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    Subject[PATCH 4.14 246/293] kbuild: Add -Werror=unknown-warning-option to CLANG_FLAGS
    Date
    [ Upstream commit 589834b3a0097a4908f4112eac0ca2feb486fa32 ]

    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@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>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    Makefile | 1 +
    1 file changed, 1 insertion(+)

    diff --git a/Makefile b/Makefile
    index 97c744513af0..bbc9b6acb177 100644
    --- a/Makefile
    +++ b/Makefile
    @@ -488,6 +488,7 @@ ifneq ($(GCC_TOOLCHAIN),)
    CLANG_FLAGS += --gcc-toolchain=$(GCC_TOOLCHAIN)
    endif
    CLANG_FLAGS += -no-integrated-as
    +CLANG_FLAGS += -Werror=unknown-warning-option
    KBUILD_CFLAGS += $(CLANG_FLAGS)
    KBUILD_AFLAGS += $(CLANG_FLAGS)
    export CLANG_FLAGS
    --
    2.20.1


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