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

[Vincent Fazio]

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
>     address.
>
> so yes we should avoid short circuiting the return address check.
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>