Re: [PATCH] kbuild: Add -Werror=unknown-warning-option to CLANG_FLAGS

[Masahiro Yamada]

On Wed, Jun 12, 2019 at 3:43 AM Nathan Chancellor
<natechancellor@xxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> ---

Applied to linux-kbuild.
Thanks!

-- 
Best Regards
Masahiro Yamada