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Date:   Wed, 22 Nov 2017 08:52:37 -0800
From:   Dan Williams <>
To:     Vlastimil Babka <>
Cc:     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 <>,
        Matthew Wilcox <>,
        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 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?

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