[PATCH] i386: Issue an error for baseless @GOT relocation with relaxation

[Jan Beulich]

On 05.11.2025 23:59, H.J. Lu wrote:
> On Wed, Nov 5, 2025 at 11:31 PM Rainer Orth <ro@cebitec.uni-bielefeld.de> wrote:
>>
>> Hi Jan,
>>
>>> On 27.10.2025 11:09, H.J. Lu wrote:
>>>> On Mon, Oct 27, 2025 at 5:52 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>
>>>>> On 27.10.2025 10:31, H.J. Lu wrote:
>>>>>> On Mon, Oct 27, 2025 at 4:41 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>>
>>>>>>> On 26.10.2025 00:28, H.J. Lu wrote:
>>>>>>>> When -mrelax-relocations=no is used, don't generate baseless R_386_GOT32X
>>>>>>>> relocation.  Issue an error instead.
>>>>>>>>
>>>>>>>> PR gas/33552
>>>>>>>> * config/tc-i386.c (output_disp): Issue an error for baseless @GOT
>>>>>>>> relocation without relaxation in 32-bit.
>>>>>>>> * testsuite/gas/i386/got-no-relax-error.l: New file.
>>>>>>>> * testsuite/gas/i386/got-no-relax-error.s: Likewise.
>>>>>>>> * testsuite/gas/i386/got-no-relax.d: Don't use got.s.  Remove
>>>>>>>> R_386_GOT32X entries.
>>>>>>>> * testsuite/gas/i386/i386.exp: Run got-no-relax-error.
>>>>>>>>
>>>>>>>
>>>>>>> Please can you add a reference to the part of the spec that puts in place
>>>>>>> this restriction?
>>>>>>
>>>>>> It is more of an ISA restriction.  In i386 psABI, there are 2 kinds of PLTs,
>>>>>> one with EBX for PIC and the other without EBX for non-PIC.  The baseless
>>>>>> @GOT reloc can only be used with non-PIC PLT.
>>>>>
>>>>> But the two PLT kinds must be specified somewhere. As must be the relocation
>>>>
>>>> It is in i386 psABI.   They are implemented in elf32-i386.c.
>>>>
>>>>> types. Furthermore, how does the PLT come into play here when we're talking
>>>>> of a GOT reloc? R_386-GOT32X is G + A - GOT / G + A, i.e. nothing PLT-ish in
>>>>
>>>> jmp *foo@GOT
>>>>
>>>> was added for -fno-plt.  For the same reason as non-PIC PLT, it only works
>>>> without PIC.
>>>>
>>>>> there. The description in the psABI also clearly mentions forms with and
>>>>> without base register.
>>>>
>>>> It is covered in:
>>>>
>>>> https://gitlab.com/x86-psABIs/i386-ABI
>>>
>>> Hmm, yes, I see the text there was extended at some point. Having a single
>>> reloc with two different calculations is, however, a no-go. Even more so with
>>> an arch like x86, where you can't reliably move backwards in the instruction
>>> byte stream.
>>
>> apparently this patch has never been tested either natively on Solaris
>> or with --enable-x86-relax-relocations=no, the only configurations where
>> it makes a noticeable difference.  When doing one finds that it causes
>> quite a number of testsuite regressions:
> 
> I withdrew my patch.  Please see
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=33552
> 
> for why R_386_GOT32X was generated for
> 
> jmp *_start@GOT

What does this mean for Rainer's patch, though? I think it is correct to avoid
emitting this reloc when --enable-x86-relax-relocations=no. This is the only
way to ensure objects generated by gas can be processed by GOT32X unaware
linkers, for example. Besides addressing the Solaris issues Rainer points out.

Jan