libbpf: Fix build on read-only filesystems

I got the following error when I tried to build perf on a read-only
filesystem with O=dir option.

  $ cd /some/where/ro/linux/tools/perf
  $ make O=$HOME/build/perf
    CC       /home/namhyung/build/perf/lib.o
  /bin/sh: bpf_helper_defs.h: Read-only file system
  make[3]: *** [Makefile:184: bpf_helper_defs.h] Error 1
  make[2]: *** [Makefile.perf:778: /home/namhyung/build/perf/libbpf.a] Error 2
  make[2]: *** Waiting for unfinished jobs....
    LD       /home/namhyung/build/perf/libperf-in.o
    AR       /home/namhyung/build/perf/libperf.a
    PERF_VERSION = 5.4.0
  make[1]: *** [Makefile.perf:225: sub-make] Error 2
  make: *** [Makefile:70: all] Error 2

It was becaused bpf_helper_defs.h was generated in current directory.
Move it to OUTPUT directory.

Signed-off-by: Namhyung Kim <>
Signed-off-by: Daniel Borkmann <>
Tested-by: Andrii Nakryiko <>
Acked-by: Andrii Nakryiko <>
3 files changed