On Mon, Feb 05, 2024 at 02:31:04PM +0100, 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
Yes, this follows now. This is a leftover from extending the helpers to work with pointers, which I don't have any current use for now. I'll fix this. > 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). I could change that separately, yeah. -- Kees Cook
