linux-kernel - Re: [PATCH v2 12/20] linux/must_be.h: Add must_be() to improve readability of BUILD_BUG_ON

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Date:   Sat, 20 Nov 2021 16:22:39 +0100
From:   "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Kees Cook <keescook@...omium.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v2 12/20] linux/must_be.h: Add must_be() to improve
 readability of BUILD_BUG_ON_ZERO()

Hi Alexey,

On 11/20/21 16:05, Alexey Dobriyan wrote:
> On Sat, Nov 20, 2021 at 02:00:55PM +0100, Alejandro Colomar wrote:
>> Historically, BUILD_BUG_ON_ZERO() has been hard to read.
>> __must_be_array() is based on BUILD_BUG_ON_ZERO(),
>> and unlike BUILD_BUG_ON_*(),
>> it has a pretty readable name.
> 
> The best name is assert() which userspace uses and is standartised.

Yes, assert() is almost the same thing.
In this case, it would be better named static_assert(),
since it's a compile-time assert().

However,
there's still one slight difference
between static_assert() and must_be():

static_assert() is limited;
it cannot be used in some places,
such as in the implementation of ARRAY_SIZE().
The following doesn't compile:

 #define __arraycount(a)  (sizeof((arr)) / sizeof((arr)[0])
 #define ARRAY_SIZE(a)    (__arraycount(a) + static_assert(is_array(a)))

And if you change it to be:

 #define ARRAY_SIZE(a)    (		\
 {					\
	static_assert(is_array(a));	\
	__arraycount(a);		\
 }					\
 )

then the macro can't be used at file scope
(since ({}) can't be used at file scope).

The good thing about __must_be() is that it evaluates to 0,
which allows you to use it
everywhere a 0 can be used.

My own implementation of __must_be() is
more standards compliant,
and is:

#define must_be(e)  (                      \
        0 * (int)sizeof(                   \
                struct {                   \
                        static_assert(e);  \
                        char ISO_C_forbids_a_struct_with_no_members__; \
                }                          \
        )                                  \
)

I would like this to superseed
the kernel's BUILD_BUG_ON_ZERO(),
but it makes use of C2X static_assert().
I don't know how much that can be a problem.

But please consider this proposal.

Thanks,
Alex


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/