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Date:   Wed, 18 Aug 2021 13:15:36 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Philip Li <philip.li@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] kbuild: Enable -Wimplicit-fallthrough for clang 14.0.0+

On Wed, Aug 18, 2021 at 8:23 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On 8/17/2021 4:06 PM, Kees Cook wrote:
> > On Tue, Aug 17, 2021 at 04:33:25PM -0500, Gustavo A. R. Silva wrote:
> >>
> >>
> >> On 8/17/21 16:17, Masahiro Yamada wrote:
> >>> On Wed, Aug 18, 2021 at 3:25 AM Nathan Chancellor <nathan@...nel.org> 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.
> >
> > I thought one of the problems (which is quickly draining away) that
> > needed to be solved here is that some Clang trunk builds (that report
> > as version 14) don't yet have support for -Wunreachable-code-fallthrough
> > since they aren't new enough.
>
> Philip, how often is the kernel test robot's clang version rebuilt?
> Would it be possible to bump it to latest ToT or at least
> 9ed4a94d6451046a51ef393cd62f00710820a7e8 so that we do not get bit by
> this warning when we go to enable this flag?
>
> I do not know of any other CI aside from ours that is testing with tip
> of tree clang and ours should already have a clang that includes my
> patch since it comes from apt.llvm.org.
>
> >>> # 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
>
> Very clever and nifty trick! I have verified that it works for clang 13
> and 14 along with a theoretical clang 15. Gustavo, feel free to stick a


I am not the inventor of this code, though :-)

I mimicked the code in Buildroot:
https://github.com/buildroot/buildroot/blob/2021.05/Makefile#L104





-- 
Best Regards
Masahiro Yamada

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