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

[Jason Wang]

On 2018å05æ18æ 21:13, Michael S. Tsirkin wrote:
> On Fri, May 18, 2018 at 09:00:43PM +0800, Jason Wang wrote:
> > 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@xxxxxxxxxxxxxxxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >   drivers/net/tun.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index d45ac37..1b29761 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1734,8 +1734,10 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> >   	int skb_xdp = 1;
> >   	bool frags = tun_napi_frags_enabled(tun);
> >   
> > -	if (!(tun->dev->flags & IFF_UP))
> > +	if (!(tun->dev->flags & IFF_UP)) {
> Isn't this racy?  What if flag is cleared at this point?

I think you mean "set at this point"? Then yes, so we probably need to 
set the bit during tun_net_close().

Thanks

> > +		set_bit(SOCKWQ_ASYNC_NOSPACE, &tfile->socket.flags);
> >   		return -EIO;
> > +	}
> >   
> >   	if (!(tun->flags & IFF_NO_PI)) {
> >   		if (len < sizeof(pi))
> > --
> > 2.7.4