Re: [PATCH] [PR83782] i386 PIE: avoid @GOTOFF for ifuncs and their aliases

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Jul 28, 2022 at 9:31 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Thu, Jul 28, 2022 at 1:26 AM Alexandre Oliva <oliva@adacore.com> wrote:
> >
> > On Jul 27, 2022, "H.J. Lu" <hjl.tools@gmail.com> wrote:
> >
> > > On Tue, Jul 26, 2022 at 10:14 PM Alexandre Oliva <oliva@adacore.com> wrote:
> >
> > >> The use of @GOTOFF for locally-bound but externally-visible symbols
> > >> (e.g. protected visibility) also breaks pointer identity if the
> > >> canonical address ends up preempted by a PLT entry.
> >
> > > Here is a different fix:
> >
> > > https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598667.html
> >
> > Oh, thanks, I'd missed that.
> >
> > It doesn't seem to fix the part of the problem I quoted above, though.
> > I think fixing that requires testing the visibility, to make sure the
> > symbol's canonical address cannot be preempted, which may occur with
> > local binding, if the symbol is protected and referenced in the main
> > program, otherwise pointer identity is broken again, admittedly for a
> > more obscure case, but pointer identity was the point of the PR.
>
> The protected symbol issue isn't IFUNC specific.   The protected
> symbol handling is changed in glibc 2.36 and binutils 2.39.   Only
> the address of the protected symbol definition should be used as
> its address.
>
> > >> * config/i386/i386.cc (ix86_call_use_plt_p): Follow the alias
> > >> chain looking for an ifunc, as in gcc.target/i386/mvc10.c.
> >
> > You may also need to do something like this bit for mvc10.c on ia32 PIE.
> > Because the ifunc is called through an alias, AFAICT we don't even
> > notice that the call target is (an alias to) an ifunc.  GCC's
> > gotoff_operand predicate accepts it, but binutils (the linker, IIRC)
> > then rejects that reference, because the named symbol is an alias to an
> > ifunc.
>
> Yes, this change is needed.

I think this fix should be applied to default_binds_local_p_3:

  if (lookup_attribute ("weakref", DECL_ATTRIBUTES (exp))
      || (!targetm.ifunc_ref_local_ok ()
          && TREE_CODE (exp) == FUNCTION_DECL
          && cgraph_node::get (exp)
          && cgraph_node::get (exp)->ifunc_resolver))
    return false;

since the ifunc_resolver check won't work on aliases.

-- 
H.J.