linux-kernel - Re: [PATCH] build_bug.h: add wrapper for _Static

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Date:   Mon, 4 Feb 2019 15:09:16 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] build_bug.h: add wrapper for _Static_assert

On Sun,  3 Feb 2019 20:24:00 +0100 Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> BUILD_BUG_ON() is a little annoying, since it cannot be used outside
> function scope. So one cannot put assertions about the sizeof() a
> struct next to the struct definition, but has to hide that in some
> more or less arbitrary function.
> 
> Since gcc 4.6 (which is now also the required minimum), there is
> support for the C11 _Static_assert in all C modes, including gnu89. So
> add a simple wrapper for that.
> 
> _Static_assert() requires a message argument, which is usually quite
> redundant (and I believe that bug got fixed at least in newer C++
> standards), but we can easily work around that with a little macro
> magic, making it optional.
> 
> For example, adding
> 
>   static_assert(sizeof(struct printf_spec) == 8);
> 
> in vsprintf.c and modifying that struct to violate it, one gets
> 
> ./include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(struct printf_spec) == 8"
>  #define __static_assert(expr, msg, ...) _Static_assert(expr, "" msg "")
> 
> godbolt.org suggests that _Static_assert() has been support by clang
> since at least 3.0.0.
> 

It would be (very) nice to actually use this macro in a few places so
it gets its build testing while in -next.