[PATCH] ld: Allow R_X86_64_GOTPCREL for call *__tls_get_addr@GOTPCREL(%rip)

[H.J. Lu]

On Tue, Jan 10, 2023 at 1:16 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 09.01.2023 22:14, H.J. Lu wrote:
> > On Mon, Jan 9, 2023 at 12:15 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 06.01.2023 18:03, H.J. Lu via Binutils wrote:
> >>> On Thu, Jan 5, 2023 at 1:06 PM Fangrui Song via Binutils
> >>> <binutils@sourceware.org> wrote:
> >>>>
> >>>> _Thread_local int a;
> >>>> int main() { return a; }
> >>>>
> >>>> % gcc -fno-plt -fpic a.c -fuse-ld=bfd -Wa,-mrelax-relocations=no
> >>>> /usr/bin/ld.bfd: /tmp/ccSSBgrg.o: TLS transition from R_X86_64_TLSGD to R_X86_64_GOTTPOFF against `a' at 0xd in section `.text' failed
> >>>> /usr/bin/ld.bfd: failed to set dynamic section sizes: bad value
> >>>> collect2: error: ld returned 1 exit status
> >>>>
> >>>> This commit fixes the issue.
> >>>>
> >>>>     PR ld/24784
> >>>>     * bfd/elf64-x86-64.c (elf_x86_64_check_tls_transition): Allow
> >>>>       R_X86_64_GOTPCREL.
> >>>> ---
> >>>>  bfd/elf64-x86-64.c | 2 +-
> >>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/bfd/elf64-x86-64.c b/bfd/elf64-x86-64.c
> >>>> index 914f82d0151..095fe2e0fe6 100644
> >>>> --- a/bfd/elf64-x86-64.c
> >>>> +++ b/bfd/elf64-x86-64.c
> >>>> @@ -1241,7 +1241,7 @@ elf_x86_64_check_tls_transition (bfd *abfd,
> >>>>           if (largepic)
> >>>>             return r_type == R_X86_64_PLTOFF64;
> >>>>           else if (indirect_call)
> >>>> -           return r_type == R_X86_64_GOTPCRELX;
> >>>> +           return (r_type == R_X86_64_GOTPCRELX || r_type == R_X86_64_GOTPCREL);
> >>>>           else
> >>>>             return (r_type == R_X86_64_PC32 || r_type == R_X86_64_PLT32);
> >>>>         }
> >>>> --
> >>>> 2.39.0.314.g84b9a713c41-goog
> >>>>
> >>>
> >>> Since the new TLS sequence was added after R_X86_64_GOTPCRELX was
> >>> required for call, R_X86_64_GOTPCREL should be invalid in this TLS sequence.
> >>
> >> While this may well be, would you mind pointing out (more to Fangrui than to
> >> me) what bad his proposed change would do?
> >
> > The problem is caused by the combination of -fno-plt and
> > -Wa,-mrelax-relocations=no.
> > -Wa,-mrelax-relocations=no was added to generate object files to be
> > consumed by the
> > older linkers.   On the other hand, -fno-plt requires newer linkers.
> > As the result,
> >  -fno-plt -Wa,-mrelax-relocations=no generates object files which
> > aren't compatible
> > with neither older linkers nor newer linkers.
> > -Wa,-mrelax-relocations shouldn't be used
> > together with -fno-plt.
>
> Imo use of such option combinations should either be disallowed (warned
> about at the very least) or produce sensible output. I guess only the
> latter would help Fangrui ...
>

This isn't a supported combination.  I believe -Wa,-mrelax-relocations=no
should be removed.

-- 
H.J.