Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

[Kees Cook]

On Wed, Nov 29, 2017 at 7:13 AM, Rasmus Villemoes
<rasmus.villemoes@xxxxxxxxx> 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