Re: [PATCH] Makefile.extrawarn: enable -Wmissing-variable-declarations for W=1

[Nathan Chancellor <nathan@kernel.org>]

On Mon, Aug 07, 2023 at 09:50:32AM -0700, Nick Desaulniers wrote:
> I noticed Tom was sending patches where smatch was recommending
> annotating functions as static when no previous declaration existed.
> Surely the compiler could make such recommendations as well, I thought.
> 
> Looks like -Wmissing-variable-declarations can make such
> recommendations.
> 
> GCC just added support for this warning (gcc 14.1.0 will ship with
> support), and all versions of clang relevant to building the kernel
> support this flag.
> 
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Any idea how many instances of this there are? At least x86_64 defconfig
fails immediately with:

  In file included from scripts/mod/devicetable-offsets.c:3:
  In file included from include/linux/mod_devicetable.h:14:
  In file included from include/linux/uuid.h:11:
  In file included from include/linux/string.h:20:
  In file included from arch/x86/include/asm/string.h:5:
  In file included from arch/x86/include/asm/string_64.h:6:
  In file included from include/linux/jump_label.h:112:
  In file included from arch/x86/include/asm/jump_label.h:7:
  arch/x86/include/asm/asm.h:208:24: error: no previous extern declaration for non-static variable 'current_stack_pointer' [-Werror,-Wmissing-variable-declarations]
    208 | register unsigned long current_stack_pointer asm(_ASM_SP);
        |                        ^
  arch/x86/include/asm/asm.h:208:10: note: declare 'static' if the variable is not intended to be used outside of this translation unit
    208 | register unsigned long current_stack_pointer asm(_ASM_SP);
        |          ^
  1 error generated.

> ---
>  scripts/Makefile.extrawarn | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 40cd13eca82e..617739eb84e2 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -32,6 +32,7 @@ KBUILD_CFLAGS += $(call cc-option, -Wunused-but-set-variable)
>  KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
>  KBUILD_CFLAGS += $(call cc-option, -Wpacked-not-aligned)
>  KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
> +KBUILD_CFLAGS += $(call cc-option, -Wmissing-varibale-declarations)
                                                ^ variable
>  # The following turn off the warnings enabled by -Wextra
>  KBUILD_CFLAGS += -Wno-missing-field-initializers
>  KBUILD_CFLAGS += -Wno-sign-compare
> 
> ---
> base-commit: 52a93d39b17dc7eb98b6aa3edb93943248e03b2f
> change-id: 20230807-missing_proto-0cb90ec6454c
> 
> Best regards,
> -- 
> Nick Desaulniers <ndesaulniers@google.com>
>