Subject: Re: [PATCH bpf-next v2 0/5] samples: bpf: fix build after move to full libbpf

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 15 May 2018 08:10:09 +0200

On Mon, 14 May 2018 22:57:53 -0700
Alexei Starovoitov <[email-masked]> wrote:

> On Mon, May 14, 2018 at 10:35:01PM -0700, Jakub Kicinski wrote:
> > Hi!
> > 
> > Following patches address build issues after recent move to libbpf.
> > For out-of-tree builds we would see the following error:
> > 
> > gcc: error: samples/bpf/../../tools/lib/bpf/libbpf.a: No such file or directory
> > 
> > libbpf build system is now always invoked explicitly rather than
> > relying on building single objects most of the time.  We need to
> > resolve the friction between Kbuild and tools/ build system.
> > 
> > Mini-library called libbpf.h in samples is renamed to bpf_insn.h,
> > using linux/filter.h seems not completely trivial since some samples
> > get upset when order on include search path in changed.  We do have
> > to rename libbpf.h, however, because otherwise it's hard to reliably
> > get to libbpf's header in out-of-tree builds.
> > 
> > v2:
> >  - fix the build error harder (patch 3);
> >  - add patch 5 (make clang less noisy).  
> 
> Applied, Thanks

I just tried it out, but the build fails.

$ make samples/bpf/
  CHK     include/config/kernel.release
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
  CHK     include/generated/bounds.h
  CHK     include/generated/timeconst.h
  CHK     include/generated/asm-offsets.h
  CALL    scripts/checksyscalls.sh
  DESCEND  objtool
  CHK     scripts/mod/devicetable-offsets.h
make -C /home/jbrouer/git/kernel/bpf-next/samples/bpf/../../tools/lib/bpf/ RM='rm -rf' LDFLAGS= srctree=/home/jbrouer/git/kernel/bpf-next/samples/bpf/../../ O=

Auto-detecting system features:
...                        libelf: [ OFF ]
...                           bpf: [ OFF ]

No libelf found
make[2]: *** [Makefile:212: elfdep] Error 1
make[1]: *** [samples/bpf/Makefile:207: /home/jbrouer/git/kernel/bpf-next/samples/bpf/../../tools/lib/bpf/libbpf.a] Error 2
make: *** [Makefile:1743: samples/bpf/] Error 2

But it might not be related to this change, as the problem seems to the
with the "Auto-detecting system features".  It is the build of libbpf
that is the problem.

$ cd tools/lib/bpf/
$ make

Auto-detecting system features:
...                        libelf: [ OFF ]
...                           bpf: [ OFF ]

No libelf found
make: *** [Makefile:212: elfdep] Error 1

And do have 'libelf' installed on this system... the same build/make
commands works on net-next.

On net-next I see:

$ cd tools/lib/bpf/
$ make

Auto-detecting system features:
...                        libelf: [ on  ]
...                           bpf: [ on  ]

Warning: Kernel ABI header at 'tools/include/uapi/linux/bpf.h' differs from latest version at 'include/uapi/linux/bpf.h'
Warning: Kernel ABI header at 'tools/include/uapi/linux/if_link.h' differs from latest version at 'include/uapi/linux/if_link.h'
  CC       libbpf.o
  CC       bpf.o
  CC       nlattr.o
  CC       btf.o
  LD       libbpf-in.o
  LINK     libbpf.a
  LINK     libbpf.so

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer