In commit ebcc592 ("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 c3f0d0b ("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: ClangBuiltLinux#511 Link: ClangBuiltLinux#517 Suggested-by: Peter Smith <peter.smith@linaro.org> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> Tested-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Add -Werror=unknown-warning-option to CLANG_FLAGS #517

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