOSPACE is set when
socket is not writable or device is down to guarantee EPOLLOUT will be
raised in either tun_chr_poll() or tun_sock_write_space() after device
is up.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Eric Dumazet <edumazet@google.com>
Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d45ac37..45d8077 100644 (file)
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1423,6 +1423,13 @@ static void tun_net_init(struct net_device *dev)
        dev->max_mtu = MAX_MTU - dev->hard_header_len;
 }
 
+static bool tun_sock_writeable(struct tun_struct *tun, struct tun_file *tfile)
+{
+       struct sock *sk = tfile->socket.sk;
+
+       return (tun->dev->flags & IFF_UP) && sock_writeable(sk);
+}
+
 /* Character device part */
 
 /* Poll */
@@ -1445,10 +1452,14 @@ static __poll_t tun_chr_poll(struct file *file, poll_table *wait)
        if (!ptr_ring_empty(&tfile->tx_ring))
                mask |= EPOLLIN | EPOLLRDNORM;
 
-       if (tun->dev->flags & IFF_UP &&
-           (sock_writeable(sk) ||
-            (!test_and_set_bit(SOCKWQ_ASYNC_NOSPACE, &sk->sk_socket->flags) &&
-             sock_writeable(sk))))
+       /* Make sure SOCKWQ_ASYNC_NOSPACE is set if not writable to
+        * guarantee EPOLLOUT to be raised by either here or
+        * tun_sock_write_space(). Then process could get notification
+        * after it writes to a down device and meets -EIO.
+        */
+       if (tun_sock_writeable(tun, tfile) ||
+           (!test_and_set_bit(SOCKWQ_ASYNC_NOSPACE, &sk->sk_socket->flags) &&
+            tun_sock_writeable(tun, tfile)))
                mask |= EPOLLOUT | EPOLLWRNORM;
 
        if (tun->dev->reg_state != NETREG_REGISTERED)