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

[Gustavo A. R. Silva]

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@xxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Co-developed-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
 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