Re: [PATCH v2] userfaultfd: allow registration of ranges below mmap_min_addr

Liam R. Howlett
2026-04-09T20:17:07Z

* Denis M. Karpov <komlomal@gmail.com> [260409 11:54]:
> On Thu, Apr 9, 2026 at 6:17 PM Mike Rapoport <rppt@kernel.org> wrote:
> > On Thu, Apr 09, 2026 at 01:30:07PM +0100, Pedro Falcato wrote:
> > > This looks relatively safe. However, I'm not sure if we want this in stable.
> > > This has been broken for 11 years now, with no complaints.
> >
> > I believe Denis has a new usecase that wasn't there for those 11 years :)
> >
> > Denis, can you share more details about your usecase for us to better
> > understand importance of backporting this to stable?
> Hello Mike.
> Actually, there is nothing new about the use case. We simply started using
> UFFD instead of the classic mprotect approach in the binary translator to
> track application writes. During development, we encountered this bug.
> The translator cannot control where the translated application chooses
> to map its
> memory and if the app requires a low-address area, UFFD fails, whereas
> mprotect would work just fine. I believe this is a genuine logic bug rather than
> an improvement, and I would appreciate including the fix in stable.

Then you should have cc'ed stable (with the tag too).

I'm fine with it backported or not.

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>