Re: [PATCH 4/5] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap()

Rasmus Villemoes
2024-01-29T20:08:50Z

On 29/01/2024 19.34, 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:
>
> add_wrap(int, 50, 50) == 2500
> add_wrap(u8, 50, 50) == 196

s/add/mul/g I suppose.

> Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 54 insertions(+)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 3c46c648d2e8..4f945e9e7881 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -120,6 +120,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> check_add_overflow(var, offset, &__result); \
> }))
>
> +/**
> + * add_wrap() - Intentionally perform a wrapping addition
> + * @type: type to check overflow against

Well, nothing is "checked", so why not just say "type of result"?

>
> +/**
> + * sub_wrap() - Intentionally perform a wrapping subtraction
> + * @type: type to check underflow against

The terminology becomes muddy, is (INT_MAX) - (-1) an underflow or
overflow? Anyway, see above.

>
> +/**
> + * mul_wrap() - Intentionally perform a wrapping multiplication
> + * @type: type to check underflow against

And here there's definitely a copy-pasto.

The code itself looks fine.

Rasmus