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From: Masahiro Yamada <yamada.masahiro@socionext.com>
Date: Fri, 27 Oct 2017 20:20:15 +0900
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Subject: Re: [PATCH] kbuild: fix linker feature test macros when cross
 compiling with Clang
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Michael Davidson <md@google.com>, Greg Hackmann <ghackmann@google.com>,
        pirama@google.com, Ingo Molnar <mingo@kernel.org>,
        Matthias Kaehlcke <mka@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
        Mark Charlebois <charlebm@gmail.com>,
        Cao jin <caoj.fnst@cn.fujitsu.com>,
        Marcin Nowakowski <marcin.nowakowski@imgtec.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Nick

2017-10-27 5:17 GMT+09:00 Nick Desaulniers <ndesaulniers@google.com>:
> I was not seeing my linker flags getting added when using ld-option when
> cross compiling with Clang. Upon investigation, this seems to be due to
> a difference in how GCC vs Clang handle cross compilation.
>
> GCC is configured at build time to support one backend, that is implicit
> when compiling.  Clang is explicit via the use of `-target <triple>` and
> ships with all supported backends by default.
>
> GNU Make feature test macros that compile then link will always fail
> when cross compiling with Clang unless Clang's triple is passed along to
> the compiler. For example:
>
> $ clang -x c /dev/null -c -o temp.o
> $ aarch64-linux-android/bin/ld -E temp.o
> aarch64-linux-android/bin/ld:
> unknown architecture of input file `temp.o' is incompatible with
> aarch64 output
> aarch64-linux-android/bin/ld:
> warning: cannot find entry symbol _start; defaulting to
> 0000000000400078
> $ echo $?
> 1
>
> $ clang -target aarch64-linux-android- -x c /dev/null -c -o temp.o
> $ aarch64-linux-android/bin/ld -E temp.o
> aarch64-linux-android/bin/ld:
> warning: cannot find entry symbol _start; defaulting to 00000000004002e4
> $ echo $?
> 0
>
> This causes conditional checks that invoke $(CC) without the target
> triple, then $(LD) on the result, to always fail.
>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
> ---
>  scripts/Kbuild.include | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
> index 9ffd3dda3889..23c4df90e8ff 100644
> --- a/scripts/Kbuild.include
> +++ b/scripts/Kbuild.include
> @@ -160,12 +160,12 @@ cc-if-fullversion = $(shell [ $(cc-fullversion) $(1) $(2) ] && echo $(3) || echo
>  # cc-ldoption
>  # Usage: ldflags += $(call cc-ldoption, -Wl$(comma)--hash-style=both)
>  cc-ldoption = $(call try-run,\
> -       $(CC) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
> +       $(CC) $(CLANG_TARGET) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
>
>  # ld-option
>  # Usage: LDFLAGS += $(call ld-option, -X)
>  ld-option = $(call try-run,\
> -       $(CC) -x c /dev/null -c -o "$$TMPO" ; $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
> +       $(CC) $(CLANG_TARGET) -x c /dev/null -c -o "$$TMPO" ; $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
>
>  # ar-option
>  # Usage: KBUILD_ARFLAGS := $(call ar-option,D)
> --

I do not like to add $(CLANG_TARGET) to a place for common helpers.


Instead of $(CLANG_TARGET), please add

$(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS)

to cc-ldoption and ld-option.



I have two requests next time:
  - please include  linux-kbuild@vger.kernel.org in your To list
  - please base your patch on linux-kbuild/kbuild branch


The URL of the tree is described in MAINTAINERS.


Thanks.


-- 
Best Regards
Masahiro Yamada