On 2/5/24 07:31, Marco Elver wrote:
On Mon, 5 Feb 2024 at 10:12, Kees Cook <[email protected]> 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: mul_wrap(int, 50, 50) == 2500 mul_wrap(u8, 50, 50) == 196 Add to the selftests to validate behavior and lack of side-effects. Cc: Rasmus Villemoes <[email protected]> Cc: Mark Rutland <[email protected]> Cc: [email protected] Signed-off-by: Kees Cook <[email protected]> --- include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++ lib/overflow_kunit.c | 23 ++++++++++++++--- 2 files changed, 73 insertions(+), 4 deletions(-) diff --git a/include/linux/overflow.h b/include/linux/overflow.h index 4e741ebb8005..9b8c05bdb788 100644 --- a/include/linux/overflow.h +++ b/include/linux/overflow.h @@ -64,6 +64,24 @@ 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)) +/** + * add_wrap() - 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 add_wrap(type, a, b) \ + ({ \ + type __val; \ + if (check_add_overflow(a, b, &__val)) { \ + /* do nothing */ \The whole reason check_*_overflow() exists is to wrap the builtin in a function with __must_check. Here the result is explicitly ignored, so do we have to go through the check_add_overflow indirection? Why not just use the builtin directly? It might make sense to make the compiler's job a little easier, because I predict that __must_check_overflow will be outlined with enough instrumentation (maybe it should have been __always_inline).
Yeah; I think that directly calling __builtin_*_overflow() is a bit cleaner. I wonder if there is any particular reason for not doing that. In any case, this version of the add_wrap() helper with the `type` as parameter looks much better than the v1 that relied on `typeof(a)`. :) So, Reviewed-by: Gustavo A. R. Silva <[email protected]> Thanks! -- Gustavo
