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

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

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?
>
> Jan

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.

-- 
H.J.