Re: Retpolines and CFI

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

On Mon, Jan 22, 2018 at 4:21 AM, Florian Weimer <fweimer@redhat.com> wrote:
> I tried this:
>
> struct C {
>   virtual ~C();
>   virtual void f();
> };
>
> void
> f (C *p)
> {
>   p->f();
>   p->f();
> }
>
> with r256939 and -mindirect-branch=thunk -O2 on x86-64 GNU/Linux, and got
> this:
>
> _Z1fP1C:
> .LFB0:
>         .cfi_startproc
>         pushq   %rbx
>         .cfi_def_cfa_offset 16
>         .cfi_offset 3, -16
>         movq    (%rdi), %rax
>         movq    %rdi, %rbx
>         jmp     .LIND1
> .LIND0:
>         pushq   16(%rax)
>         jmp     __x86_indirect_thunk
> .LIND1:
>         call    .LIND0
>         movq    (%rbx), %rax
>         movq    %rbx, %rdi
>         popq    %rbx
>         .cfi_def_cfa_offset 8
>         movq    16(%rax), %rax
>         jmp     __x86_indirect_thunk_rax
>         .cfi_endproc
>
> This doesn't look quite right.  x86-64 is supposed to have asynchronous
> unwind tables by default, but there is nothing that reflects the change in
> the (relative) frame address after .LIND0.  I think that region really has
> to be moved outside of the .cfi_startproc/.cfi_endproc bracket.

I'd like to remove __x86_indirect_thunk since it can't be made compatible
with CET:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83970

That means  -mindirect-branch=thunk should imply  -mindirect-branch-register.
When -fno-plt is used with __x86_indirect_thunk_reg, linker can convert call
via GOT to direct branch if function is locally defined:

https://groups.google.com/forum/#!topic/x86-64-abi/eED5lzn3_Mg


H.J.