From:   Nathan Chancellor <nathan@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org
Cc:     "H. Peter Anvin" <hpa@zytor.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        Nathan Chancellor <nathan@kernel.org>
Subject: [PATCH] x86/boot/compressed: Move CLANG_FLAGS to beginning of KBUILD_CFLAGS
Date:   Tue, 21 Dec 2021 17:52:45 -0700
Message-Id: <20211222005245.3081136-1-nathan@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

When cross compiling i386_defconfig on an arm64 host with clang, there
are a few instances of '-Waddress-of-packed-member' and
'-Wgnu-variable-sized-type-not-at-end' in arch/x86/boot/compressed/,
which should both be disabled with the cc-disable-warning calls in that
directory's Makefile, which indicates that cc-disable-warning is failing
at the point of testing these flags.

The cc-disable-warning calls fail because at the point that the flags
are tested, KBUILD_CFLAGS has '-march=i386' without $(CLANG_FLAGS),
which has the '--target=' flag to tell clang what architecture it is
targeting. Without the '--target=' flag, the host architecture (arm64)
is used and i386 is not a valid value for '-march=' in that case. This
error can be seen by adding some logging to try-run:

clang-14: error: the clang compiler does not support '-march=i386'

Invoking the compiler has to succeed prior to calling cc-option or
cc-disable-warning in order to accurately test whether or not the flag
is supported; if it doesn't, the requested flag can never be added to
the compiler flags. Move $(CLANG_FLAGS) to the beginning of KBUILD_FLAGS
so that any new flags that might be added in the future can be
accurately tested.

Fixes: d5cbd80e302d ("x86/boot: Add $(CLANG_FLAGS) to compressed KBUILD_CFLAGS")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 arch/x86/boot/compressed/Makefile | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 431bf7f846c3..169186dba1d7 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -28,7 +28,7 @@ KCOV_INSTRUMENT		:= n
 targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
 	vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4 vmlinux.bin.zst
 
-KBUILD_CFLAGS := -m$(BITS) -O2
+KBUILD_CFLAGS := -m$(BITS) -O2 $(CLANG_FLAGS)
 KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
 KBUILD_CFLAGS += -Wundef
 KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
@@ -47,7 +47,6 @@ KBUILD_CFLAGS += -D__DISABLE_EXPORTS
 # Disable relocation relaxation in case the link is not PIE.
 KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
 KBUILD_CFLAGS += -include $(srctree)/include/linux/hidden.h
-KBUILD_CFLAGS += $(CLANG_FLAGS)
 
 # sev.c indirectly inludes inat-table.h which is generated during
 # compilation and stored in $(objtree). Add the directory to the includes so

base-commit: 2f47a9a4dfa3674fad19a49b40c5103a9a8e1589
-- 
2.34.1