Compatibility issues around <sys/platform/x86.h>

[H.J. Lu]

On Mon, Dec 21, 2020 at 9:05 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Mon, Dec 21, 2020 at 5:00 AM Florian Weimer <fweimer@redhat.com> wrote:
> >
> > I finally had some time to review the <sys/platform/x86.h> interface.
> >
> > I have two major concerns:
> >
> > (a) Placement of struct cpu_features in _rtld_global_ro
> >
> > This means that backporting new feature support changes the internal
> > GLIBC_PRIVATE ABI. Consequently, there's a race condition during
> > in-place updates where loading binaries can fail in obscure ways, due to
> > the change in _rtld_global_ro offsets.  We should really avoid this.
>
> This applies to all members in _rtld_global_ro.
>
> > (c) The COMMON_CPUID_INDEX_MAX handshake does not work.
> >
> > __x86_get_cpu_features returns NULL if its argument is too large.  This
> > effectively disables *all* feature bits, not just those beyond
> > COMMON_CPUID_INDEX_MAX. This means that if you build with a header that
>
> Can you open a glibc bug?  It shouldn't be too hard to fix.  I can take
> a look if you don't have time.

https://sourceware.org/bugzilla/show_bug.cgi?id=27104

A patch is posted at

https://sourceware.org/pipermail/libc-alpha/2020-December/120968.html

> > is too new, an application may fail due to the lack of detected features
> > when running on an older glibc version.  Ideally, only the newly added
> > CPU features would be detected as missing in this scenario.  Simply
> > recompiling an application with a newer header should not result in any
> > detection changes.
> >
> >
> > Not quite a major concern is that this feature is not usable from IFUNC
> > resolvers with BIND_NOW, which I consider an important use of CPU
> > feature selection.  I do not have a good solution here that can be
> > implemented in time for glibc 2.33.  I expect that a proper solution
> > will require some binutils work.
>
> Can you open a glibc bug?
>
> > I'm going to work on this with priority this week and hope to have
> > patches posted by the 24th.
> >
> > Thanks,
> > Florian
> > --
> > Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
> > Commercial register: Amtsgericht Muenchen, HRB 153243,
> > Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill
> >
>
>
> --
> H.J.



-- 
H.J.