linux-hardening - Re: [PATCH v6 1/3] overflow: Adjust check_*

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Date: Wed, 14 Feb 2024 11:38:57 -0800
From: Kees Cook <keescook@...omium.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Andy Shevchenko <andy@...nel.org>,
	"Gustavo A . R . Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org,
	Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
	Marco Elver <elver@...gle.com>, Eric Biggers <ebiggers@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/3] overflow: Adjust check_*_overflow() kern-doc to
 reflect results

On Wed, Feb 14, 2024 at 11:57:28AM +0000, Mark Rutland wrote:
> On Tue, Feb 13, 2024 at 02:10:57PM -0800, Kees Cook wrote:
> > The check_*_overflow() helpers will return results with potentially
> > wrapped-around values. These values have always been checked by the
> > selftests, so avoid the confusing language in the kern-doc. The idea of
> > "safe for use" was relative to the expectation of whether or not the
> > caller wants a wrapped value -- the calculation itself will always follow
> > arithmetic wrapping rules.
> > 
> > Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > Cc: linux-hardening@...r.kernel.org
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  include/linux/overflow.h | 18 ++++++------------
> >  1 file changed, 6 insertions(+), 12 deletions(-)
> > 
> > diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> > index 7b5cf4a5cd19..4e741ebb8005 100644
> > --- a/include/linux/overflow.h
> > +++ b/include/linux/overflow.h
> > @@ -57,11 +57,9 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> >   * @b: second addend
> >   * @d: pointer to store sum
> >   *
> > - * Returns 0 on success.
> > + * Returns 0 on success, 1 on wrap-around.
> 
> Sorry for the last minute bikeshedding, but could we clarify 'success' here?
> e.g. I think it'd be clearer to say:
> 
>   Returns true on wrap-around, false otherwise.
> 
> Note that also uses true/false since these all return bool (as do the
> underlying __builtin_*_overflow() functions).

Yeah, that's a good point. I'll update this.

> >   *
> > - * *@d holds the results of the attempted addition, but is not considered
> > - * "safe for use" on a non-zero return value, which indicates that the
> > - * sum has overflowed or been truncated.
> > + * *@d holds the results of the attempted addition, which may wrap-around.
> 
> How about:
> 
>   @d holds the results of the attempted addition, regardless of whether
>   wrap-around occurred.
> 
> ... and likewise for the others below?

Yeah, that's more clear. Thanks!

-Kees

-- 
Kees Cook