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

On 31.07.2026 10:55, H.J. Lu wrote:
> On Fri, Jul 31, 2026 at 4:26 PM Jan Beulich <[email protected]> wrote:
>>
>> On 31.07.2026 01:09, H.J. Lu wrote:
>>> Disallow ".long foo@PLT - .L4" and only allow ".slong foo@PLT - .L4" for
>>> PLT32 relocation.
>>
>> Why would you enforce this also for 32-bit, where the difference between
>> .slong and .long is entirely benign? The existing cons_sign should be
>> sufficient to achieve what's needed for 64-bit code.
>>
> 
> I compared ".long foo@PLT - .L4":
> 
>         .section .rodata
>         .p2align 2
> .L4:
>         .long foo1@PLT - .L4
>         .long foo2@PLT - .L4
>         .long foo3@PLT - .L4
>         .long foo4@PLT - .L4
>         .long foo1@PLT - .L5
>         .long foo2@PLT - .L5
>         .long foo3@PLT - .L5
>         .long foo4@PLT - .L5
> .L5:
> 
> vs ".slong foo@PLT - .L4":
>         .section .rodata
>         .p2align 2
> .L4:
>         .slong foo1@PLT - .L4
>         .slong foo2@PLT - .L4
>         .slong foo3@PLT - .L4
>         .slong foo4@PLT - .L4
>         .slong foo1@PLT - .L5
>         .slong foo2@PLT - .L5
>         .slong foo3@PLT - .L5
>         .slong foo4@PLT - .L5
> .L5:
> 
> in 64-bit.   The output object files are identical.  If we don't allow
> ".long foo@PLT - .L4" in 64-bit, we shouldn't allow it in 32-bit.

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.

Jan