fix a verifier bug in check_attach_btf_id()

Commit 5b92a28aae4d ("bpf: Support attaching tracing BPF program to other BPF programs") added support to attach tracing bpf program to other bpf programs. It had a bug when trying to get the address of the jited image if the main program does not have any callees, resulting in the following kernel segfault:

      ......
      [79162.619208] BUG: kernel NULL pointer dereference, address:
      0000000000000000
      ......
      [79162.634255] Call Trace:
      [79162.634974]  ? _cond_resched+0x15/0x30
      [79162.635686]  ? kmem_cache_alloc_trace+0x162/0x220
      [79162.636398]  ? selinux_bpf_prog_alloc+0x1f/0x60
      [79162.637111]  bpf_prog_load+0x3de/0x690
      [79162.637809]  __do_sys_bpf+0x105/0x1740
      [79162.638488]  do_syscall_64+0x5b/0x180
      [79162.639147]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Patch #1 fixed the problem with more explanation in the commit message. Patch #2 added a selftest which will fail without this patch.

Yonghong Song (2):
bpf: fix a bug to get subprog 0 jited image in check_attach_btf_id selftests/bpf: add a fexit/bpf2bpf test with target bpf prog no
callees

 kernel/bpf/verifier.c                         |  5 +-
 .../selftests/bpf/prog_tests/fexit_bpf2bpf.c  | 70 ++++++++++++++-----
 .../bpf/progs/fexit_bpf2bpf_simple.c          | 26 +++++++
 .../selftests/bpf/progs/test_pkt_md_access.c  |  4 +-
 4 files changed, 85 insertions(+), 20 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/fexit_bpf2bpf_simple.c

2.17.1