Re: [PATCH] i386: Honour -mdirect-extern-access when calling __fentry__

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, May 10, 2023 at 2:17 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Tue, May 9, 2023 at 10:58 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > The small and medium PIC code models generate profiling calls that
> > always load the address of __fentry__() via the GOT, even if
> > -mdirect-extern-access is in effect.
> >
> > This deviates from the behavior with respect to other external
> > references, and results in a longer opcode that relies on linker
> > relaxation to eliminate the GOT load. In this particular case, the
> > transformation replaces an indirect 'CALL *__fentry__@GOTPCREL(%rip)'
> > with either 'CALL __fentry__; NOP' or 'NOP; CALL __fentry__', where the
> > NOP is a 1 byte NOP that preserves the 6 byte length of the sequence.
> >
> > This is problematic for the Linux kernel, which generally relies on
> > -mdirect-extern-access and hidden visibility to eliminate GOT based
> > symbol references in code generated with -fpie/-fpic, without having to
> > depend on linker relaxation.
> >
> > The Linux kernel relies on code patching to replace these opcodes with
> > NOPs at runtime, and this is complicated code that we'd prefer not to
> > complicate even more by adding support for patching both 5 and 6 byte
> > sequences as well as parsing the instruction stream to decide which
> > variant of CALL+NOP we are dealing with.
> >
> > So let's honour -mdirect-extern-access, and only load the address of
> > __fentry__ via the GOT if direct references to external symbols are not
> > permitted.
> >
> > Note that the GOT reference in question is in fact a data reference: we
> > explicitly load the address of __fentry__ from the GOT, which amounts to
> > eager binding, rather than emitting a PLT call that could bind eagerly,
> > lazily or directly at link time.
> >
> > gcc/ChangeLog:
> >
> >         * config/i386/i386.cc (x86_function_profiler): Take
> >           ix86_direct_extern_access into account when generating calls
> >           to __fentry__()
>
> HJ, is the patch OK with you?

LGTM.

Thanks.

> Uros.
>
> >
> > Cc: H.J. Lu <hjl.tools@gmail.com>
> > Cc: Jakub Jelinek <jakub@redhat.com>
> > Cc: Richard Biener <rguenther@suse.de>
> > Cc: Uros Bizjak <ubizjak@gmail.com>
> > Cc: Hou Wenlong <houwenlong.hwl@antgroup.com>
> > ---
> >  gcc/config/i386/i386.cc | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> > index b1d08ecdb3d44729..69b183abb4318b0a 100644
> > --- a/gcc/config/i386/i386.cc
> > +++ b/gcc/config/i386/i386.cc
> > @@ -21836,8 +21836,12 @@ x86_function_profiler (FILE *file, int labelno ATTRIBUTE_UNUSED)
> >               break;
> >             case CM_SMALL_PIC:
> >             case CM_MEDIUM_PIC:
> > -             fprintf (file, "1:\tcall\t*%s@GOTPCREL(%%rip)\n", mcount_name);
> > -             break;
> > +             if (!ix86_direct_extern_access)
> > +               {
> > +                 fprintf (file, "1:\tcall\t*%s@GOTPCREL(%%rip)\n", mcount_name);
> > +                 break;
> > +               }
> > +             /* fall through */
> >             default:
> >               x86_print_call_or_nop (file, mcount_name);
> >               break;
> > --
> > 2.39.2
> >



-- 
H.J.