Re: [PATCH v3 2/3] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap()

[Kees Cook <keescook@chromium.org>]

On Mon, Feb 05, 2024 at 02:31:04PM +0100, 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

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