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 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.

>
> --
> Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
>    Free Software Activist                       GNU Toolchain Engineer
> Disinformation flourishes because many people care deeply about injustice
> but very few check the facts.  Ask me about <https://stallmansupport.org>



-- 
H.J.