linux-hardening - Re: [PATCH v4 1/3] overflow: Adjust check_*

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Date: Tue, 6 Feb 2024 10:36:15 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <keescook@...omium.org>,
 Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 linux-hardening@...r.kernel.org, Marco Elver <elver@...gle.com>,
 Eric Biggers <ebiggers@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] overflow: Adjust check_*_overflow() kern-doc to
 reflect results



On 2/6/24 04:31, Kees Cook wrote:
> The check_*_overflow() helpers will return results with potentially
> wrapped-around values. These values have always been checked by the
> selftests, so avoid the confusing language in the kern-doc. The idea of
> "safe for use" was relative to the expectation of whether or not the
> caller wants a wrapped value -- the calculation itself will always follow
> arithmetic wrapping rules.
> 
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Better to be concise and direct. 🙂

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks!
-- 
Gustavo

> ---
>   include/linux/overflow.h | 18 ++++++------------
>   1 file changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 7b5cf4a5cd19..4e741ebb8005 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -57,11 +57,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>    * @b: second addend
>    * @d: pointer to store sum
>    *
> - * Returns 0 on success.
> + * Returns 0 on success, 1 on wrap-around.
>    *
> - * *@d holds the results of the attempted addition, but is not considered
> - * "safe for use" on a non-zero return value, which indicates that the
> - * sum has overflowed or been truncated.
> + * *@d holds the results of the attempted addition, which may wrap-around.
>    */
>   #define check_add_overflow(a, b, d)	\
>   	__must_check_overflow(__builtin_add_overflow(a, b, d))
> @@ -72,11 +70,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>    * @b: subtrahend; value to subtract from @a
>    * @d: pointer to store difference
>    *
> - * Returns 0 on success.
> + * Returns 0 on success, 1 on wrap-around.
>    *
> - * *@d holds the results of the attempted subtraction, but is not considered
> - * "safe for use" on a non-zero return value, which indicates that the
> - * difference has underflowed or been truncated.
> + * *@d holds the results of the attempted subtraction, which may wrap-around.
>    */
>   #define check_sub_overflow(a, b, d)	\
>   	__must_check_overflow(__builtin_sub_overflow(a, b, d))
> @@ -87,11 +83,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>    * @b: second factor
>    * @d: pointer to store product
>    *
> - * Returns 0 on success.
> + * Returns 0 on success, 1 on wrap-around.
>    *
> - * *@d holds the results of the attempted multiplication, but is not
> - * considered "safe for use" on a non-zero return value, which indicates
> - * that the product has overflowed or been truncated.
> + * *@d holds the results of the attempted multiplication, which may wrap-around.
>    */
>   #define check_mul_overflow(a, b, d)	\
>   	__must_check_overflow(__builtin_mul_overflow(a, b, d))