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From: Masahiro Yamada <email@example.com>
To: Nathan Chancellor <firstname.lastname@example.org>
Cc: Kees Cook <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 06:17:24 +0900 [thread overview]
Message-ID: <CAK7LNAQFgYgavTP2ZG9Y16XBVdPuJ98J_Ty1OrQy1GXHq6JjQQ@mail.gmail.com> (raw)
On Wed, Aug 18, 2021 at 3:25 AM Nathan Chancellor <email@example.com> 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.
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 $(shell test ...) is less expensive than cc-option.
If you want to make it even faster, you can use only
built-in functions, like this:
# Warn about unmarked fall-throughs in switch statement.
# Clang prior to 14.0.0 warned on unreachable fallthroughs with
# -Wimplicit-fallthrough, which is unacceptable due to IS_ENABLED().
ifeq ($(firstword $(sort $(CONFIG_CLANG_VERSION) 140000)),140000)
KBUILD_CFLAGS += -Wimplicit-fallthrough
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.
> > 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
next prev parent reply other threads:[~2021-08-17 21:18 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 [this message]
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
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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