linux-kernel - Re: [PATCH] lib/bitmap: waive const

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Date:   Mon, 17 Jul 2023 14:31:24 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Tom Rix <trix@...hat.com>, oe-kbuild-all@...ts.linux.dev
Subject: Re: [PATCH] lib/bitmap: waive const_eval test as it breaks the build

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@...nel.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@...il.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?