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

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Date:   Mon, 29 Aug 2022 23:14:56 +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] overflow: Allow mixed type arguments

On 29/08/2022 22.47, 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,
> once the fallback macros were 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.

Makes sense. I'm a little worried about the implications for -stable
backports to kernels that can still be built with gcc < 5.1, but we
can't let that dictate what is done in mainline. And even people
building old kernels shouldn't be using ancient compilers.

>  
> -#define DEFINE_TEST_ARRAY(t)			\
> -	static const struct test_ ## t {	\
> -		t a, b;				\
> -		t sum, diff, prod;		\
> -		bool s_of, d_of, p_of;		\
> -	} t ## _tests[]
> +#define DEFINE_TEST_ARRAY_TYPED(t1, t2, t)		\
> +	static const struct test_ ## t1 ## t2 ## t {	\
> +		t1 a;					\
> +		t2 b;					\
> +		t sum, diff, prod;			\
> +		bool s_of, d_of, p_of;			\
> +	} t1 ## t2 ## t ## _tests[]

Can I get you to throw in some extra _, because this...

> +DEFINE_TEST_FUNC_TYPED(u32u32int, int, "%d");

...makes my eyes hurt a little. Maybe even make it u32_u32__int, so it's
emphasized that the order is [src op src -> tgt] and not [tgt = src op src].

Rasmus