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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 19 Nov 2018 10:00:54 -0800
Message-ID: <CAKwvOdk7WAeLPibZojsPFjvnnWWL8xE0D8=SxxWjNeoO3EY9rQ@mail.gmail.com>
Subject: Re: [PATCH v3 3/3] build_bug.h: remove most of dummy BUILD_BUG_ON
 stubs for sparse
To:     Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc:     Masahiro Yamada <yamada.masahiro@socionext.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>, josh@joshtriplett.org,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Nov 19, 2018 at 4:37 AM Luc Van Oostenryck
<luc.vanoostenryck@gmail.com> wrote:
>
> 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>

Clang builds not affected. Tested a quick arm64 defconfig build with
Clang + this patch.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

-- 
Thanks,
~Nick Desaulniers