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Date:   Wed, 22 Nov 2017 11:53:18 -0800
From:   Matthew Wilcox <>
To:     Dan Williams <>
Cc:     Vlastimil Babka <>, Jan Kara <>,
        Ross Zwisler <>,
        Christoph Hellwig <>,
        linux-fsdevel <>,
        "" <>,
        Linux MM <>,
        Linux API <>,
        linux-ext4 <>,
        linux-xfs <>,
        "Darrick J . Wong" <>,
        Arnd Bergmann <>,
        Andy Lutomirski <>,
        Andrew Morton <>,
        Michal Hocko <>,
        Kees Cook <>,
        Florian Weimer <>,
        John Hubbard <>,
        "Kirill A. Shutemov" <>,
Subject: Re: [PATCH 01/18] mm: introduce MAP_SHARED_VALIDATE, a mechanism to
 safely define new mmap flags

On Wed, Nov 22, 2017 at 08:52:37AM -0800, Dan Williams wrote:
> On Wed, Nov 22, 2017 at 4:02 AM, Vlastimil Babka <> wrote:
> > On 11/01/2017 04:36 PM, Jan Kara wrote:
> >> From: Dan Williams <>
> >>
> >> The mmap(2) syscall suffers from the ABI anti-pattern of not validating
> >> unknown flags. However, proposals like MAP_SYNC need a mechanism to
> >> define new behavior that is known to fail on older kernels without the
> >> support. Define a new MAP_SHARED_VALIDATE flag pattern that is
> >> guaranteed to fail on all legacy mmap implementations.
> >
> > So I'm trying to make sense of this together with Michal's attempt for
> > MAP_FIXED_SAFE [1] where he has to introduce a completely new flag
> > instead of flag modifier exactly for the reason of not validating
> > unknown flags. And my conclusion is that because MAP_SHARED_VALIDATE
> > implies MAP_SHARED and excludes MAP_PRIVATE, MAP_FIXED_SAFE as a
> > modifier cannot build on top of this. Wouldn't thus it be really better
> > long-term to introduce mmap3 at this point? ...
> We have room to define MAP_PRIVATE_VALIDATE in MAP_TYPE on every arch
> except parisc. Can we steal an extra bit for MAP_TYPE from somewhere
> else on parisc?

It looks like 0x08 should work.  But I don't have an HPUX machine around
to check that HP didn't use that bit for something else.

It'd probably help to cc the linux-parisc mailing list when asking
questions about PARISC, eh?

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