linux-kernel - Re: [PATCH v4 3/3] build_bug.h: remove most of dummy BUILD_BUG

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Message-ID: <CANiq72n25pSbQgHqYtLOFYMkWyzfFLfsMRmzpHNZHh=APXoFww@mail.gmail.com>
Date:   Sat, 24 Nov 2018 09:29:19 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>, josh@...htriplett.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] build_bug.h: remove most of dummy BUILD_BUG_ON
 stubs for Sparse

On Thu, Nov 22, 2018 at 5:08 PM Masahiro Yamada
<yamada.masahiro@...ionext.com> 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 enabled only very recently),
> 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 positives, and noisier than
> it should be at that time.
>
> 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.
>
> 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@...ionext.com>
> Acked-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@...il.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>

Nice to see those CHECKER blocks are being reduced!

Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>

Cheers,
Miguel