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

From: Vincent Fazio
Subject: Re: [PATCH] linux-user/elfload: do not assume MAP_FIXED_NOREPLACE kernel support
Date: Thu, 25 Feb 2021 08:26:33 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2/15/21 3:52 AM, Alex Bennée wrote:

Vincent Fazio <vfazio@gmail.com> writes:

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?
Yes, my builds were happening in a container to eventually have the statically built binaries run in another container. I discovered this issue (and the two others reviewed) while trying to debootstrap Debian Bullseye in a container.

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>

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