Re: [PATCH v3] overflow: Allow mixed type arguments

From: Kees Cook <keescook@chromium.org>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>,
	Andrzej Hajda <andrzej.hajda@intel.com>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-hardening@vger.kernel.org,
	Daniel Latypov <dlatypov@google.com>,
	Vitor Massaru Iha <vitor@massaru.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] overflow: Allow mixed type arguments
Date: Wed, 31 Aug 2022 10:49:07 -0700	[thread overview]
Message-ID: <202208311040.C6CA8253@keescook> (raw)
In-Reply-To: <787894a0-b1b7-43c2-c509-f246f94f58f7@rasmusvillemoes.dk>

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