Messages in this thread | | | Date | Mon, 5 Feb 2024 13:53:12 -0600 | Subject | Re: [PATCH v3 2/3] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap() | From | "Gustavo A. R. Silva" <> |
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On 2/5/24 07:31, Marco Elver wrote: > On Mon, 5 Feb 2024 at 10:12, Kees Cook <keescook@chromium.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: >> >> 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 <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 ++++++++++++++++++++++++++++++++++++++++ >> 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 <gustavoars@kernel.org>
Thanks! -- Gustavo
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