linux-kernel - [PATCH AUTOSEL 4.9 21/45] kbuild: Add -Werror=unknown-warning-option to CLANG

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Message-Id: <20190719041304.18849-21-sashal@kernel.org>
Date:   Fri, 19 Jul 2019 00:12:40 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Peter Smith <peter.smith@...aro.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Sasha Levin <sashal@...nel.org>, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: [PATCH AUTOSEL 4.9 21/45] kbuild: Add -Werror=unknown-warning-option to CLANG_FLAGS

From: Nathan Chancellor <natechancellor@...il.com>

[ 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@...aro.org>
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Makefile b/Makefile
index c80dad45334e..60b562dbd97f 100644
--- a/Makefile
+++ b/Makefile
@@ -515,6 +515,7 @@ 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)
 endif
-- 
2.20.1