linux-hardening - Re: [PATCH v3] overflow: Allow mixed type arguments

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Date:   Wed, 31 Aug 2022 10:49:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-hardening@...r.kernel.org,
        Daniel Latypov <dlatypov@...gle.com>,
        Vitor Massaru Iha <vitor@...saru.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] overflow: Allow mixed type arguments

On Tue, Aug 30, 2022 at 09:52:32PM +0200, Rasmus Villemoes wrote:
> On 30/08/2022 21.21, Kees Cook wrote:
> > [...]
> > + * *@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.
> 
> I don't like that wording. It makes it sound like there's some ambiguity
> or (implementation|un)-definedness involved in what the destination
> holds on overflow. The gcc documentation is perfectly clear that the
> result is the infinite-precision result truncated to N bits, with N
> being the bitwidth of d.

Hm, well, I think use of *d should be strongly discouraged on overflow.
How about just adding the specifics to the end?

 * *@d holds the result 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 will contain the
 * infinite-precision result truncated to the bitwidth of *@d.)

-- 
Kees Cook