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 Intel Golden Cove CPU (see the
> "Misprediction for Branches >2GB" section in Intel 64 and IA-32
> Architectures Optimization Reference Manual.)  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 in shared
> libraries 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.  LD_PREFER_MAP_32BIT_EXEC works only between
> shared libraries or between shared libraries and executables with
> addresses below 2GB.  PIEs are usually mapped above 4GB by the kernel.

I still think we should fix this in the kernel, using MAP_DENYWRITE as a
hint for placement.  This way, it's easier to turn it on unconditionally
for the whole system because the lower 4 GiB will not be polluted by
code mappings.

Thanks,
Florian