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

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).

Jan