[PATCH] arm: Handle undefweak with ST_BRANCH_UNKNOWN

[Christophe Lyon]

On Tue, 10 Sept 2024 at 14:49, Richard Earnshaw (lists)
<Richard.Earnshaw@arm.com> wrote:
>
> On 06/09/2024 18:21, Christophe Lyon wrote:
> > A previous patch made ld fail early on Thumb-only where branch_type is
> > ST_BRANCH_UNKNOWN.
> >
> > However, this fails erroneously when the target is undefweak: in that
> > case the branch should be replaced by a branch to the next instruction
> > (or nop.w on thumb2).  This patch accepts this case and restores the
> > previous behaviour in such cases.
> >
> > This was reported by failures in the GCC testsuite, where we fail to
> > link executables because __deregister_frame_info is undefweak:
> >
> > (__deregister_frame_info): Unknown destination type (ARM/Thumb) in ...crtbegin.o
> > crtbegin.o: in function `__do_global_dtors_aux':
> > crtstuff.c:(.text+0x52): dangerous relocation: unsupported relocation
> > ---
> >  bfd/elf32-arm.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
> > index 7441ee2cc38..17df8b30eb6 100644
> > --- a/bfd/elf32-arm.c
> > +++ b/bfd/elf32-arm.c
> > @@ -10512,7 +10512,12 @@ elf32_arm_final_link_relocate (reloc_howto_type *        howto,
> >    if (using_thumb_only (globals)
> >        && (r_type == R_ARM_THM_CALL
> >         || r_type == R_ARM_THM_JUMP24)
> > -      && branch_type == ST_BRANCH_UNKNOWN)
> > +      && branch_type == ST_BRANCH_UNKNOWN
> > +      /* Exception to the rule above: a branch to an undefined weak
> > +      symbol is turned into a jump to the next instruction unless a
> > +      PLT entry will be created (see below).  */
> > +      && !(h && h->root.type == bfd_link_hash_undefweak
> > +        && plt_offset == (bfd_vma) -1))
> >      {
> >        if (sym_sec != NULL
> >         && sym_sec->owner != NULL)
>
> Did you look at the other case in elf32_arm_get_plt_info where we handle (I think) i-realtive relocs?
> It might be that these must never be weak, but this is not really my area of expertise.
>  It might be worth a comment here if they don't need to be covered.
>  CCing Richard Sandiford as he did the original i-relative support for Arm (though some time ago now).
>
I did not look specifically at that, but the code which converts the
branch into nop says:
"A branch to an undefined weak symbol is turned into a jump to
   the next instruction unless a PLT entry will be created."
If elf32_arm_get_plt_info returns true, plt_offset != -1, so we
wouldn't convert the branch into a nop anyway?

So, in presence of i-relative & undefweak, we would now  enter "Handle
calls via the PLT"
because we now have stub_type == arm_stub_none, and thus try to jump
directly to the PLT in thumb mode,
and the normal code flow will catch any overflow.   An overflow means
that a long-branch stub is needed,
so if we still have branch_type == ST_BRANCH_UNKNOWN at this point,
it's good that the user
is informed that something is wrong?


> A testcase would also be useful, so that we don't need to run GCC to find validate this.
>
Sure I'll add one.

Thanks,

Christophe

> R.