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

[Mark Rutland <mark.rutland@arm.com>]

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
>