Re: [PATCH] mm/userfaultfd: provide unmasked address on page-fault

Mike Rapoport
2021-10-10T05:31:35Z

On Fri, Oct 08, 2021 at 10:05:36AM +0200, David Hildenbrand wrote:
> On 08.10.21 01:50, Nadav Amit wrote:
> > From: Nadav Amit <namit@vmware.com>
> >
> > Userfaultfd is supposed to provide the full address (i.e., unmasked) of
> > the faulting access back to userspace. However, that is not the case for
> > quite some time.
> >
> > Even running "userfaultfd_demo" from the userfaultfd man page provides
> > the wrong output (and contradicts the man page). Notice that
> > "UFFD_EVENT_PAGEFAULT event" shows the masked address.
> >
> > Address returned by mmap() = 0x7fc5e30b3000
> >
> > fault_handler_thread():
> > poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
> > UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
> > (uffdio_copy.copy returned 4096)
> > Read address 0x7fc5e30b300f in main(): A
> > Read address 0x7fc5e30b340f in main(): A
> > Read address 0x7fc5e30b380f in main(): A
> > Read address 0x7fc5e30b3c0f in main(): A
> >
> > Add a new "real_address" field to vmf to hold the unmasked address. It
> > is possible to keep the unmasked address in the existing address field
> > (and mask whenever necessary) instead, but this is likely to cause
> > backporting problems of this patch.
>
> Can we be sure that no existing users will rely on this behavior that has
> been the case since end of 2016 IIRC, one year after UFFD was upstreamed? I
> do wonder what the official ABI nowadays is, because man pages aren't
> necessarily the source of truth.
>
> I checked QEMU (postcopy live migration), and I think it should be fine with
> this change.

CRIU is Ok with this change, we anyway mask the address.

--
Sincerely yours,
Mike.