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

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Date:   Tue, 30 Jul 2019 09:49:59 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        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 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.