Re: [PATCH] kconfig: Add support for -Wimplicit-fallthrough

[Nathan Chancellor <nathan@kernel.org>]

On Sat, Nov 13, 2021 at 06:57:25PM -0600, Gustavo A. R. Silva wrote:
> Add Kconfig support for -Wimplicit-fallthrough for both GCC and Clang.
> 
> The compiler option is under configuration CC_IMPLICIT_FALLTHROUGH,
> which is enabled by default.
> 
> Special thanks to Nathan Chancellor who fixed the Clang bug[1][2]. This
> bugfix only appears in Clang 14.0.0, so older versions still contain
> the bug and -Wimplicit-fallthrough won't be enabled for them, for now.
> 
> This concludes a long journey and now we are finally getting rid
> of the unintentional fallthrough bug-class in the kernel, entirely. :)
> 
> [1] https://github.com/llvm/llvm-project/commit/9ed4a94d6451046a51ef393cd62f00710820a7e8
> [2] https://bugs.llvm.org/show_bug.cgi?id=51094
> 
> Link: https://github.com/KSPP/linux/issues/115
> Link: https://github.com/ClangBuiltLinux/linux/issues/236
> Co-developed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Co-developed-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

This appears to do the right thing with both clang-13 and clang-14.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

It feels a little odd to have this in Kconfig but if it works and gets
the warning enabled, then so be it.

> ---
>  Makefile     | 6 +-----
>  init/Kconfig | 5 +++++
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 30c7c81d0437..f18a50daad00 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -786,7 +786,7 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
>  KBUILD_CFLAGS += $(stackp-flags-y)
>  
>  KBUILD_CFLAGS-$(CONFIG_WERROR) += -Werror
> -KBUILD_CFLAGS += $(KBUILD_CFLAGS-y)
> +KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
>  
>  ifdef CONFIG_CC_IS_CLANG
>  KBUILD_CPPFLAGS += -Qunused-arguments
> @@ -798,10 +798,6 @@ KBUILD_CFLAGS += -Wno-gnu
>  KBUILD_CFLAGS += -mno-global-merge
>  else
>  
> -# Warn about unmarked fall-throughs in switch statement.
> -# Disabled for clang while comment to attribute conversion happens and
> -# https://github.com/ClangBuiltLinux/linux/issues/636 is discussed.
> -KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough=5,)
>  # gcc inanely warns about local variables called 'main'
>  KBUILD_CFLAGS += -Wno-main
>  endif
> diff --git a/init/Kconfig b/init/Kconfig
> index 11f8a845f259..b0582cd3e096 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -885,6 +885,11 @@ config ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
>  config CC_HAS_INT128
>  	def_bool !$(cc-option,$(m64-flag) -D__SIZEOF_INT128__=0) && 64BIT
>  
> +config CC_IMPLICIT_FALLTHROUGH
> +	string
> +	default "-Wimplicit-fallthrough=5" if CC_IS_GCC
> +	default "-Wimplicit-fallthrough" if CC_IS_CLANG && $(cc-option,-Wunreachable-code-fallthrough)
> +
>  #
>  # For architectures that know their GCC __int128 support is sound
>  #
> -- 
> 2.27.0
> 
>