linux-kernel - Re: [PATCH] kbuild: Add -Werror=unknown-warning-option to CLANG

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Message-ID: <CAK7LNARM510X5JL9T6J7Mc3ZG_TnGxpxaLndR044f6nFFgY3VA@mail.gmail.com>
Date:   Thu, 20 Jun 2019 23:23:52 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Peter Smith <peter.smith@...aro.org>
Subject: Re: [PATCH] kbuild: Add -Werror=unknown-warning-option to CLANG_FLAGS

On Wed, Jun 12, 2019 at 3:43 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> In commit ebcc5928c5d9 ("arm64: Silence gcc warnings about arch ABI
> drift"), the arm64 Makefile added -Wno-psabi to KBUILD_CFLAGS, which is
> a GCC only option so clang rightfully complains:
>
> warning: unknown warning option '-Wno-psabi' [-Wunknown-warning-option]
>
> https://clang.llvm.org/docs/DiagnosticsReference.html#wunknown-warning-option
>
> However, by default, this is merely a warning so the build happily goes
> on with a slew of these warnings in the process.
>
> Commit c3f0d0bc5b01 ("kbuild, LLVMLinux: Add -Werror to cc-option to
> support clang") worked around this behavior in cc-option by adding
> -Werror so that unknown flags cause an error. However, this all happens
> silently and when an unknown flag is added to the build unconditionally
> like -Wno-psabi, cc-option will always fail because there is always an
> unknown flag in the list of flags. This manifested as link time failures
> in the arm64 libstub because -fno-stack-protector didn't get added to
> KBUILD_CFLAGS.
>
> To avoid these weird cryptic failures in the future, make clang behave
> like gcc and immediately error when it encounters an unknown flag by
> adding -Werror=unknown-warning-option to CLANG_FLAGS. This can be added
> unconditionally for clang because it is supported by at least 3.0.0,
> according to godbolt [1] and 4.0.0, according to its documentation [2],
> which is far earlier than we typically support.
>
> [1]: https://godbolt.org/z/7F7rm3
> [2]: https://releases.llvm.org/4.0.0/tools/clang/docs/DiagnosticsReference.html#wunknown-warning-option
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/511
> Link: https://github.com/ClangBuiltLinux/linux/issues/517
> Suggested-by: Peter Smith <peter.smith@...aro.org>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---

Applied to linux-kbuild.
Thanks!

-- 
Best Regards
Masahiro Yamada