Trying the bpf trace a bpf xdp program

["Eelco Chaudron" <echaudro@xxxxxxxxxx>]

Trying out the BPF trace to trace a BPF program, but I’m already 
getting stuck loading the object with the fexit  :(

I’ve loaded a simple pass program on my XDP interface:

```
bpftool prog list
137: xdp  name xdp_prog_simple  tag db89f03479f5c101  gpl
        loaded_at 2019-11-28T16:17:31+0000  uid 0
        xlated 208B  jited 143B  memlock 4096B
        btf_id 108
```

The trace I would like to attach is the following:

```
#define bpf_debug(fmt, ...)                         \
{                                                   \
  char __fmt[] = fmt;                               \
  bpf_trace_printk(__fmt, sizeof(__fmt),            \
                   ##__VA_ARGS__);                  \
}

BPF_TRACE_2("fexit/xdp_prog_simple", trace_on_exit,
             struct xdp_md *, ctx, int, ret)
{
  bpf_debug("Debug: %d [ifindex = %u, queue =  %u, ret = %d]\n",
            ctx->ingress_ifindex, ctx->rx_queue_index, ret);

  return 0;
}

char _license[] SEC("license") = "GPL";
```

Now I have a simple program trying to open the object file (modeled 
after fexit_bpf2bpf.c):

```
  prog_fe = bpf_prog_get_fd_by_id(137);  //HERE YOU GET THE ID FROM 
bpftool list
  if (prog_fd < 0) {
    printf("ERROR: Cant cat program fd, make sure hardcoded ID is ok, 
and ran as root!\n");
    return 0;
  }

  DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts, .attach_prog_fd = 
prog_fd, );

  obj = bpf_object__open_file("./xdp_sample_fentry_fexit_kern.o", 
&opts);
  if (IS_ERR_OR_NULL(obj)) {
    perror("ERROR: Failed to open trace object file");
    return 0;
  }

  err = bpf_object__load(obj);
  if (err < 0) {
    perror("ERROR: Failed to load object file");
    return 0;
  }
```

The load fails:

```
libbpf: load bpf program failed: Argument list too long
libbpf: failed to load program 'fexit/xdp_prog_simple'
libbpf: failed to load object './xdp_sample_fentry_fexit_kern.o'
ERROR: Failed to load object file: Operation not permitted
```

I tried using more or fewer arguments to the 
“fexit/xdp_prog_simple”, but the same problem.
If I change the name to something like “fexit/foo” it complains it 
can not find foo in the program I try to attach too.

Any idea? this is with the latests bpd-next kernel…

Cheers,

Eelco