[PATCH] x86/boot/compressed: Move CLANG_FLAGS to beginning of KBUILD_CFLAGS

[PATCH] x86/boot/compressed: Move CLANG_FLAGS to beginning of KBUILD_CFLAGS

[Nathan Chancellor]

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

Re: [PATCH] x86/boot/compressed: Move CLANG_FLAGS to beginning of KBUILD_CFLAGS

[Borislav Petkov]

On Tue, Dec 21, 2021 at 05:52:45PM -0700, Nathan Chancellor wrote:
> 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)

I don't mind but this makes the placement of that variable magical so it
needs a comment above it that it cannot be moved around freely and why
that is the case.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette