On Wed, Aug 18, 2021 at 3:25 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > > On 8/17/2021 11:03 AM, Kees Cook wrote: > > On Mon, Aug 16, 2021 at 09:55:28PM -0700, Nathan Chancellor wrote: > >> If you/Gustavo would prefer, I can upgrade that check to > >> > >> ifneq ($(call cc-option, -Wunreachable-code-fallthrough),) > >> > >> I was just trying to save a call to the compiler, as that is more expensive > >> than a shell test call. > > > > I prefer the option test -- this means no changes are needed on the > > kernel build side if it ever finds itself backported to earlier versions > > (and it handles the current case of "14" not meaning "absolute latest"). > > > > More specifically, I think you want this (untested): > > That should work but since -Wunreachable-code-fallthrough is off by > default, I did not really see a reason to include it in KBUILD_CFLAGS. I > do not have a strong opinion though, your version is smaller than mine > is so we can just go with that. I'll defer to Gustavo on it since he has > put in all of the work cleaning up the warnings. https://github.com/llvm/llvm-project/commit/9ed4a94d6451046a51ef393cd62f00710820a7e8 did two things: (1) Change the -Wimplicit-fallthrough behavior so that it fits to our use in the kernel (2) Add a new option -Wunreachable-code-fallthrough that works like the previous -Wimplicit-fallthrough of Clang <= 13.0.0 They are separate things. Checking the presence of -Wunreachable-code-fallthrough does not make sense since we are only interested in (1) here. So, checking the Clang version is sensible and matches the explanation in the comment block. Moreover, using $(shell test ...) is less expensive than cc-option. If you want to make it even faster, you can use only built-in functions, like this: # Warn about unmarked fall-throughs in switch statement. # Clang prior to 14.0.0 warned on unreachable fallthroughs with # -Wimplicit-fallthrough, which is unacceptable due to IS_ENABLED(). # https://bugs.llvm.org/show_bug.cgi?id=51094 ifeq ($(firstword $(sort $(CONFIG_CLANG_VERSION) 140000)),140000) KBUILD_CFLAGS += -Wimplicit-fallthrough endif The $(sort ...) is alphabetical sort, not numeric sort. It works for us because the minimum Clang version is 10.0.1 (that is CONFIG_CLANG_VERSION is always 6-digit) It will break when Clang version 100.0.0 is released. But, before that, we will raise the minimum supported clang version, and this conditional will go away. > Cheers, > Nathan > > > diff --git a/Makefile b/Makefile > > index b5fd51e68ae9..9845ea50a368 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -859,11 +859,11 @@ KBUILD_CFLAGS += -Wno-gnu > > # source of a reference will be _MergedGlobals and not on of the whitelisted names. > > # See modpost pattern 2 > > KBUILD_CFLAGS += -mno-global-merge > > +# Warn about unmarked fall-throughs in switch statement only if we can also > > +# disable the bogus unreachable code warnings. > > +KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough -Wno-unreachable-code-fallthrough,) > > 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,) > > endif > > > > -- Best Regards Masahiro Yamada
