Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

[Michael Matz]

Hello,

On Wed, 14 Sep 2022, Josh Poimboeuf wrote:

> > >    This information is needed because the
> > >    code after the call to such a function is optimized out as
> > >    unreachable and objtool has no way of knowing that.
> > 
> > Since June we (GCC) have -funreachable-traps.  This creates a trap insn
> > wherever control flow would otherwise go into limbo.
> 
> Ah, that's interesting, though I'm not sure if we'd be able to
> distinguish between "call doesn't return" traps and other traps or
> reasons for UD2.

There are two reasons (which will turn out to be the same) for a trap (say 
'UD2' on x86-64) directly after a call insn:
1) "the call shall not have returned"
2) something else jumps to that trap because it was __builtin_unreachable 
   (or equivalent), and the compiler happened to put that ud2 directly 
   after the call.  It could have done that only when the call itself was 
   noreturn:
     cmp $foo, %rax
     jne do_trap
     call noret
    do_trap:
     ud2

So, it's all the same.  If there's an ud2 (or whatever the trap maker is) 
after a call then it was because it's noreturn.

(But, of course this costs (little) code size, unlike the non-alloc 
checker sections)


Ciao,
Michael.