resolve_btfids breaks kernel cross-compilation

Seth Forshee
2020-09-16T19:50:53Z

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?

Thanks,
Seth