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

Hello,

On Wed, 14 Sep 2022, Josh Poimboeuf wrote:

quoted

quoted

   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.