Re: linux-next: Tree for Apr 22 (objtool warnings)

Josh Poimboeuf
2020-04-23T19:51:29Z

On Thu, Apr 23, 2020 at 12:32:32PM -0700, Kees Cook wrote:
> On Thu, Apr 23, 2020 at 01:28:45PM -0500, Josh Poimboeuf wrote:
> > On Thu, Apr 23, 2020 at 11:06:14AM -0700, Kees Cook wrote:
> > > Regardless, the type_mismatch it triggered for misalignment and
> > > object-size checks, and the alignment check is likely going to always
> > > misfire on x86. The randconfig includes that config:
> > >
> > > CONFIG_UBSAN_ALIGNMENT=y
> > >
> > > So perhaps the config should be strengthened to disallow it under
> > > COMPILE_TEST?
> > >
> > > config UBSAN_ALIGNMENT
> > > def_bool !UBSAN_NO_ALIGNMENT
> > > depends on !COMPILE_TEST
> >
> > But this issue basically makes UBSAN_TRAP unusable, right? Should
> > UBSAN_ALIGNMENT and UBSAN_TRAP be made mutually exclusive?
>
> Well, no, not UBSAN_TRAP alone. It seems that -fsanitize=alignment (and
> possibly -fsanitize=object-size) is likely to produce false positives. And
> that was already well understood. With UBSAN_TRAP, though, those functions
> become unusable. Part of the goal of adding UBSAN_TRAP was to be able to
> extract the non-false-positive sanitizer options out of UBSAN_MISC so
> they could be used in production without the large debug text overhead.
> (And the "alignment" checker had long since already been separate and
> set to false by default on x86, since it's going to trip over everything
> since it shouldn't get run on such hardware for any sane reason that
> I know of.) So, while UBSAN_TRAP with UBSAN_ALIGNMENT is going to be
> especially bad, UBSAN_ALIGNMENT at all is likely bad enough on x86, so
> I think it'd be best to make it a negative depends on COMPILE_TEST (but
> leave it available if someone is doing some really unusual UBSAN work).

I think I'm still missing why UBSAN_ALIGNMENT is even possible on x86.
But at least it sounds like your fix will keep the objtool reports from
coming in :-)

--
Josh