linux-kernel - Re: [PATCH v2] kbuild: Check for unknown options with cc-option usage in Kconfig and clang

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Message-ID: <CAD=FV=UZQEqNyJ_3Corx0iro-OB5E==d8qZCxgrMvvKon4yAxw@mail.gmail.com>
Date:   Thu, 25 Jul 2019 09:41:17 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux@...glegroups.com,
        Peter Smith <peter.smith@...aro.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] kbuild: Check for unknown options with cc-option usage
 in Kconfig and clang

Hi,

On Thu, Jul 25, 2019 at 8:47 AM Stephen Boyd <swboyd@...omium.org> 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>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Stephen Boyd <swboyd@...omium.org>

Fixes: 589834b3a009 ("kbuild: Add -Werror=unknown-warning-option to
CLANG_FLAGS")
Reviewed-by: Douglas Anderson <dianders@...omium.org>