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

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Date:   Sun, 10 Oct 2021 08:29:18 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jan Kara <jack@...e.cz>, stable@...r.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: provide unmasked address on page-fault

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@...are.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.