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

[Jan Beulich <jbeulich@suse.com>]

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 ...

Jan