linux-kernel - Re: [PATCH] [net-next, wrong] make BPFILTER

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Date:   Wed, 30 May 2018 11:17:38 -0400
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kbuild@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [net-next, wrong] make BPFILTER_UMH depend on X86

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@...ionext.com>
> Cc: linux-kbuild@...r.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@...db.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.