linux-hardening - Re: [PATCH v2] overflow: Introduce overflows_type() and castable_to

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Message-ID: <202209261405.619441AC2F@keescook>
Date:   Mon, 26 Sep 2022 14:07:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        Daniel Latypov <dlatypov@...gle.com>,
        Vitor Massaru Iha <vitor@...saru.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-hardening@...r.kernel.org, llvm@...ts.linux.dev,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-sparse@...r.kernel.org, Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH v2] overflow: Introduce overflows_type() and
 castable_to_type()

On Mon, Sep 26, 2022 at 01:17:18PM -0700, Nick Desaulniers wrote:
> + Arnd
> 
> On Mon, Sep 26, 2022 at 12:11 PM Kees Cook <keescook@...omium.org> wrote:
> > ---
> > v2:
> >  - fix comment typo
> >  - wrap clang pragma to avoid GCC warnings
> >  - style nit cleanups
> >  - rename __castable_to_type() to castable_to_type()
> >  - remove prior overflows_type() definition
> > v1: https://lore.kernel.org/lkml/20220926003743.409911-1-keescook@chromium.org
> > diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> > index f385ca652b74..fffc3f86181d 100644
> > --- a/lib/overflow_kunit.c
> > +++ b/lib/overflow_kunit.c
> > @@ -16,6 +16,11 @@
> >  #include <linux/types.h>
> >  #include <linux/vmalloc.h>
> >
> > +/* We're expecting to do a lot of "always true" or "always false" tests. */
> > +#ifdef CONFIG_CC_IS_CLANG
> > +#pragma clang diagnostic ignored "-Wtautological-constant-out-of-range-compare"
> > +#endif
> 
> Any chance we can reuse parts of __diag_ignore or __diag_clang from
> include/linux/compiler_types.h or include/linux/compiler-clang.h
> respectively?

Hm, I'm not sure how those are supposed to be used. Those defines don't
seem to be used externally?

> Those are needed for pragmas within preprocessor macros, which we
> don't have here, but I suspect they may be more concise to use here.

Yeah, I was surprised when I had to wrap it in #ifdef given "clang" is
part of the string.

> 
> > +#define TEST_SAME_TYPE(t1, t2, same)                   do {    \
> > +       typeof(t1) __t1h = type_max(t1);                        \
> > +       typeof(t1) __t1l = type_min(t1);                        \
> > +       typeof(t2) __t2h = type_max(t2);                        \
> > +       typeof(t2) __t2l = type_min(t2);                        \
> 
> Can we use __auto_type here rather than typeof(macro expansion)?

I'd rather it stay explicit -- otherwise we start to wander into "oops,
we got lucky" territory for what should be a really distinct test case.

-- 
Kees Cook