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Date:   Tue, 17 Aug 2021 11:03:03 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] kbuild: Enable -Wimplicit-fallthrough for clang 14.0.0+
Message-ID: <202108171056.EDCE562@keescook>
References: <20210817005624.1455428-1-nathan@kernel.org>
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 <CAHk-=wgFXOf9OUh3+vmWjhp1PC47RVsUkL0NszBxSWhbGzx4tw@mail.gmail.com>
 <5c856f36-69a7-e274-f72a-c3aef195adeb@kernel.org>
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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):

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
 

-- 
Kees Cook