Re: [PATCH net] tuntap: correctly set SOCKWQ_ASYNC_NOSPACE

[David Miller]

From: Jason Wang 
Date: Tue, 22 May 2018 14:21:04 +0800

> 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 
> Cc: Eric Dumazet 
> Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
> Signed-off-by: Jason Wang 

Applied and queued up for -stable, thanks Jason.

[PATCH net] tuntap: correctly set SOCKWQ_ASYNC_NOSPACE

[Jason Wang]

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 
Cc: Eric Dumazet 
Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
Signed-off-by: Jason Wang 
---
 drivers/net/tun.c | 19 +++
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d45ac37..45d8077 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 +1452,14 @@ static __poll_t tun_chr_poll(struct file *file, 
poll_table *wait)
if (!ptr_ring_empty(>tx_ring))
mask |= EPOLLIN | EPOLLRDNORM;
 
-   if (tun->dev->flags & IFF_UP &&
-   (sock_writeable(sk) ||
-(!test_and_set_bit(SOCKWQ_ASYNC_NOSPACE, >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_socket->flags) &&
+tun_sock_writeable(tun, tfile)))
mask |= EPOLLOUT | EPOLLWRNORM;
 
if (tun->dev->reg_state != NETREG_REGISTERED)
-- 
2.7.4