Re: resolve_btfids breaks kernel cross-compilation

Jiri Olsa
2020-09-17T09:14:16Z

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

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

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/