linux-kernel - Re: [PATCH v2] kbuild: fix linker feature test macros when cross compiling with Clang

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Date:   Sun, 29 Oct 2017 00:00:36 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Pirama Arumuga Nainar <pirama@...gle.com>,
        Douglas Anderson <dianders@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Marcin Nowakowski <marcin.nowakowski@...tec.com>,
        Mark Charlebois <charlebm@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kbuild: fix linker feature test macros when cross
 compiling with Clang

2017-10-28 5:13 GMT+09:00 Nick Desaulniers <ndesaulniers@...gle.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.
>
> Suggested-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reviewed-by: Matthias Kaehlcke <mka@...omium.org>
> ---
> Changes since v1:
> * base patch off of
>   git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
>   kbuild branch, per Masahiro.
> * Use $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) rather than $(CLANG_TRIPLE), per
>   Masahiro.
>
>  scripts/Kbuild.include | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
> index 064f477dfdca..0f09e4508554 100644
> --- a/scripts/Kbuild.include
> +++ b/scripts/Kbuild.include
> @@ -228,12 +228,13 @@ 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-cached,\
> -       $(CC) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
> +       $(CC) $(1) $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
>
>  # ld-option
>  # Usage: LDFLAGS += $(call ld-option, -X)
>  ld-option = $(call try-run-cached,\
> -       $(CC) -x c /dev/null -c -o "$$TMPO" ; $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
> +       $(CC) $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) -x c /dev/null -c -o "$$TMPO"; \
> +       $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2))
>
>  # ar-option
>  # Usage: KBUILD_ARFLAGS := $(call ar-option,D)
> --
> 2.15.0.rc2.357.g7e34df9404-goog
>
> --
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Applied to linux-kbuild/kbuild.  Thanks!


-- 
Best Regards
Masahiro Yamada