Messages in this thread | | | From | Marco Elver <> | Date | Mon, 5 Feb 2024 14:31:04 +0100 | Subject | Re: [PATCH v3 2/3] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap() |
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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).
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