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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Thu, 31 May 2018 10:42:24 +0900
Message-ID: <CAK7LNATdw+nTQ0O7jBUDTKHaL53ejZ=oTPocFOJ1brtDr-GB1Q@mail.gmail.com>
Subject: Re: [PATCH] [net-next, wrong] make BPFILTER_UMH depend on X86
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        "David S. Miller" <davem@davemloft.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        netdev@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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2018-05-31 0:17 GMT+09:00 Alexei Starovoitov <alexei.starovoitov@gmail.com>:
> On Mon, May 28, 2018 at 05:31:01PM +0200, Arnd Bergmann wrote:
>> When build testing across architectures, I run into a build error on
>> all targets other than X86:
>>
>> gcc-8.1.0-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-objdump: net/bpfilter/bpfilter_umh: File format not recognized
>> gcc-8.1.0-nolibc/arm-linux-gnueabi/bin/arm-linux-gnueabi-objcopy:net/bpfilter/bpfilter_umh.o: Invalid bfd target
>>
>> The problem is that 'hostprogs' get built with 'gcc' rather than
>> '$(CROSS_COMPILE)gcc', and my default gcc (as most people's) targets x86.
>>
>> To work around it, adding an X86 dependency gets randconfigs building
>> again on my box.
>>
>> Clearly, this is not a good solution, since it should actually work fine
>> when building native kernels on other architectures but that is now
>> disabled, while cross building an x86 kernel on another host is still
>> broken after my patch.
>>
>> What we probably want here is to try out if the compiler is able to build
>> executables for the target architecture and not build the helper otherwise,
>> at least when compile-testing. No idea how to do that though.
>>
>> Link: http://www.kernel.org/pub/tools/crosstool/
>> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
>> Cc: linux-kbuild@vger.kernel.org
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>> ---
>>  net/bpfilter/Kconfig | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/net/bpfilter/Kconfig b/net/bpfilter/Kconfig
>> index 60725c5f79db..61cc4fcbb4d0 100644
>> --- a/net/bpfilter/Kconfig
>> +++ b/net/bpfilter/Kconfig
>> @@ -9,6 +9,7 @@ menuconfig BPFILTER
>>  if BPFILTER
>>  config BPFILTER_UMH
>>       tristate "bpfilter kernel module with user mode helper"
>> +     depends on X86 # actually depends on native builds
>
> depends on X86 will break it on arm.
> I think the better short term fix would be to test that HOSTCC == CC
> It doesn't have to be the same compiler. HOSTCC's arch == kernel ARCH
> Not sure how to hack makefile to do that.
> Long term we need to get rid of HOSTCC dependency.
> --
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Hmm.
For cross-compiling, we set 'ARCH' via the environment variable or the
command line.

ARCH is not explicitly set, the top-level Makefile sets it to $(SUBARCH)


ARCH ?= $(SUBARCH)


Maybe, we can assume the native build if $(ARCH) and $(SUBARCH) are the same?


-- 
Best Regards
Masahiro Yamada