[PATCH] elf: Drop GRP_COMDAT when localizing a global symbol

[Alan Modra]

On Tue, Dec 07, 2021 at 04:46:29AM -0800, H.J. Lu wrote:
> On Tue, Dec 7, 2021 at 12:00 AM Alan Modra <amodra@gmail.com> wrote:
> >
> > On Mon, Dec 06, 2021 at 08:54:42AM -0800, H.J. Lu via Binutils wrote:
> > > --- a/binutils/objcopy.c
> > > +++ b/binutils/objcopy.c
> > > @@ -1696,6 +1696,21 @@ filter_symbols (bfd *abfd, bfd *obfd, asymbol **osyms,
> > >                     && ! is_specified_symbol (name, keepglobal_specific_htab))
> > >                 || (localize_hidden && is_hidden_symbol (sym))))
> > >           {
> > > +           if (bfd_get_flavour (abfd) == bfd_target_elf_flavour)
> > > +             {
> > > +               sec_ptr s = sym->section;
> > > +               if (s != NULL
> > > +                   && elf_section_data (s) != NULL
> > > +                   && elf_group_name (s) != NULL)
> > > +                 {
> > > +                   /* When a global symbol in a COMDAT group was
> > > +                      changed to local, drop GRP_COMDAT on the
> > > +                      group.  */
> > > +                   s = elf_sec_group (s);
> > > +                   if (s != NULL && s->output_section != NULL)
> > > +                     s->output_section->flags &= ~SEC_LINK_ONCE;
> > > +                 }
> > > +             }
> > >             sym->flags &= ~ (BSF_GLOBAL | BSF_WEAK);
> > >             sym->flags |= BSF_LOCAL;
> > >           }
> >
> > I don't think this is quite correct.  Surely you only want to change
> > the group semantics if the group symbol changes?  This patch would
> > change group semantics if *any* symbol defined in the group is
> > localised.
> 
> This is done on purpose.  When any global symbol in a comdat group
> is localized, this comdat group is no longer interchangeable with other
> comdat groups of the same group signature.

The ELF gABI says of GRP_COMDAT that "duplication is defined as having
the same group signature".  There is no other requirement.  Things
that you might imagine are necessary to make groups work in practice
are not relevant.  objcopy is a tool that can destroy object files.

Besides that, someone may be using objcopy on multiple object files
localising one symbol for every object, leaving all the comdat groups
with a given signature the same.

Please read https://groups.google.com/g/generic-abi/c/2X6mR-s2zoc
again.  The only symbol change that should affect the comdat status of
the group is the group symbol.

-- 
Alan Modra
Australia Development Lab, IBM