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Date:   Mon, 19 Nov 2018 13:37:33 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
To:     Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        Josh Triplett <josh@joshtriplett.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/3] build_bug.h: remove most of dummy BUILD_BUG_ON
 stubs for sparse
Message-ID: <20181119123732.zcb6axl25bixgtly@ltop.local>
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 <1542623503-3755-3-git-send-email-yamada.masahiro@socionext.com>
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On Mon, Nov 19, 2018 at 07:31:43PM +0900, Masahiro Yamada wrote:
> The introduction of these dummy BUILD_BUG_ON stubs dates back to
> commit 903c0c7cdc21 ("sparse: define dummy BUILD_BUG_ON definition
> for sparse").
> 
> At that time, BUILD_BUG_ON() was implemented with the negative array
> trick *and* the link-time trick, like this:
> 
>   extern int __build_bug_on_failed;
>   #define BUILD_BUG_ON(condition)                                \
>           do {                                                   \
>                   ((void)sizeof(char[1 - 2*!!(condition)]));     \
>                   if (condition) __build_bug_on_failed = 1;      \
>           } while(0)
> 
> Sparse is more strict about the negative array trick than GCC because
> Sparse requires the array length to be really constant.
> 
> Here is the simple test code for the macro above:
> 
>   static const int x = 0;
>   BUILD_BUG_ON(x);
> 
> GCC is absolutely fine with it (-Wvla was not enabled at that time),
> but Sparse warns like this:
> 
>   error: bad constant expression
>   error: cannot size expression
> 
> (If you are using a newer version of Sparse, you will see a different
> warning message, "warning: Variable length array is used".)
> 
> Anyway, Sparse was producing many false positive warnings, hence
> silenced.
> 
> With the previous commit, the leftover negative array trick is gone.
> Sparse is fine with the current BUILD_BUG_ON(), which is implemented
> by using the 'error' attribute. (assuming your Sparse version supports
> -Wno-unknown-attribute option)
> 
> I am keeping the stub for BUILD_BUG_ON_ZERO(). Otherwise, Sparse
> would complain about the following code, which GCC is fine with:
> 
>   static const int x = 0;
>   int y = BUILD_BUG_ON_ZERO(x);
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> Acked-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>