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

[Adhemerval Zanella]

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?