[RFC 2/5] aarch64: check if clone3 supports shadow stack

[Yury Khrustalev]

On Tue, Jun 10, 2025 at 02:34:28PM -0300, Adhemerval Zanella Netto wrote:
> 
> 
> On 10/06/25 12:13, Yury Khrustalev wrote:
> > Rely on the version of the currently running kernel and use
> > existing mechanism to determine this version (same that was
> > used for checks related to SVE).
> > 
> > Since we only need this information in certain case, we try
> > to guard this additional code with macros and extra flag.
> > ...
> >
> Since this is WIP feature, this would be simpler if kernel advertise it through
> a new HWCAP bits.  The kernel parsing is somewhat fragile and thinking

I agree that it's fragile, I only used it to facilitate building and testing
of this patch series.

> twice we should have done it as well for the SVE issue that required to add
> this back on 2e94e2f5d2bf2de124c8ad7da85463355e54ccb2.
> 
> The powerpc did a similar thing with PPC_FEATURE2_HTM_NOSC to avoid some
> performance issues with the initial HTM enablement on pthread (which made
> us tune it better, check f0458cf4f9ff3d870c43b624e6dccaaf657d5e83).


HWCAP or AT_ value in auxv could be a solution, but they require some
thinking through because it's easy to do a stopgap fix and then have even
more issues in the future.

Looking at other extensions of struct clone_args in the past, I see that
it's happened a couple of times, however the new fields are not used in
Glibc, so we've never had this issue before.

One exception is the cgroup field that is used by posix_spawn() and friends.
However, the implementation doesn't check for E2BIG after clone3 syscall,
so we seem to ignore possibility of an error here.

Thanks,
Yury