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Re: [PATCH] linux-user/elfload: do not assume MAP_FIXED_NOREPLACE kernel

From: Alex Bennée
Subject: Re: [PATCH] linux-user/elfload: do not assume MAP_FIXED_NOREPLACE kernel support
Date: Mon, 15 Feb 2021 09:52:26 +0000
User-agent: mu4e 1.5.8; emacs 28.0.50

Vincent Fazio <vfazio@gmail.com> writes:

> On Sun, Feb 14, 2021 at 6:50 AM Laurent Vivier <laurent@vivier.eu> wrote:
>> Le 14/02/2021 à 12:24, Alex Bennée a écrit :
>> >
>> > Vincent Fazio <vfazio@xes-inc.com> writes:
>> >
>> >> From: Vincent Fazio <vfazio@gmail.com>
>> >>
>> >> Previously, pgd_find_hole_fallback assumed that if the build host's libc
>> >> had MAP_FIXED_NOREPLACE defined that the address returned by mmap would
>> >> match the requested address. This is not a safe assumption for Linux
>> >> kernels prior to 4.17
>> >
>> > It doesn't as we have in osdep.h:
>> >
>> >   #ifndef MAP_FIXED_NOREPLACE
>> >   #define MAP_FIXED_NOREPLACE 0
>> >   #endif
>> >
>> > which is to say to assume if MAP_FIXED_NOREPLACE is defined the kernel
>> > should have given us what we want otherwise we do the check.
>> But what is the purpose of the "if (MAP_FIXED_NOREPLACE != 0 ||"?
>> Can't we rely only on "mmap_start == (void *) align_start"?
>> Thanks,
>> Laurent
> I think we have to rely on address matching. The problem is
> specifically when MAP_FIXED_NOREPLACE is defined and is passed to mmap
> but the running kernel doesn't know what to do with the flag so
> returns a value that is not what was hinted at. Previously the code
> assumed that if MAP_FIXED_NOREPLACE was defined that the returned
> address would match, but that isn't always the case if the kernel
> doesn't have support for the flag. The 4.4, 4.9 and 4.14 LTS kernels
> are still in use and could run into this problem.

Ahh right so I think this is a case of binaries being built on a
different platform than kernel they are running on. In which case the
flag would be defined but the underlying kernel fails to identify it. Is
this a container like case by any chance?

If I'd read the man page closer:

   Note   that   older   kernels   which   do   not  recognize  the
   MAP_FIXED_NOREPLACE flag will typically (upon detecting a colli‐
   sion  with a preexisting mapping) fall back to a "non-MAP_FIXED"
   type of behavior: they will return an address that is  different
   from  the  requested  address.   Therefore,  backward-compatible
   software should check the returned address against the requested

so yes we should avoid short circuiting the return address check.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

Alex Bennée

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