Re: [PATCH] x86: Disallow ".long foo@PLT - .L4" for PLT32 relocation

On 03.08.2026 09:22, H.J. Lu wrote:
> On Mon, Aug 3, 2026 at 2:44 PM Jan Beulich <[email protected]> wrote:
>>
>> On 01.08.2026 15:47, Maciej W. Rozycki wrote:
>>> On Sat, 1 Aug 2026, H.J. Lu wrote:
>>>
>>>>>>> Why? .slong was introduced because of a peculiarity of x86-64. If that
>>>>>>> was straightforward to achieve, it may better not have been exposed to
>>>>>>> 16- and 32-bit code in the first place. While that ship has sailed,
>>>>>>> suddenly making its use mandatory for certain constructs even outside
>>>>>>> of 64-bit code feels outright wrong.
>>>>>>
>>>>>> What is wrong with using " .long foo4@PLT - .L4" in 64-bit?
>>>>>> The possible reason could be wrong code.  You don't like it
>>>>>> doesn't count.
>>>>>
>>>>> It's not a matter of liking it (or not). The distinction between both is
>>>>> what relocation is (supposed to be) used when one is needed. .long should
>>>>> only ever use zero-extending ones; .slong was added such that want for a
>>>>> sign-extending one can be expressed. Without distinct directives, the
>>>>> assembler has to guess, and such guessing can only go wrong. I'm pretty
>>>>> certain this isn't properly dealt with everywhere, but when adding new
>>>>> functionality we shouldn't extend existing flaws.
>>>>
>>>> Given that there are no issues with ".long foo - .L4",
>>>> ".long foo1@PLT - .L4" should have no issues at all.
>>>> If you believe that it isn't true, please show me a case.
>>>
>>>  I gather this is about the overflow case, handled differently depending
>>> on the signedness or the lack of, of the datum relocated by the static
>>> linker or dynamic loader as appropriate.
>>>
>>>  Jan, please correct me if I'm wrong since commit d182319b0955 has no
>>> change description and there's no documentation for `.slong' in the GAS
>>> manual either.  What are the x86 psABI ELF relocation types corresponding
>>> to `.long' and `.slong' respectively in this context?
>>
>> For .long there simply is no correct relocation type to use for PLT (and,
>> fwiw, simple PC-relative) expressions. The correct type to use is
>> R_X86_64_PLT32 (and R_X86_64_PC32 for the simple PC-rel case).
>>
>> For the simple PC-rel case we simply cannot require use of .slong, as we've
>> been accepting .long (then guessing that .slong was meant). Else we'd very
>> likely break existing code. Since @PLT was rejected so far when used with
>> .long and .slong, doing better here is imo very desirable (if not
>> imperative).
>>
> 
> Because assembler has been correctly generating R_X86_64_PC32 for
> ".long foo - .L4",

What, again, does "correctly" mean here? We need to retain this incorrect
behavior, yes, but that doesn't mean we should extend it to other constructs.

Jan

> it is perfectly correct to generate R_X86_64_PLT32 for
> ".long foo@PLT - .L4".
> 
> 
> --
> H.J.