linux-kernel - Re: [PATCH v3] kbuild: fix linker feature test macros when cross compiling with Clang

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Message-ID: <CAK7LNATKn+8g=6QWoK3HXb9dk35myNq=xt0vnrHamckhDAhtLA@mail.gmail.com>
Date:   Tue, 7 Nov 2017 12:41:00 +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>,
        Matthias Kaehlcke <mka@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Charlebois <charlebm@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] kbuild: fix linker feature test macros when cross
 compiling with Clang

2017-11-07 3:47 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 v2:
> * Add LDFLAGS to ld-option, as per Masahiro, and spotted by 0-day bot:
>   https://lists.01.org/pipermail/lkp/2017-October/007427.html
>
>  scripts/Kbuild.include | 5 +++--


Applied to linux-kbuild/kbuild.


-- 
Best Regards
Masahiro Yamada