Re: [PATCH v2] Fixes a race in iopt_unmap_iova_range

[Sina Hassani]

On Mon, Apr 6, 2026 at 6:27 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Mon, Apr 06, 2026 at 06:17:24PM -0700, Sina Hassani wrote:
> > On Mon, Apr 6, 2026 at 6:12 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > >
> > > On Mon, Apr 06, 2026 at 04:07:01PM -0700, Sina Hassani wrote:
> > >
> > > > io_pagetable *iopt, unsigned long start,
> > > >                 unmapped_bytes += area_last - area_first + 1;
> > > >
> > > >                 down_write(&iopt->iova_rwsem);
> > > > +
> > > > +               /* Do not reconsider things already unmapped in case of
> > > > +                * concurrent allocation */
> > > > +               start = area_last + 1;
> > >
> > > area_last can be ULONG_MAX so this literally overflows to 0. It is why
> > > I formed the suggestion I gave as I did
> > >
> > Yes, in which case the  if (start < area_last) that follows will catch
> > it. Are you suggesting I compare against ULONG_MAX instead?
>
> iommufd does not have any overflows to 0 and rely on it tricks like
> this. You should just compare to the existing iteration last
>
Just to confirm that I understand correctly, like this?

+               if (area_last >= last) {
+                       break;
+.              } else {
+.                      start = area_last + 1;
+               }

> Jason