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Subject: Re: [PATCH] [net-next, wrong] make BPFILTER_UMH depend on X86
To:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Alexei Starovoitov <ast@kernel.org>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        linux-kbuild <linux-kbuild@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <20180528153222.2037158-1-arnd@arndb.de>
 <CAMuHMdXy0uHXaGkFBh9_tr2VzmJF+8KtraTuFHJwcmqhcHs4Dg@mail.gmail.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <8c0f8cc4-b919-9b69-4144-fd216253aa24@iogearbox.net>
Date:   Fri, 8 Jun 2018 11:33:15 +0200
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Hi Geert,

On 06/08/2018 10:57 AM, Geert Uytterhoeven wrote:
> On Mon, May 28, 2018 at 5:31 PM, Arnd Bergmann <arnd@arndb.de> 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.
> 
> So that was done in commit 819dd92b9c0bc7bc ("bpfilter: switch to CC
> from HOSTCC"), but it is not sufficient:
> 
>       GEN net/bpfilter/bpfilter_umh.o
>     Usage: m68k-linux-gnu-objcopy [option(s)] in-file [out-file]
>      Copies a binary file, possibly transforming it in the process
>      The options are:
>     [...]
> 
> net/bpfilter/Makefile:29: recipe for target 'net/bpfilter/bpfilter_umh.o' failed
> make[5]: *** [net/bpfilter/bpfilter_umh.o] Error 1
> 
>> --- 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
> 
> No, (currently) it does depend on X86, due to its use of:
> 
>     $(OBJCOPY) -I binary -O $(CONFIG_OUTPUT_FORMAT)
> 
> with CONFIG_OUTPUT_FORMAT being defined on X86 only...

That hard dependency should have been fixed with the following patch in -net tree:

https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/commit/?id=8d97ca6b6755bf7ef57d323642ca9ee80d689782

Thanks,
Daniel