Re: [REGRESSION] virtio: reject shm region if length is zero

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 15, 2025 at 09:19:34PM +0200, Alyssa Ross wrote:
> Alyssa Ross <hi@alyssa.is> writes:
> 
> > On Mon, May 12, 2025 at 07:51:53AM +0930, SamiUddinsami.md.ko@gmail.com wrote:
> >> From: Sami Uddin <sami.md.ko@gmail.com>
> >>
> >> Prevent usage of shared memory regions where the length is zero,
> >> as such configurations are not valid and may lead to unexpected behavior.
> >>
> >> Signed-off-by: Sami Uddin <sami.md.ko@gmail.com>
> >> ---
> >> v3:
> >> - Use idiomatic 'if (!region->len)' as suggested by reviewer
> >> v2:
> >> - Fixed coding style issue: added space after 'if' statement
> >>
> >>  include/linux/virtio_config.h | 2 ++
> >>  1 file changed, 2 insertions(+)
> >
> > Hi, I'm sorry to be the bearer of bad news, but since this patch my VM
> > no longer works.  The system is running wayland-proxy-virtwl[1] inside
> > a crosvm[2] VM, using crosvm's virtio-gpu device to do cross-domain
> > Wayland forwarding.
> >
> > Since this change, wayland-proxy-virtwl crashes with the following log
> > message:
> >
> > 	wl-proxy [WARNING]: Error handling client: Unix.Unix_error(Unix.EINVAL, "DRM_IOCTL_VIRTGPU_RESOURCE_CREATE_BLOB", "")
> >
> > I'm pretty confused by what this change was supposed to do in the first
> > place…  Looking at how virtio_get_shm_region() is used in
> > virtio_gpu_init(), it's called with a pointer to zeroed memory, and then
> > the get_shm_region() implementation is supposed to write to the region,
> > without ever reading from it as far as I can tell.  Why is the initial
> > value of an out parameter being checked at all?  How does this prevent
> > using zero-length shared memory regions?
> >
> > [1]: https://crosvm.dev/
> > [2]: https://github.com/talex5/wayland-proxy-virtwl
> >
> > #regzbot introduced: 206cc44588f72b49ad4d7e21a7472ab2a72a83df
> 
> Okay, just found that it's already been reverted:
> https://lore.kernel.org/all/20250808072533-mutt-send-email-mst@kernel.org/
> 
> Still, I'm confused how this was supposed to fix anything…
> 
> #regzbot fix: Revert "virtio: reject shm region if length is zero"



Are you asking why was the patch applied in the 1st place?
It seemed like an invalid behaviour to me, and I thought it's
not too late to block it so we don't need to support it
down the road.

-- 
MST