linux-kernel - Re: [PATCH v10 3/9] compiler_types.h: Add assert_type to catch type mis-match while compiling

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Date:   Sun, 11 Sep 2022 13:04:55 +0200
From:   Andi Shyti <andi.shyti@...ux.intel.com>
To:     Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>
Cc:     intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, mchehab@...nel.org,
        chris@...is-wilson.co.uk, matthew.auld@...el.com,
        thomas.hellstrom@...ux.intel.com, jani.nikula@...el.com,
        nirmoy.das@...el.com, airlied@...hat.com, daniel@...ll.ch,
        andi.shyti@...ux.intel.com, andrzej.hajda@...el.com,
        keescook@...omium.org, mauro.chehab@...ux.intel.com,
        linux@...musvillemoes.dk, vitor@...saru.org, dlatypov@...gle.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH v10 3/9] compiler_types.h: Add assert_type to catch type
 mis-match while compiling

Hi Gwan-gyeong,

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

/Add/It adds/, please use the imperative form.

> 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@...omium.org>
> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>
> Cc: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
> Cc: Matthew Auld <matthew.auld@...el.com>
> Cc: Nirmoy Das <nirmoy.das@...el.com>
> Cc: Jani Nikula <jani.nikula@...el.com>
> Cc: Andi Shyti <andi.shyti@...ux.intel.com>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Cc: Andrzej Hajda <andrzej.hajda@...el.com>
> Cc: Kees Cook <keescook@...omium.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

I would use /aborts compilation/break compile/

> + *

nowhere is written that this extra blank line is not needed, but
just checking the style in compiler_types.h it is not used.

I personally like the blank line, but standing to the general
taste, it should be removed also for keeping a coherent style.

> + * @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))

In C11 _Static_assert is defined as:

  _Static_assert ( constant-expression , string-literal ) ;

While

  _Static_assert ( constant-expression ) ;

is defined in C17 along with the previous. I think you should add
the error message as a 'string-literal'.

Andi

> +/**
> + * 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
> + * 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)))
> +
>  /*
>   * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
>   *			       non-scalar types unchanged.
> -- 
> 2.37.1