Re: [PATCH net-next v2] tap: report IFF_DETACH_QUEUE in TUNGETIFF

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Umang Pokhriyal wrote:
> tun sets IFF_DETACH_QUEUE in the flags returned by TUNGETIFF when the
> queue is detached, since commit 3d407a80b62f ("tun: Report whether the
> queue is attached or not"). tap does not, so userspace cannot tell a
> detached macvtap or ipvtap queue from an attached one.
> 
> Cloud Hypervisor ran into this when checking queue state on macvtap.
> 
> Report the flag from q->enabled, as tun does. Unlike tun, tap accepts
> TUNSETIFF on a bound fd, so ignore IFF_DETACH_QUEUE there to keep
> writing the TUNGETIFF flags back working.
> 
> Assisted-by: LLM
> Signed-off-by: Umang Pokhriyal <umangpokhriyall@gmail.com>
> ---
> 
> Notes:
>     v2:
>       - ignore IFF_DETACH_QUEUE in TUNSETIFF so the TUNGETIFF flags can be
>         written back, and describe the change as parity with tun (Sashiko)
>       - drop Willem's Reviewed-by because of the new TUNSETIFF hunk
>     v1: https://lore.kernel.org/netdev/20260929142238.41742-1-umangpokhriyall@gmail.com/
> 
>  drivers/net/tap.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> index ff67d99deb39e..832439b8a8988 100644
> --- a/drivers/net/tap.c
> +++ b/drivers/net/tap.c
> @@ -932,6 +932,9 @@ static long tap_ioctl(struct file *file, unsigned int cmd,
>  		if (get_user(u, &ifr->ifr_flags))
>  			return -EFAULT;
>  
> +		/* TUNGETIFF may report IFF_DETACH_QUEUE, ignore it here */
> +		u &= ~IFF_DETACH_QUEUE;
> +

I suppose we want tap to expose this state through TUNGETIFF, like tun,
because applications already use that?

Else cleaner than working around bits would be a whole new TUNGETQUEUE
call to match TUNSETQUEUE. Rather than to squish this into TUNGETIFF,
while that has no equivalent in TUNSETIFF.

But, that approach does not help existing applications.

So this is probably the right way. Just want to quickly check.

>  		ret = 0;
>  		if ((u & ~TAP_IFFEATURES) != (IFF_NO_PI | IFF_TAP))
>  			ret = -EINVAL;
> @@ -950,6 +953,8 @@ static long tap_ioctl(struct file *file, unsigned int cmd,
>  
>  		ret = 0;
>  		u = q->flags;
> +		if (!q->enabled)
> +			u |= IFF_DETACH_QUEUE;
>  		if (copy_to_user(&ifr->ifr_name, tap->dev->name, IFNAMSIZ) ||
>  		    put_user(u, &ifr->ifr_flags))
>  			ret = -EFAULT;
> 
> base-commit: cfb7793d1bc0f7d90571611979654cf1b3886b29
> -- 
> 2.53.0
>