linux-kernel - Re: resolve_btfids breaks kernel cross-compilation

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Date:   Thu, 17 Sep 2020 11:14:06 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Seth Forshee <seth.forshee@...onical.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: resolve_btfids breaks kernel cross-compilation

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/