[PATCH] x86-64: Remove sframe relocs against discarded sections

[H.J. Lu]

On Tue, Jul 15, 2025 at 2:16 PM Indu Bhagat <indu.bhagat@oracle.com> wrote:
>
> On 7/15/25 12:20 AM, Jan Beulich wrote:
> > On 15.07.2025 07:51, Sam James wrote:
> >> From: "H.J. Lu" <hjl.tools@gmail.com>
> >>
> >> Since unlike eh_frame editing code, sframe editing code keeps
> >> R_X86_64_NONE reloc as is, its r_offset is wrong, we must not
> >> generate R_X86_64_NONE reloc in sframe section against discarded
> >> sections for "ld -r".
> >>
> >> bfd/
> >>
> >>      PR ld/33156
> >>      * elf64-x86-64.c (elf_x86_64_relocate_section): Also remove
> >>      sframe relocations against discarded sections for "ld -r".
> >>
> >> ld/
> >>
> >>      PR ld/33156
> >>      * testsuite/ld-elf/eh-group.exp (as_gsframe): New.
> >>      Assemble eh-group.o with $as_gsframe.
> >>
> >> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> >> ---
> >> Sent on H.J.'s behalf. OK?
> >
> > What's the relationship with what it now 5f9bf0cf711a ("sframe: Allow
> > input R_*_NONE relocations")? Indu's reply there suggests to me that
> > altering the number of relocs may be a problem.
> >
>
> For SFrame sections, at parsing time, the code creates a cache of which
> functions are discarded via _bfd_elf_discard_section_sframe (called via
> bfd_elf_discard_info).  So for input sections, there is reliance on
> relocs being in order of the functions.
>
> At the time of relocation in elf_x86_64_relocate_section (), deleting
> the R_*_NONE relocs will not pose problem, because we use the cache
> created above. So the patch that H.J. proposes should be OK (as far as I
> can reason so far).

I am checking in my patch.

> That aside, taking a step back, the reason why these R_*_NONE are
> causing issues is (as HJ said) that their r_offset is wrong.  We get
> bfd_reloc_outofrange in elf_x86_64_relocate_section () -->
> _bfd_final_link_relocate () --> bfd_reloc_offset_in_range ().  To fix it
> the generic code in elf_link_input_bfd, for deleted relas, this would mean:
>
>    last_offset = ...
>    last_offset += ...
>
>    irela->r_offset = _bfd_elf_section_offset (output_bfd,
>                                               flinfo->info, o,
>                                               irela->r_offset);
>
>    if (irela->r_offset >= (bfd_vma) -2)
>      {
>        /* This is a reloc for a deleted entry or somesuch.
>           Turn it into an R_*_NONE reloc, at the same
>           offset as the last reloc.  elf_eh_frame.c and
>           bfd_elf_discard_info rely on reloc offsets
>           being ordered.  */
> -     irela->r_offset = last_offset;
> +     if (elf_section_type (o) == SHT_GNU_SFRAME)
> +       irela->r_offset = sframe_last_dummy_offset;
> +     else
> +       irela->r_offset = last_offset;
>        irela->r_info = 0;
>        irela->r_addend = 0;
>        continue;
>      }
>
>     last_offset = irela->r_offset;
> +  sframe_last_dummy_offset = irela->r_offset;
>
> Indu



-- 
H.J.