linux-kernel - Re: [PATCH v3] kbuild: Check for unknown options with cc-option usage in Kconfig and clang

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Date:   Wed, 31 Jul 2019 01:51:50 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Stephen Boyd <swboyd@...omium.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Peter Smith <peter.smith@...aro.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH v3] kbuild: Check for unknown options with cc-option usage
 in Kconfig and clang

On Wed, Jul 31, 2019 at 1:50 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Tue, Jul 30, 2019 at 09:48:03AM -0700, Stephen Boyd wrote:
> > If the particular version of clang a user has doesn't enable
> > -Werror=unknown-warning-option by default, even though it is the
> > default[1], then make sure to pass the option to the Kconfig cc-option
> > command so that testing options from Kconfig files works properly.
> > Otherwise, depending on the default values setup in the clang toolchain
> > we will silently assume options such as -Wmaybe-uninitialized are
> > supported by clang, when they really aren't.
> >
> > A compilation issue only started happening for me once commit
> > 589834b3a009 ("kbuild: Add -Werror=unknown-warning-option to
> > CLANG_FLAGS") was applied on top of commit b303c6df80c9 ("kbuild:
> > compute false-positive -Wmaybe-uninitialized cases in Kconfig"). This
> > leads kbuild to try and test for the existence of the
> > -Wmaybe-uninitialized flag with the cc-option command in
> > scripts/Kconfig.include, and it doesn't see an error returned from the
> > option test so it sets the config value to Y. Then the Makefile tries to
> > pass the unknown option on the command line and
> > -Werror=unknown-warning-option catches the invalid option and breaks the
> > build. Before commit 589834b3a009 ("kbuild: Add
> > -Werror=unknown-warning-option to CLANG_FLAGS") the build works fine,
> > but any cc-option test of a warning option in Kconfig files silently
> > evaluates to true, even if the warning option flag isn't supported on
> > clang.
> >
> > Note: This doesn't change cc-option usages in Makefiles because those
> > use a different rule that includes KBUILD_CFLAGS by default (see the
> > __cc-option command in scripts/Kbuild.incluide). The KBUILD_CFLAGS
> > variable already has the -Werror=unknown-warning-option flag set. Thanks
> > to Doug for pointing out the different rule.
> >
> > [1] https://clang.llvm.org/docs/DiagnosticsReference.html#wunknown-warning-option
> > Cc: Peter Smith <peter.smith@...aro.org>
> > Cc: Nathan Chancellor <natechancellor@...il.com>
> > Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> > Cc: Douglas Anderson <dianders@...omium.org>
> > Signed-off-by: Stephen Boyd <swboyd@...omium.org>
>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
>
> > ---
> >  Makefile                | 1 +
> >  scripts/Kconfig.include | 2 +-
> >  2 files changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 9be5834073f8..517d0a3f6539 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -536,6 +536,7 @@ KBUILD_AFLAGS     += $(CLANG_FLAGS)
> >  export CLANG_FLAGS
> >  endif
> >
> > +
>
> Not sure it's worth sending a v4 for but I don't think this should be
> there.


I will remove it when I apply this.



-- 
Best Regards
Masahiro Yamada