netdev - Re: [PATCH] bpfilter: fix user mode helper cross compilation

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Date:   Thu, 28 Jun 2018 09:23:30 +0000
From:   Matteo Croce <mcroce@...hat.com>
To:     akpm@...ux-foundation.org
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] bpfilter: fix user mode helper cross compilation

On Thu, Jun 28, 2018 at 6:17 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 20 Jun 2018 16:04:34 +0200 Matteo Croce <mcroce@...hat.com> wrote:
>
> > Use $(OBJDUMP) instead of literal 'objdump' to avoid
> > using host toolchain when cross compiling.
> >
>
> I'm still having issues here, with ld.
>
> x86_64 machine, ARCH=i386:
>
> y:/usr/src/25> make V=1 M=net/bpfilter
> test -e include/generated/autoconf.h -a -e include/config/auto.conf || (       \
> echo >&2;                                                       \
> echo >&2 "  ERROR: Kernel configuration is invalid.";           \
> echo >&2 "         include/generated/autoconf.h or include/config/auto.conf are missing.";\
> echo >&2 "         Run 'make oldconfig && make prepare' on kernel src to fix it.";      \
> echo >&2 ;                                                      \
> /bin/false)
> mkdir -p net/bpfilter/.tmp_versions ; rm -f net/bpfilter/.tmp_versions/*
> make -f ./scripts/Makefile.build obj=net/bpfilter
> (cat /dev/null;   echo kernel/net/bpfilter/bpfilter.ko;) > net/bpfilter/modules.order
>   ld -m elf_i386   -r -o net/bpfilter/bpfilter.o net/bpfilter/bpfilter_kern.o net/bpfilter/bpfilter_umh.o ; scripts/mod/modpost net/bpfilter/bpfilter.o
> ld: i386:x86-64 architecture of input file `net/bpfilter/bpfilter_umh.o' is incompatible with i386 output
> scripts/Makefile.build:530: recipe for target 'net/bpfilter/bpfilter.o' failed
> make[1]: *** [net/bpfilter/bpfilter.o] Error 1
> Makefile:1518: recipe for target '_module_net/bpfilter' failed
> make: *** [_module_net/bpfilter] Error 2
>
> y:/usr/src/25> ld --version
> GNU ld (GNU Binutils for Ubuntu) 2.29.1
>
>

Hi Andrew,

That's because the Makefile does `HOSTCC:=$(CC)` which replaces the
tools compiler with the target one.
The problem is that for i386 and x86_64 the compiler is the same, it's
just called with different arguments, -m32 and -m64.
This ends up with mixed i386 and x86_64 binaries which obviously can't
link together.

Personally I think that we should add infrastructure to build target
progs like we do with hostprogs-y instead of keeping messing with
variables and flags. If you want a quick and dirty hack to build it,
I'm using this.

Regards,

diff --git a/net/bpfilter/Makefile b/net/bpfilter/Makefile
index 051dc18b8ccb..5de353cfd26b 100644
--- a/net/bpfilter/Makefile
+++ b/net/bpfilter/Makefile
@@ -5,8 +5,9 @@

 hostprogs-y := bpfilter_umh
 bpfilter_umh-objs := main.o
-HOSTCFLAGS += -I. -Itools/include/ -Itools/include/uapi
 HOSTCC := $(CC)
+HOSTCFLAGS := $(KBUILD_CFLAGS) -I. -Itools/include/ -Itools/include/uapi
+HOSTLOADLIBES_bpfilter_umh := $(KBUILD_CFLAGS)

 ifeq ($(CONFIG_BPFILTER_UMH), y)
 # builtin bpfilter_umh should be compiled with -static

--
Matteo Croce
per aspera ad upstream