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

[Gustavo A. R. Silva]

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@xxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> Reviewed-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>   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.

Thanks
--
Gustavo