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Date:   Tue, 13 Sep 2022 05:01:23 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc:     intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, mchehab@kernel.org,
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        thomas.hellstrom@linux.intel.com, jani.nikula@intel.com,
        nirmoy.das@intel.com, airlied@redhat.com, daniel@ffwll.ch,
        andi.shyti@linux.intel.com, andrzej.hajda@intel.com,
        mauro.chehab@linux.intel.com, linux@rasmusvillemoes.dk,
        vitor@massaru.org, dlatypov@google.com, ndesaulniers@google.com
Subject: Re: [PATCH v10 3/9] compiler_types.h: Add assert_type to catch type
 mis-match while compiling
Message-ID: <202209130455.E7CF976A@keescook>
References: <20220909105913.752049-1-gwan-gyeong.mun@intel.com>
 <20220909105913.752049-4-gwan-gyeong.mun@intel.com>
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On Fri, Sep 09, 2022 at 07:59:07PM +0900, Gwan-gyeong Mun wrote:
> It adds assert_type and assert_typable macros to catch type mis-match while
> compiling. The existing typecheck() macro outputs build warnings, but the
> newly added assert_type() macro uses the _Static_assert() keyword (which is
> introduced in C11) to generate a build break when the types are different
> and can be used to detect explicit build errors.
> Unlike the assert_type() macro, assert_typable() macro allows a constant
> value as the second argument.
> 
> Suggested-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
> Cc: Thomas Hellstr?m <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Nirmoy Das <nirmoy.das@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Andi Shyti <andi.shyti@linux.intel.com>
> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> Cc: Andrzej Hajda <andrzej.hajda@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> ---
>  include/linux/compiler_types.h | 39 ++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 4f2a819fd60a..19cc125918bb 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -294,6 +294,45 @@ struct ftrace_likely_data {
>  /* Are two types/vars the same type (ignoring qualifiers)? */
>  #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
>  
> +/**
> + * assert_type - break compile if the first argument's data type and the second
> + *               argument's data type are not the same
> + *
> + * @t1: data type or variable
> + * @t2: data type or variable
> + *
> + * The first and second arguments can be data types or variables or mixed (the
> + * first argument is the data type and the second argument is variable or vice
> + * versa). It determines whether the first argument's data type and the second
> + * argument's data type are the same while compiling, and it breaks compile if
> + * the two types are not the same.
> + * See also assert_typable().
> + */
> +#define assert_type(t1, t2) _Static_assert(__same_type(t1, t2))
> +
> +/**
> + * assert_typable - break compile if the first argument's data type and the
> + *                  second argument's data type are not the same
> + *
> + * @t: data type or variable
> + * @n: data type or variable or constant value
> + *
> + * The first and second arguments can be data types or variables or mixed (the
> + * first argument is the data type and the second argument is variable or vice
> + * versa). Unlike the assert_type() macro, this macro allows a constant value
> + * as the second argument. And if the second argument is a constant value, it
> + * always passes. And it doesn't mean that the types are explicitly the same.
> + * When a constant value is used as the second argument, if you need an
> + * overflow check when assigning a constant value to a variable of the type of
> + * the first argument, you can use the overflows_type() macro. When a constant

I wonder if the overflows_type() check should happen in this test? It
seems weird that assert_typable(u8, 1024) would pass...

> + * value is not used as a second argument, it determines whether the first
> + * argument's data type and the second argument's data type are the same while
> + * compiling, and it breaks compile if the two types are not the same.
> + * See also assert_type() and overflows_type().
> + */
> +#define assert_typable(t, n) _Static_assert(__builtin_constant_p(n) ||	\
> +					    __same_type(t, typeof(n)))

Totally untested -- I'm not sure if this gets the right semantics for
constant expressoins, etc...

static_assert(__builtin_choose_expression(__builtin_constant_p(n), \
			overflows_type(n, typeof(t)), \
			__same_type(t, typeof(n))))


Also, can you please add KUnit tests for these new helpers into
lib/overflow_kunit.c?

-- 
Kees Cook