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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Tue, 5 Feb 2019 17:05:15 +0900
Message-ID: <CAK7LNAQqHgLTJU1tBU+oU+1CrWBrDpnui=dQCrNpzW4N2R-Omw@mail.gmail.com>
Subject: Re: [PATCH] build_bug.h: add wrapper for _Static_assert
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Kees Cook <keescook@chromium.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Feb 4, 2019 at 4:24 AM Rasmus Villemoes
<linux@rasmusvillemoes.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.
>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  include/linux/build_bug.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
> index faeec7433aab..4bf9ba847b44 100644
> --- a/include/linux/build_bug.h
> +++ b/include/linux/build_bug.h
> @@ -58,4 +58,23 @@
>   */
>  #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
>
> +/**
> + * static_assert - check integer constant expression at build time
> + *
> + * static_assert() is a wrapper for the C11 _Static_assert, with a
> + * little macro magic to make the message optional (defaulting to the
> + * stringification of the tested expression).
> + *
> + * Contrary to BUILD_BUG_ON(), static_assert() can be used at global
> + * scope, but requires the expression to be an integer constant
> + * expression (i.e., it is not enough that __builtin_constant_p() is
> + * true for expr).
> + *
> + * Also note that BUILD_BUG_ON() fails the build if the condition is
> + * true, while static_assert() fails the build if the expression is
> + * false.
> + */
> +#define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
> +#define __static_assert(expr, msg, ...) _Static_assert(expr, "" msg "")

What is the "" "" for?


Bikeshed:

There might be room for argument about
where this macro should go.

Another possible place is <linux/compiler.h>
where compiletime_assert() is defined.


Thanks.


-- 
Best Regards
Masahiro Yamada