Re: [PATCH] lib/bitmap: waive const_eval test as it breaks the build

[Nathan Chancellor <nathan@kernel.org>]

On Mon, Jul 17, 2023 at 02:31:24PM -0700, Yury Norov wrote:
> On Mon, Jul 17, 2023 at 01:57:40PM -0700, Nick Desaulniers wrote:
> > On Mon, Jul 17, 2023 at 1:31 PM Nathan Chancellor <nathan@kernel.org> wrote:
> > >
> > > Hi Yury,
> > >
> > > On Mon, Jul 17, 2023 at 12:58:13PM -0700, Yury Norov wrote:
> > > > When building with clang, and when KASAN and GCOV_PROFILE_ALL are both
> > > > enabled, the test fails to build [1]:
> > > >
> > > > >> lib/test_bitmap.c:920:2: error: call to '__compiletime_assert_239' declared with 'error' attribute: BUILD_BUG_ON failed: !__builtin_constant_p(res)
> > > >            BUILD_BUG_ON(!__builtin_constant_p(res));
> > > >            ^
> > > >    include/linux/build_bug.h:50:2: note: expanded from macro 'BUILD_BUG_ON'
> > > >            BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
> > > >            ^
> > > >    include/linux/build_bug.h:39:37: note: expanded from macro 'BUILD_BUG_ON_MSG'
> > > >    #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> > > >                                        ^
> > > >    include/linux/compiler_types.h:352:2: note: expanded from macro 'compiletime_assert'
> > > >            _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> > > >            ^
> > > >    include/linux/compiler_types.h:340:2: note: expanded from macro '_compiletime_assert'
> > > >            __compiletime_assert(condition, msg, prefix, suffix)
> > > >            ^
> > > >    include/linux/compiler_types.h:333:4: note: expanded from macro '__compiletime_assert'
> > > >                            prefix ## suffix();                             \
> > > >                            ^
> > > >    <scratch space>:185:1: note: expanded from here
> > > >    __compiletime_assert_239
> > > >
> > > > Originally it was attributed to s390, which now looks seemingly wrong. The
> > > > issue is also not related to bitmap code itself, but it breaks build for
> > > > a given configuration. So, disabling the test unless the compiler will
> > > > get fixed.
> > > >
> > > > [1] https://github.com/ClangBuiltLinux/linux/issues/1874
> > > >
> > > > Fixes: dc34d5036692 ("lib: test_bitmap: add compile-time optimization/evaluations assertions")
> > > > Signed-off-by: Yury Norov <yury.norov@gmail.com>
> > > > ---
> > > >  lib/test_bitmap.c | 17 +++++------------
> > > >  1 file changed, 5 insertions(+), 12 deletions(-)
> > > >
> > > > diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
> > > > index 187f5b2db4cf..a791fdb7a8c9 100644
> > > > --- a/lib/test_bitmap.c
> > > > +++ b/lib/test_bitmap.c
> > > > @@ -1163,6 +1163,9 @@ static void __init test_bitmap_print_buf(void)
> > > >
> > > >  static void __init test_bitmap_const_eval(void)
> > > >  {
> > > > +#if defined(CONFIG_CC_IS_CLANG) && defined(CONFIG_KASAN) && defined(CONFIG_GCOV_PROFILE_ALL)
> > > > +#warning "FIXME: Clang breaks compile time evaluations when KASAN and GCOV are enabled"
> > >
> > > Making this a '#warning' will basically just replace the current error
> > > with a different one in the face of CONFIG_WERROR, which seems pointless
> > > to me:
> > >
> > >   lib/test_bitmap.c:1167:2: error: "FIXME: Clang breaks compile time evaluations when KASAN and GCOV are enabled" [-Werror,-W#warnings]
> > >    1167 | #warning "FIXME: Clang breaks compile time evaluations when KASAN and GCOV are enabled"
> > >         |  ^
> > >   1 error generated.
> > >
> > > Could we just opt out of GCOV for test_bitmap.c if KASAN is enabled with
> > > clang? That does not seem too bad of a workaround. I highly doubt there
> > > are many people who are interested in debugging test_bitmap.c with KASAN
> > > while profiling it with GCOV when building with clang, since they would
> > > have hit this already and reported it already; as far as I can tell,
> > > only the Intel robot has reported this with a randconfig.
> > 
> > Yury, Nathan,
> > Thanks for the patches and discussion.  Yes, I think a combo of:
> > 
> > 1. Nathan's lib/Makefile change plus
> > 2. Yury's removal of the current preprocessor guards in lib/test_bitmap.c
> > 
> > make most sense.
> 
> OK, then I'll send a v2 shortly. Nathan, don't you mind if I add your
> co-developed-by for this?

Sure, no worries!

Co-developed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>