Re: [PATCH RFA] ubsan: do return check with -fsanitize=unreachable

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jun 22, 2022 at 12:04:59AM -0400, Jason Merrill wrote:
> On 6/20/22 16:16, Jason Merrill wrote:
> > On 6/20/22 07:05, Jakub Jelinek wrote:
> > > On Fri, Jun 17, 2022 at 05:20:02PM -0400, Jason Merrill wrote:
> > > > Related to PR104642, the current situation where we get less
> > > > return checking
> > > > with just -fsanitize=unreachable than no sanitize flags seems
> > > > undesirable; I
> > > > propose that we do return checking when -fsanitize=unreachable.
> > > 
> > > __builtin_unreachable itself (unless turned into trap or
> > > __ubsan_handle_builtin_unreachable) is not any kind of return
> > > checking, it
> > > is just an optimization.
> > 
> > Yes, but I'm talking about "when -fsanitize=unreachable".

The usual case is that people just use -fsanitize=undefined
and get both return and unreachable sanitization, for fall through
into end of functions returning non-void done through return sanitization.

In the rare case people use something different like
-fsanitize=undefined -fno-sanitize=return
or
-fsanitize=unreachable
etc., they presumably don't want the fall through from end of function
diagnosed at runtime.

I think the behavior we want is:
1) -fsanitize=return is on -> use ubsan_instrument_return
   (__ubsan_missing_return_data or __builtin_trap depending on
    -fsanitize-trap=return); otherwise
2) -funreachable-traps is on (from -O0/-Og by default or explicit),
   emit __builtin_trap; otherwise
3) -fsanitize=unreachable is on, not emit anything (__builtin_unreachable
   would be just an optimization, but user asked not to instrument returns,
   only unreachable, so honor user's decision and avoid confusion); otherwise
4) -O0 is on, not emit anything (__builtin_unreachable wouldn't be much
   of an optimization, just surprising and hard to debug effect); otherwise
5) emit __builtin_unreachable

Current trunk with your PR104642 fix in implements 1), will do 2)
only if -fsanitize=unreachable is not on, will do 3), will do 4) and 5).

So, I'd change cp-gimplify.cc (cp_maybe_instrument_return), change:
  if (!sanitize_flags_p (SANITIZE_RETURN, fndecl)
      && ((!optimize && !flag_unreachable_traps)
	  || sanitize_flags_p (SANITIZE_UNREACHABLE, fndecl)))
    return;
to
  if (!sanitize_flags_p (SANITIZE_RETURN, fndecl)
      && !flag_unreachable_traps
      && (!optimize || sanitize_flags_p (SANITIZE_UNREACHABLE, fndecl)))
    return;
and
  if (sanitize_flags_p (SANITIZE_RETURN, fndecl))
    t = ubsan_instrument_return (loc);
  else
    t = build_builtin_unreachable (BUILTINS_LOCATION);
to
  if (sanitize_flags_p (SANITIZE_RETURN, fndecl))
    t = ubsan_instrument_return (loc);
  else if (flag_unreachable_traps)
    t = build_call_expr_loc (BUILTINS_LOCATION,
			     builtin_decl_explicit (BUILT_IN_TRAP), 0);
  else
    t = build_builtin_unreachable (BUILTINS_LOCATION);

	Jakub