Re: [PATCH] x86-64: Restore LD_PREFER_MAP_32BIT_EXEC support [BZ #28656]

[Florian Weimer <fweimer@redhat.com>]

* H. J. Lu via Libc-alpha:

> Crossing 2GB boundaries with indirect calls and jumps can use more
> branch prediction resources on several Intel CPUs.  There is visible
> performance improvement on workloads with many PLT calls when executable
> and shared libraries are mmapped below 2GB.  Add the Prefer_MAP_32BIT_EXEC
> bit so that mmap will try to map executable or denywrite pages with
> MAP_32BIT first.
>
> NB: Prefer_MAP_32BIT_EXEC reduces bits available for address space
> layout randomization (ASLR), which is always disabled for SUID programs
> and can only be enabled by setting environment variable,
> LD_PREFER_MAP_32BIT_EXEC.

If the performance benefits are significant, this should be handled at
the kernel level.  Only the kernel can put the main program, ld.so and
the vDSO into the same 2GB window (presumably with the main program at
the top, so that the heap can grow almost indefinitely).

For mapping shared objects, we can give the kernel a hint that they will
eventually contain an executable mapping.  If the kernel could reuse
MAP_DENYWRITE for that, no glibc changes would be needed after all.

Doing this is in glibc is only a very partial solution, and so I'd
appreciate if it could be fixed properly in the kernel.

Thanks,
Florian