Re: [RFC PATCH] compiler.h: give up __compiletime_assert_fallback()

[Nick Desaulniers]

+ gbiv who wrote this cool paste (showing alternatives to
_Static_assert, which is supported by both compilers in -std=gnu89,
but not until gcc 4.6): https://godbolt.org/g/DuLsxu

I can't help but think that BUILD_BUG_ON_MSG should use
_Static_assert, then have fallbacks for gcc < 4.6.

On Sun, Aug 19, 2018 at 9:14 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Sat, Aug 18, 2018 at 10:51 PM, Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
> > __compiletime_assert_fallback() is supposed to stop building earlier
> > by using the negative-array-size method in case the compiler does not
> > support "error" attribute, but has never worked like that.
> >
> > You can try this simple code:
> >
> >     #include <linux/build_bug.h>
> >     void foo(void)
> >     {
> >             BUILD_BUG_ON(1);
> >     }
> >
> > GCC (precisely, GCC 4.3 or later) immediately terminates the build,
> > but Clang does not report anything because Clang does not support the
> > "error" attribute now.  It will eventually fail in the link stage,
> > but at least __compiletime_assert_fallback() is not working.
> >
> > The root cause is commit 1d6a0d19c855 ("bug.h: prevent double evaluation
> > of `condition' in BUILD_BUG_ON").  Prior to that commit, BUILD_BUG_ON()
> > was checked by the negative-array-size method *and* the link-time trick.
> > Since that commit, the negative-array-size is not effective because
> > '__cond' is no longer constant.  As the comment in <linux/build_bug.h>
> > says, GCC (and Clang as well) only emits the error for obvious cases.
> >
> > When '__cond' is a variable,
> >
> >     ((void)sizeof(char[1 - 2 * __cond]))
> >
> > ... is not obvious for the compiler to know the array size is negative.
> >
> > One way to fix this is to stop the variable assignment, i.e. to pass
> > '!(condition)' directly to __compiletime_error_fallback() at the cost
> > of the double evaluation of 'condition'.  However, all calls of
> > BUILD_BUG() would be turned into errors even if they are called from
> > dead-code.
> >
> > This commit does not change the current behavior since it just rips
> > off the code that has not been effective for some years.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
>
> Yeah, Clang would only complain about the VLA (and not error) and then
> later fail at link time. This seems like a reasonable change to me.

Heh, we were just talking about this (-Wvla warnings from this macro).
Was there a previous thread before this patch?

>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> -Kees
>
> > ---
> >
> >  include/linux/compiler.h | 17 +----------------
> >  1 file changed, 1 insertion(+), 16 deletions(-)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 42506e4..c062238f4 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -295,29 +295,14 @@ unsigned long read_word_at_a_time(const void *addr)
> >  #endif
> >  #ifndef __compiletime_error
> >  # define __compiletime_error(message)
> > -/*
> > - * Sparse complains of variable sized arrays due to the temporary variable in
> > - * __compiletime_assert. Unfortunately we can't just expand it out to make
> > - * sparse see a constant array size without breaking compiletime_assert on old
> > - * versions of GCC (e.g. 4.2.4), so hide the array from sparse altogether.
> > - */
> > -# ifndef __CHECKER__
> > -#  define __compiletime_error_fallback(condition) \
> > -       do { ((void)sizeof(char[1 - 2 * condition])); } while (0)

Note that there are a few definitions of BUILD_BUG_ON that still use
this negative array size trick.  Should that pattern be removed from
them as well?  See:
* arch/x86/boot/boot.h#L33
* include/linux/build_bug.h#L66
* tools/include/linux/kernel.h#L38

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
-- 
Thanks,
~Nick Desaulniers