On Tue, Aug 17, 2021 at 2:18 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote: > > 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 $(sort ...) is alphabetical sort, not numeric sort. > It works for us because the minimum Clang version is 10.0.1 > (that is CONFIG_CLANG_VERSION is always 6-digit) > > It will break when Clang version 100.0.0 is released. > > But, before that, we will raise the minimum supported clang version, > and this conditional will go away. I'd much rather pay the cost of cc-option to have a more precise check; Linus is right: when I upgrade AOSP's fork of LLVM, it may not be the fully released version of clang-14 though we have already moved the version numbers upstream to clang-14. I think we should strive to prefer feature tests over version tests, which are brittle. ``` # Clang would warn about unreachable fall throughs until clang-14. ifdef CONFIG_CC_IS_CLANG ifneq ($(call cc-option,-Wunreachable-code-fallthrough),) KBUILD_CFLAGS += -Wimplicit-fallthrough endif endif ``` Is precisely what we want. -- Thanks, ~Nick Desaulniers
