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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 18 May 2018 16:13:36 +0300

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 <[email-masked]>
> Cc: Eric Dumazet <[email-masked]>
> Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  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?

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