[PATCH] x86-64: Stack alignment in _dl_tlsdesc_dynamic and red zone usage (bug 31501)

[H.J. Lu]

On Sat, Mar 16, 2024 at 8:04 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu:
>
> > On Sat, Mar 16, 2024 at 7:43 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >>
> >> On Sat, Mar 16, 2024 at 7:37 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >> >
> >> > On Sat, Mar 16, 2024 at 7:33 AM Florian Weimer <fweimer@redhat.com> wrote:
> >> > >
> >> > > In sysdeps/x86_64/dl-tlsdesc-dynamic.h, the stack pointer is
> >> > > realigned for some variants (notably _dl_tlsdesc_dynamic_xsavec).
> >> > > This realignment does not take into account that the function has
> >> > > already used part of the red zone at this point, thus clobbering
> >> > > the initally saved register values located there if the stack
> >> > > alignment inherited from the caller is unfortunate.
> >> > >
> >> > > (Note: I do not know to write a good test case for this in the existing
> >> > > framework.  We saw this as a random LTO plugin crash when building GCC
> >> > > with -mtls-dialect=gnu2.  The existing tests on pass on x86_64-linux-gnu
> >> > > with this change here.)
> >
> > We should try to find a testcase.  Can you provide a backtrace when it
> > happens?  It should be possible to write a testcase with the backtrace.
>
> In my reproducer, when %rbx is about to be clobbered, I see
> (%rsp % 64) == 8 at the start of _dl_tlsdesc_dynamic_xsavec.
>
> The %rbx register does not get clobbered if (%rsp % 64) == 56.
>
> Does this help?
>

Yes.  I am working on a testcase.


-- 
H.J.