Re: [PATCH 2/2] vduse: fix compat handling for VDUSE_IOTLB_GET_FD/VDUSE_VQ_GET_INFO

[Eugenio Perez Martin <eperezma@redhat.com>]

On Tue, Feb 3, 2026 at 11:35 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Feb 02, 2026 at 11:54:17PM +0100, Arnd Bergmann wrote:
> > On Mon, Feb 2, 2026, at 17:45, Michael S. Tsirkin wrote:
> > > On Mon, Feb 02, 2026 at 12:59:03PM +0100, Arnd Bergmann wrote:
> > >
> > > I think .compat_ioctl would be cleaner frankly. Just look at
> > > all the ifdefery. And who knows what broken-ness userspace
> > > comes up with with this approach. Better use the standard approach.
> >
> > Sent now.
> >
> > I'm not sure it's much better because there is quite a bit of
> > code duplication, and reducing that would be a larger rework.
>
> yes but on the flip side, we can put it all inside ifdef CONFIG_COMPAT
> (which this code did not do, but should IMHO).
>
> > It may be best to hold off on patch 2 for the coming merge window
> > since the compat ioctl code has apparently always been broken for
> > x86 here.
>
> And it needs testing.
>
> > I hope we can at least get patch 1/2 merged along with the
> > new code though, otherwise it would get a lot harder to sort
> > it out properly, with the v2 struct members overlapping the
> > old padding fields.
> >
> >      Arnd
>
> Along with it or no, surely before the release.
> Given 32 on 64 with this apparently has been broken forever,
> I will merge this just based on even you did not bother testing compat, I am
> inclined to say I am merging this but not rebasing because
> of this.
>
> Oh and we got lucky this didn't leak kernel stack info.
>
> Eugenio, note for the future: please help make sure UAPI
> structs do not have hidden padding.
>

Sure. I'm trying to find an automatic way to check for this but with
no luck :(. Arnd, did you use some tool for this or you just found it
by visual inspection? I'm trying pahole and -Wpadded but the output
includes a lot of struct not related to uapi.

I guess it is possible to filter it with some clang or awk machinery,
but I'm asking in case I avoid developing something already existing.

Thanks!