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From: Masahiro Yamada <email@example.com>
To: Nathan Chancellor <firstname.lastname@example.org>
Cc: Kees Cook <email@example.com>,
"Gustavo A. R. Silva" <firstname.lastname@example.org>,
Philip Li <email@example.com>,
Linus Torvalds <firstname.lastname@example.org>,
"Gustavo A. R. Silva" <email@example.com>,
Nick Desaulniers <firstname.lastname@example.org>,
Linux Kbuild mailing list <email@example.com>,
Linux Kernel Mailing List <firstname.lastname@example.org>,
Subject: Re: [PATCH] kbuild: Enable -Wimplicit-fallthrough for clang 14.0.0+
Date: Wed, 18 Aug 2021 13:15:36 +0900 [thread overview]
Message-ID: <CAK7LNAQW6LMnfU6reTNzDLneX+mBFSKHgbF5epQ+6GQZr7vWLQ@mail.gmail.com> (raw)
On Wed, Aug 18, 2021 at 8:23 AM Nathan Chancellor <email@example.com> 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 <firstname.lastname@example.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:
next prev parent reply other threads:[~2021-08-18 4:16 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-17 0:56 Nathan Chancellor
2021-08-17 4:16 ` kernel test robot
2021-08-17 4:20 ` Nathan Chancellor
2021-08-17 4:37 ` Linus Torvalds
2021-08-17 4:55 ` Nathan Chancellor
2021-08-17 18:03 ` Kees Cook
2021-08-17 18:25 ` Nathan Chancellor
2021-08-17 21:17 ` Masahiro Yamada
2021-08-17 21:33 ` Gustavo A. R. Silva
2021-08-17 23:06 ` Kees Cook
2021-08-17 23:23 ` Nathan Chancellor
2021-08-17 23:40 ` Gustavo A. R. Silva
2021-08-18 4:15 ` Masahiro Yamada [this message]
2021-08-18 4:27 ` Philip Li
2021-08-18 4:45 ` Gustavo A. R. Silva
2021-08-18 7:31 ` Philip Li
2021-08-18 12:12 ` Mark Brown
2021-08-25 21:09 ` Nick Desaulniers
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