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

[H.J. Lu]

On Mon, Dec 21, 2020 at 9:16 AM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> On 21/12/2020 14:05, H.J. Lu via Libc-alpha 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.
>
> Couldn't it be mitigated by adding member solely at the end of the struct
> and making sure the backport uses the same offset?

What if we need to extend a member in the middle?

-- 
H.J.