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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 8 Jun 2018 10:57:07 +0200
Message-ID: <CAMuHMdXy0uHXaGkFBh9_tr2VzmJF+8KtraTuFHJwcmqhcHs4Dg@mail.gmail.com>
Subject: Re: [PATCH] [net-next, wrong] make BPFILTER_UMH depend on X86
To:     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>
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Hi Arnd, Alexei,

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...

>         default m
>         help
>           This builds bpfilter kernel module with embedded user mode helper

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds