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

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 11 May 2023 at 08:08, Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Thu, May 11, 2023 at 12:04 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > 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.
>
> OK then.
>

Thanks all. Is anything expected of me at this point?