Re: resolve_btfids breaks kernel cross-compilation

Seth Forshee
2020-09-17T13:14:14Z

On Thu, Sep 17, 2020 at 11:14:06AM +0200, Jiri Olsa wrote:
> On Thu, Sep 17, 2020 at 10:38:12AM +0200, Jiri Olsa wrote:
> > On Thu, Sep 17, 2020 at 10:04:55AM +0200, Jiri Olsa wrote:
> > > On Wed, Sep 16, 2020 at 02:47:33PM -0500, Seth Forshee wrote:
> > > > The requirement to build resolve_btfids whenever CONFIG_DEBUG_INFO_BTF
> > > > is enabled breaks some cross builds. For example, when building a 64-bit
> > > > powerpc kernel on amd64 I get:
> > > >
> > > > Auto-detecting system features:
> > > > ... libelf: [ [32mon[m ]
> > > > ... zlib: [ [32mon[m ]
> > > > ... bpf: [ [31mOFF[m ]
> > > >
> > > > BPF API too old
> > > > make[6]: *** [Makefile:295: bpfdep] Error 1
> > > >
> > > > The contents of tools/bpf/resolve_btfids/feature/test-bpf.make.output:
> > > >
> > > > In file included from /home/sforshee/src/u-k/unstable/tools/arch/powerpc/include/uapi/asm/bitsperlong.h:11,
> > > > from /usr/include/asm-generic/int-ll64.h:12,
> > > > from /usr/include/asm-generic/types.h:7,
> > > > from /usr/include/x86_64-linux-gnu/asm/types.h:1,
> > > > from /home/sforshee/src/u-k/unstable/tools/include/linux/types.h:10,
> > > > from /home/sforshee/src/u-k/unstable/tools/include/uapi/linux/bpf.h:11,
> > > > from test-bpf.c:3:
> > > > /home/sforshee/src/u-k/unstable/tools/include/asm-generic/bitsperlong.h:14:2: error: #error Inconsistent word size. Check asm/bitsperlong.h
> > > > 14 | #error Inconsistent word size. Check asm/bitsperlong.h
> > > > | ^~~~~
> > > >
> > > > This is because tools/arch/powerpc/include/uapi/asm/bitsperlong.h sets
> > > > __BITS_PER_LONG based on the predefinied compiler macro __powerpc64__,
> > > > which is not defined by the host compiler. What can we do to get cross
> > > > builds working again?
> > >
> > > could you please share the command line and setup?
> >
> > I just reproduced.. checking on fix
>
> I still need to check on few things, but patch below should help

It does help with the word size problem, thanks.

> we might have a problem for cross builds with different endianity
> than the host because libbpf does not support reading BTF data
> with different endianity, and we get:
>
> BTFIDS vmlinux
> libbpf: non-native ELF endianness is not supported

Yes, I see this now when cross building for s390.

Thanks,
Seth

>
> jirka
>
>
> ---
> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index a88cd4426398..d3c818b8d8d3 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -1,5 +1,6 @@
> # SPDX-License-Identifier: GPL-2.0-only
> include ../../scripts/Makefile.include
> +include ../../scripts/Makefile.arch
>
> ifeq ($(srctree),)
> srctree := $(patsubst %/,%,$(dir $(CURDIR)))
> @@ -29,6 +30,7 @@ endif
> AR = $(HOSTAR)
> CC = $(HOSTCC)
> LD = $(HOSTLD)
> +ARCH = $(HOSTARCH)
>
> OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
>
>