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Date: Wed, 7 Feb 2024 19:06:57 +0100
From: Marco Elver <elver@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Andy Shevchenko <andy@...nel.org>, Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
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 v5 2/3] overflow: Introduce wrapping_add(),
wrapping_sub(), and wrapping_mul()
On Wed, 7 Feb 2024 at 16:24, Kees Cook <keescook@...omium.org> 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>
Reviewed-by: Marco Elver <elver@...gle.com>
> ---
> include/linux/overflow.h | 48 ++++++++++++++++++++++++++++++++++++++++
> lib/overflow_kunit.c | 24 ++++++++++++++++----
> 2 files changed, 68 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 4e741ebb8005..849a49fb496e 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -64,6 +64,22 @@ 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; \
> + __builtin_add_overflow(a, b, &__val); \
> + __val; \
> + })
> +
> /**
> * check_sub_overflow() - Calculate subtraction with overflow checking
> * @a: minuend; value to subtract from
> @@ -77,6 +93,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> #define check_sub_overflow(a, b, d) \
> __must_check_overflow(__builtin_sub_overflow(a, b, d))
>
> +/**
> + * wrapping_sub() - Intentionally perform a wrapping subtraction
> + * @type: type for result of calculation
> + * @a: minuend; value to subtract from
> + * @b: subtrahend; value to subtract from @a
> + *
> + * Return the potentially wrapped-around subtraction without
> + * tripping any wrap-around sanitizers that may be enabled.
> + */
> +#define wrapping_sub(type, a, b) \
> + ({ \
> + type __val; \
> + __builtin_sub_overflow(a, b, &__val); \
> + __val; \
> + })
> +
> /**
> * check_mul_overflow() - Calculate multiplication with overflow checking
> * @a: first factor
> @@ -90,6 +122,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> #define check_mul_overflow(a, b, d) \
> __must_check_overflow(__builtin_mul_overflow(a, b, d))
>
> +/**
> + * wrapping_mul() - Intentionally perform a wrapping multiplication
> + * @type: type for result of calculation
> + * @a: first factor
> + * @b: second factor
> + *
> + * Return the potentially wrapped-around multiplication without
> + * tripping any wrap-around sanitizers that may be enabled.
> + */
> +#define wrapping_mul(type, a, b) \
> + ({ \
> + type __val; \
> + __builtin_mul_overflow(a, b, &__val); \
> + __val; \
> + })
> +
> /**
> * check_shl_overflow() - Calculate a left-shifted value and check overflow
> * @a: Value to be shifted
> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index c527f6b75789..d3fdb906d3fe 100644
> --- a/lib/overflow_kunit.c
> +++ b/lib/overflow_kunit.c
> @@ -258,20 +258,36 @@ DEFINE_TEST_ARRAY(s64) = {
> \
> _of = check_ ## op ## _overflow(a, b, &_r); \
> KUNIT_EXPECT_EQ_MSG(test, _of, of, \
> - "expected "fmt" "sym" "fmt" to%s overflow (type %s)\n", \
> + "expected check "fmt" "sym" "fmt" to%s overflow (type %s)\n", \
> a, b, of ? "" : " not", #t); \
> KUNIT_EXPECT_EQ_MSG(test, _r, r, \
> - "expected "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
> + "expected check "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
> a, b, r, _r, #t); \
> /* Check for internal macro side-effects. */ \
> _of = check_ ## op ## _overflow(_a_orig++, _b_orig++, &_r); \
> - KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, "Unexpected " #op " macro side-effect!\n"); \
> - KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, "Unexpected " #op " macro side-effect!\n"); \
> + KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, \
> + "Unexpected check " #op " macro side-effect!\n"); \
> + KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
> + "Unexpected check " #op " macro side-effect!\n"); \
> + \
> + _r = wrapping_ ## op(t, a, b); \
> + KUNIT_EXPECT_TRUE_MSG(test, _r == r, \
> + "expected wrap "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
> + a, b, r, _r, #t); \
> + /* Check for internal macro side-effects. */ \
> + _a_orig = a; \
> + _b_orig = b; \
> + _r = wrapping_ ## op(t, _a_orig++, _b_orig++); \
> + KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, \
> + "Unexpected wrap " #op " macro side-effect!\n"); \
> + KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
> + "Unexpected wrap " #op " macro side-effect!\n"); \
> } while (0)
>
> #define DEFINE_TEST_FUNC_TYPED(n, t, fmt) \
> static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
> { \
> + /* check_{add,sub,mul}_overflow() and wrapping_{add,sub,mul} */ \
> check_one_op(t, fmt, add, "+", p->a, p->b, p->sum, p->s_of); \
> check_one_op(t, fmt, add, "+", p->b, p->a, p->sum, p->s_of); \
> check_one_op(t, fmt, sub, "-", p->a, p->b, p->diff, p->d_of); \
> --
> 2.34.1
>
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