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

["H.J. Lu" <hjl.tools@gmail.com>]

On Fri, Feb 10, 2023 at 1:54 PM Carlos O'Donell <carlos@redhat.com> wrote:
>
> On 2/10/23 11:50, H.J. Lu wrote:
> > On Thu, Feb 9, 2023 at 6:35 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * 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.
> >>
> >
> > Kernel MM change may take a long time and this mitigation may only be needed
> > for a few specific applications.   On the other hand, it is simpler to
> > use tunable
> > instead.
>
> I tend to agree with you here that a Kernel MM change is going to take too long and may
> be possibly fragile.
>
> I would like to see this use a tunable with an alias.

Done:

https://patchwork.sourceware.org/project/glibc/patch/20230210222142.17943-1-hjl.tools@gmail.com/

> Note that the Linux man pages continue to document LD_PREFER_MAP_32BIT_EXEC :-)
>
> --
> Cheers,
> Carlos.
>

Thanks.

-- 
H.J.