Re: [PATCH v2] lib/bitmap: workaround const_eval test build failure

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Yury Norov <yury.norov@gmail.com>
Date: Mon, 17 Jul 2023 15:04:35 -0700

> 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 not related to bitmap code itself, but it breaks build for a given
> configuration.
> 
> Disabling the const_eval test under that config may potentially hide other
> bugs. Instead, workaround it by disabling GCOV for the test_bitmap unless
> the compiler will get fixed.
> 
> [1] https://github.com/ClangBuiltLinux/linux/issues/1874
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202307171254.yFcH97ej-lkp@intel.com/
> Fixes: dc34d5036692 ("lib: test_bitmap: add compile-time optimization/evaluations assertions")
> Co-developed-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> ---
>  lib/Makefile      | 6 ++++++
>  lib/test_bitmap.c | 8 ++++----
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/Makefile b/lib/Makefile
> index 42d307ade225..1ffae65bb7ee 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -82,7 +82,13 @@ obj-$(CONFIG_TEST_STATIC_KEYS) += test_static_key_base.o
>  obj-$(CONFIG_TEST_DYNAMIC_DEBUG) += test_dynamic_debug.o
>  obj-$(CONFIG_TEST_PRINTF) += test_printf.o
>  obj-$(CONFIG_TEST_SCANF) += test_scanf.o
> +
>  obj-$(CONFIG_TEST_BITMAP) += test_bitmap.o
> +ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_KASAN),yy)
> +# FIXME: Clang breaks test_bitmap_const_eval when KASAN and GCOV are enabled
> +GCOV_PROFILE_test_bitmap.o := n
> +endif

Oh, quite elegant!

> +
>  obj-$(CONFIG_TEST_UUID) += test_uuid.o
>  obj-$(CONFIG_TEST_XARRAY) += test_xarray.o
>  obj-$(CONFIG_TEST_MAPLE_TREE) += test_maple_tree.o
> diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
> index 187f5b2db4cf..4a678de9c350 100644
> --- a/lib/test_bitmap.c
> +++ b/lib/test_bitmap.c
> @@ -1161,6 +1161,10 @@ static void __init test_bitmap_print_buf(void)
>  	}
>  }
>  
> +/*
> + * FIXME: Clang breaks compile-time evaluations when KASAN and GCOV are enabled.
> + * To workaround it, GCOV is force-disabled in Makefile for this configuration.
> + */
>  static void __init test_bitmap_const_eval(void)
>  {
>  	DECLARE_BITMAP(bitmap, BITS_PER_LONG);
> @@ -1186,11 +1190,7 @@ static void __init test_bitmap_const_eval(void)
>  	 * the compiler is fixed.
>  	 */
>  	bitmap_clear(bitmap, 0, BITS_PER_LONG);
> -#if defined(__s390__) && defined(__clang__)
> -	if (!const_test_bit(7, bitmap))
> -#else
>  	if (!test_bit(7, bitmap))
> -#endif

...and even allows to remove that ugly piece.

>  		bitmap_set(bitmap, 5, 2);
>  
>  	/* Equals to `unsigned long bitopvar = BIT(20)` */

I like this version the most.

Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>

Thanks,
Olek