Re: [PATCH v4 2/3] overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping_mul()

[Kees Cook <keescook@chromium.org>]

On Tue, Feb 06, 2024 at 10:54:06AM -0600, Gustavo A. R. Silva wrote:
> 
> 
> On 2/6/24 04:31, Kees Cook wrote:
> > Provide helpers that will perform wrapping addition, subtraction, or
> > multiplication without tripping the arithmetic wrap-around sanitizers. The
> > first argument is the type under which the wrap-around should happen
> > with. In other words, these two calls will get very different results:
> > 
> > 	wrapping_mul(int, 50, 50) == 2500
> > 	wrapping_mul(u8,  50, 50) ==  196
> > 
> > Add to the selftests to validate behavior and lack of side-effects.
> > 
> > Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
> > Cc: Marco Elver <elver@google.com>
> > Cc: Eric Biggers <ebiggers@kernel.org>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: linux-hardening@vger.kernel.org
> > Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >   include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
> >   lib/overflow_kunit.c     | 24 +++++++++++++++---
> >   2 files changed, 74 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> > index 4e741ebb8005..429c4d61a940 100644
> > --- a/include/linux/overflow.h
> > +++ b/include/linux/overflow.h
> > @@ -64,6 +64,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> >   #define check_add_overflow(a, b, d)	\
> >   	__must_check_overflow(__builtin_add_overflow(a, b, d))
> > +/**
> > + * wrapping_add() - Intentionally perform a wrapping addition
> > + * @type: type for result of calculation
> > + * @a: first addend
> > + * @b: second addend
> > + *
> > + * Return the potentially wrapped-around addition without
> > + * tripping any wrap-around sanitizers that may be enabled.
> > + */
> > +#define wrapping_add(type, a, b)				\
> > +	({							\
> > +		type __val;					\
> > +		if (__builtin_add_overflow(a, b, &__val)) {	\
> > +			/* do nothing */			\
> > +		}						\
> > +		__val;						\
> 
> mmh... now that __builtin_*_overflow() is directly used, I guess
> we don't need to _check_ for overflow anymore.

/me slaps his forehead

Yes indeed! I will adjust it.

-- 
Kees Cook