linux-kernel - Re: [PATCH v4 2/3] overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping

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Message-ID: <4bde6e72-c7f6-434d-9489-3a0de7804b18@embeddedor.com>
Date: Tue, 6 Feb 2024 10:54:06 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <keescook@...omium.org>,
 Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: Marco Elver <elver@...gle.com>, Eric Biggers <ebiggers@...nel.org>,
 Mark Rutland <mark.rutland@....com>, linux-hardening@...r.kernel.org,
 "Gustavo A . R . Silva" <gustavoars@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] overflow: Introduce wrapping_add(),
 wrapping_sub(), and wrapping_mul()



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@...vas.dk>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Eric Biggers <ebiggers@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: linux-hardening@...r.kernel.org
> Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.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.

Thanks
--
Gustavo