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Date:   Wed, 29 Nov 2017 14:15:22 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Florian Weimer <fweimer@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

On Wed, Nov 29, 2017 at 7:13 AM, Rasmus Villemoes
<rasmus.villemoes@...vas.dk> wrote:
> On 2017-11-29 15:42, Michal Hocko wrote:
>>
>> The first patch introduced MAP_FIXED_SAFE which enforces the given
>> address but unlike MAP_FIXED it fails with ENOMEM if the given range
>> conflicts with an existing one.
>
> [s/ENOMEM/EEXIST/, as it seems you also did in the actual patch and
> changelog]
>
>>The flag is introduced as a completely
>> new one rather than a MAP_FIXED extension because of the backward
>> compatibility. We really want a never-clobber semantic even on older
>> kernels which do not recognize the flag. Unfortunately mmap sucks wrt.
>> flags evaluation because we do not EINVAL on unknown flags. On those
>> kernels we would simply use the traditional hint based semantic so the
>> caller can still get a different address (which sucks) but at least not
>> silently corrupt an existing mapping. I do not see a good way around
>> that.
>
> I think it would be nice if this rationale was in the 1/2 changelog,
> along with the hint about what userspace that wants to be compatible
> with old kernels will have to do (namely, check that it got what it
> requested) - which I see you did put in the man page.

Okay, so ignore my other email, I must have misunderstood. It _is_,
quite intentionally, being exposed to userspace. Cool by me. :)

-Kees

-- 
Kees Cook
Pixel Security

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