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

[H.J. Lu]

On Fri, Jul 31, 2026 at 5:03 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 31.07.2026 10:55, H.J. Lu wrote:
> > On Fri, Jul 31, 2026 at 4:26 PM Jan Beulich <jbeulich@suse.com> 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.
>

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.

-- 
H.J.