Re: [PATCH v4 3/3] build_bug.h: remove most of dummy BUILD_BUG_ON stubs for Sparse

[Miguel Ojeda]

On Thu, Nov 22, 2018 at 5:08 PM Masahiro Yamada
<yamada.masahiro@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Nice to see those CHECKER blocks are being reduced!

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

Cheers,
Miguel