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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Tue, 20 Nov 2018 10:30:33 +0900
Message-ID: <CAK7LNASUgKUa+h_YuD+dyrBVB6aCw-QbJNe8MTQC9yc3MzwZPw@mail.gmail.com>
Subject: Re: [PATCH v3 3/3] build_bug.h: remove most of dummy BUILD_BUG_ON
 stubs for sparse
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Nov 20, 2018 at 3:02 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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>
>


This patch can go in only when 1/3 is acceptable.

But, I see 1/3 is controversial.


-- 
Best Regards
Masahiro Yamada