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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Tue, 5 Mar 2019 18:47:37 +0100
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Subject: Re: [PATCH v10 07/12] fs, arm64: untag user pointers in fs/userfaultfd.c
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
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On Fri, Mar 1, 2019 at 7:37 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 3/1/19 8:59 AM, Catalin Marinas wrote:
> >>> So, we have to patch all these sites before the tagged values get to the
> >>> point of hitting the vma lookup functions.  Dumb question: Why don't we
> >>> just patch the vma lookup functions themselves instead of all of these
> >>> callers?
> >> That might be a working approach as well. We'll still need to fix up
> >> places where the vma fields are accessed directly. Catalin, what do
> >> you think?
> > Most callers of find_vma*() always follow it by a check of
> > vma->vma_start against some tagged address ('end' in the
> > userfaultfd_(un)register()) case. So it's not sufficient to untag it in
> > find_vma().
>
> If that's truly the common case, sounds like we should have a find_vma()
> that does the vma_end checking as well.  Then at least the common case
> would not have to worry about tagging.

It seems that a lot of find_vma() callers indeed do different kinds of
checking/subtractions of vma->vma_start and a tagged address, which
look hardly unifiable. So untagging the addresses in find_vma()
callers looks like a more suitable solution.