Re: [PATCH v7 3/3] overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

Mark Rutland
2024-02-19T17:03:22Z

On Wed, Feb 14, 2024 at 11:46:03AM -0800, Kees Cook wrote:
> This allows replacements of the idioms "var += offset" and "var -=
> offset" with the wrapping_assign_add() and wrapping_assign_sub() helpers
> respectively. They will avoid wrap-around sanitizer instrumentation.
>
> Add to the selftests to validate behavior and lack of side-effects.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
> Cc: Marco Elver <elver@google.com>
> Cc: Eric Biggers <ebiggers@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: linux-hardening@vger.kernel.org

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
> include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++
> lib/overflow_kunit.c | 43 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 75 insertions(+)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index d3ff8e2bec29..dede374832c9 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -81,6 +81,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> __val; \
> })
>
> +/**
> + * wrapping_assign_add() - Intentionally perform a wrapping increment assignment
> + * @var: variable to be incremented
> + * @offset: amount to add
> + *
> + * Increments @var by @offset with wrap-around. Returns the resulting
> + * value of @var. Will not trip any wrap-around sanitizers.
> + *
> + * Returns the new value of @var.
> + */
> +#define wrapping_assign_add(var, offset) \
> + ({ \
> + typeof(var) *__ptr = &(var); \
> + *__ptr = wrapping_add(typeof(var), *__ptr, offset); \
> + })
> +
> /**
> * check_sub_overflow() - Calculate subtraction with overflow checking
> * @a: minuend; value to subtract from
> @@ -111,6 +127,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> __val; \
> })
>
> +/**
> + * wrapping_assign_sub() - Intentionally perform a wrapping decrement assign
> + * @var: variable to be decremented
> + * @offset: amount to subtract
> + *
> + * Decrements @var by @offset with wrap-around. Returns the resulting
> + * value of @var. Will not trip any wrap-around sanitizers.
> + *
> + * Returns the new value of @var.
> + */
> +#define wrapping_assign_sub(var, offset) \
> + ({ \
> + typeof(var) *__ptr = &(var); \
> + *__ptr = wrapping_sub(typeof(var), *__ptr, offset); \
> + })
> +
> /**
> * check_mul_overflow() - Calculate multiplication with overflow checking
> * @a: first factor
> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index d3fdb906d3fe..65e8a72a83bf 100644
> --- a/lib/overflow_kunit.c
> +++ b/lib/overflow_kunit.c
> @@ -284,6 +284,45 @@ DEFINE_TEST_ARRAY(s64) = {
> "Unexpected wrap " #op " macro side-effect!\n"); \
> } while (0)
>
> +static int global_counter;
> +static void bump_counter(void)
> +{
> + global_counter++;
> +}
> +
> +static int get_index(void)
> +{
> + volatile int index = 0;
> + bump_counter();
> + return index;
> +}
> +
> +#define check_self_op(fmt, op, sym, a, b) do { \
> + typeof(a + 0) _a = a; \
> + typeof(b + 0) _b = b; \
> + typeof(a + 0) _a_sym = a; \
> + typeof(a + 0) _a_orig[1] = { a }; \
> + typeof(b + 0) _b_orig = b; \
> + typeof(b + 0) _b_bump = b + 1; \
> + typeof(a + 0) _r; \
> + \
> + _a_sym sym _b; \
> + _r = wrapping_ ## op(_a, _b); \
> + KUNIT_EXPECT_TRUE_MSG(test, _r == _a_sym, \
> + "expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n", \
> + a, b, _a_sym, _r); \
> + KUNIT_EXPECT_TRUE_MSG(test, _a == _a_sym, \
> + "expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n", \
> + a, b, _a_sym, _a); \
> + /* Check for internal macro side-effects. */ \
> + global_counter = 0; \
> + wrapping_ ## op(_a_orig[get_index()], _b_orig++); \
> + KUNIT_EXPECT_EQ_MSG(test, global_counter, 1, \
> + "Unexpected wrapping_" #op " macro side-effect on arg1!\n"); \
> + KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
> + "Unexpected wrapping_" #op " macro side-effect on arg2!\n"); \
> +} while (0)
> +
> #define DEFINE_TEST_FUNC_TYPED(n, t, fmt) \
> static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
> { \
> @@ -293,6 +332,10 @@ static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
> check_one_op(t, fmt, sub, "-", p->a, p->b, p->diff, p->d_of); \
> check_one_op(t, fmt, mul, "*", p->a, p->b, p->prod, p->p_of); \
> check_one_op(t, fmt, mul, "*", p->b, p->a, p->prod, p->p_of); \
> + /* wrapping_assign_{add,sub}() */ \
> + check_self_op(fmt, assign_add, +=, p->a, p->b); \
> + check_self_op(fmt, assign_add, +=, p->b, p->a); \
> + check_self_op(fmt, assign_sub, -=, p->a, p->b); \
> } \
> \
> static void n ## _overflow_test(struct kunit *test) { \
> --
> 2.34.1
>