Re: [PATCH net] tuntap: raise EPOLLOUT on device up

[Jason Wang <jasowang@redhat.com>]

On 2018年05月22日 06:08, Michael S. Tsirkin wrote:
> On Mon, May 21, 2018 at 11:47:42AM -0400, David Miller wrote:
>> From: Jason Wang <jasowang@redhat.com>
>> Date: Fri, 18 May 2018 21:00:43 +0800
>>
>>> We return -EIO on device down but can not raise EPOLLOUT after it was
>>> up. This may confuse user like vhost which expects tuntap to raise
>>> EPOLLOUT to re-enable its TX routine after tuntap is down. This could
>>> be easily reproduced by transmitting packets from VM while down and up
>>> the tap device. Fixing this by set SOCKWQ_ASYNC_NOSPACE on -EIO.
>>>
>>> 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>
>> I'm no so sure what to do with this patch.
>>
>> Like Michael says, this flag bit is only checks upon transmit which
>> may or may not happen after this point.  It doesn't seem to be
>> guaranteed.

The flag is checked in tun_chr_poll() as well.

> Jason, can't we detect a link up transition and respond accordingly?
> What do you think?
>

I think we've already tried to do this, in tun_net_open() we call 
write_space(). But the problem is the bit may not be set at that time.

A second thought is to set the bit in tun_chr_poll() instead of -EIO like:

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d45ac37..46a1573 100644
--- 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 +1452,9 @@ 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))))
+       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)

Does this make more sense?

Thanks