Re: [PATCH 4/5] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap()

[Kees Cook <keescook@chromium.org>]

On Mon, Jan 29, 2024 at 09:08:43PM +0100, Rasmus Villemoes wrote:
> On 29/01/2024 19.34, Kees Cook 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:
> > 
> > 	add_wrap(int, 50, 50) == 2500
> > 	add_wrap(u8,  50, 50) ==  196
> 
> s/add/mul/g I suppose.

Oops, yes.

> > 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 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 54 insertions(+)
> > 
> > diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> > index 3c46c648d2e8..4f945e9e7881 100644
> > --- a/include/linux/overflow.h
> > +++ b/include/linux/overflow.h
> > @@ -120,6 +120,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> >  		check_add_overflow(var, offset, &__result);	\
> >  	}))
> >  
> > +/**
> > + * add_wrap() - Intentionally perform a wrapping addition
> > + * @type: type to check overflow against
> 
> Well, nothing is "checked", so why not just say "type of result"?

Yeah, that's better. I was trying to describe that @type will affect the
value of the result.

> > +/**
> > + * sub_wrap() - Intentionally perform a wrapping subtraction
> > + * @type: type to check underflow against
> 
> The terminology becomes muddy, is (INT_MAX) - (-1) an underflow or
> overflow? Anyway, see above.

Right, I should explicitly say "wrap-around".

> 
> >  
> > +/**
> > + * mul_wrap() - Intentionally perform a wrapping multiplication
> > + * @type: type to check underflow against
> 
> And here there's definitely a copy-pasto.

Ek, yes.

> The code itself looks fine.

Thanks!

-- 
Kees Cook