[PATCH] x32: Use THREAD_SELF for RSEQ_* accessors

[Uros Bizjak]

On Fri, Aug 22, 2025 at 5:46 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Fri, Aug 22, 2025 at 3:44 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > Use THREAD_SELF to get the %fs register to work around the GCC bug:
> >
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121613
> >
> > with the negative offset from the %fs register.
> >
> > Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> > ---
> >  sysdeps/x86_64/x32/nptl/rseq-access.h | 110 ++++++++++++--------------
> >  1 file changed, 49 insertions(+), 61 deletions(-)
> >
> > diff --git a/sysdeps/x86_64/x32/nptl/rseq-access.h b/sysdeps/x86_64/x32/nptl/rseq-access.h
> > index 8386ebd4bf..0baf4e8ea1 100644
> > --- a/sysdeps/x86_64/x32/nptl/rseq-access.h
> > +++ b/sysdeps/x86_64/x32/nptl/rseq-access.h
> > @@ -16,70 +16,58 @@
> >     License along with the GNU C Library; if not, see
> >     <https://www.gnu.org/licenses/>.  */
> >
> > +/* Read member of the RSEQ area directly.  */
> > +#define RSEQ_GETMEM(member) \
> > +  ({                                                           \
> > +     _Static_assert (sizeof (RSEQ_SELF()->member) == 1         \
> > +                    || sizeof (RSEQ_SELF()->member) == 4       \
> > +                    || sizeof (RSEQ_SELF()->member) == 8,      \
> > +                    "size of rseq data");                      \
> > +     void *__self = THREAD_SELF;                               \
> > +     asm ("" ::: "memory");                                    \
>
> You can use:
>
>   asm ("" : "+r" (__self));
>
> to keep the value in the register without clobbering all the memory.

OTOH, your approach will have two mem accesses. Is it really better
than assembly?

Uros.