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

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Date:   Tue, 30 Aug 2022 21:52:32 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Kees Cook <keescook@...omium.org>
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 30/08/2022 21.21, Kees Cook wrote:
> When the check_[op]_overflow() helpers were introduced, all arguments were
> required to be the same type to make the fallback macros simpler. However,
> now that the fallback macros have been removed[1], it is fine to allow
> mixed types, which makes using the helpers much more useful, as they
> can be used to test for type-based overflows (e.g. adding two large ints
> but storing into a u8), as would be handy in the drm core[2].
> 
> Remove the restriction, and add additional self-tests that exercise some
> of the mixed-type overflow cases, and double-check for accidental macro
> side-effects.
> 

> -/*
> - * For simplicity and code hygiene, the fallback code below insists on
> - * a, b and *d having the same type (similar to the min() and max()
> - * macros), whereas gcc's type-generic overflow checkers accept
> - * different types. Hence we don't just make check_add_overflow an
> - * alias for __builtin_add_overflow, but add type checks similar to
> - * below.
> +/** check_add_overflow() - Calculate addition with overflow checking
> + *
> + * @a: first addend
> + * @b: second addend
> + * @d: pointer to store sum
> + *
> + * Returns 0 on success.
> + *
> + * *@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.

Rasmus