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

[Jan Beulich <jbeulich@suse.com>]

On 10.01.2023 21:39, H.J. Lu wrote:
> 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.

But the reason it was introduced (commit 0cb4071ef9e1) hasn't disappeared.

Jan