Re: [PATCH net-next 2/3] bpf: permit multiple bpf attachments for a single perf event

[Daniel Borkmann]

On 10/23/2017 07:58 PM, Yonghong Song wrote:
[...]

        __this_cpu_dec(bpf_prog_active);
@@ -741,3 +754,63 @@ const struct bpf_verifier_ops perf_event_verifier_ops = {

  const struct bpf_prog_ops perf_event_prog_ops = {
  };
+
+static DEFINE_MUTEX(bpf_event_mutex);
+
+int perf_event_attach_bpf_prog(struct perf_event *event,
+                          struct bpf_prog *prog)
+{
+       struct bpf_prog_array __rcu *old_array;
+       struct bpf_prog_array *new_array;
+       int ret;
+
+       mutex_lock(&bpf_event_mutex);
+
+       if (event->prog)
+               return -EEXIST;

Needs to go to out here, otherwise deadlock.

+
+       old_array = rcu_dereference_protected(event->tp_event->prog_array,
+                                             
lockdep_is_held(&bpf_event_mutex));
+       ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
+       if (ret < 0)
+               goto out;
+
+       /* set the new array to event->tp_event and set event->prog */
+       event->prog = prog;
+       rcu_assign_pointer(event->tp_event->prog_array, new_array);
+
+       if (old_array)
+               bpf_prog_array_free(old_array);
+
+out:
+       mutex_unlock(&bpf_event_mutex);
+       return ret;
+}
+
+void perf_event_detach_bpf_prog(struct perf_event *event)
+{
+       struct bpf_prog_array __rcu *old_array;
+       struct bpf_prog_array *new_array;
+       int ret;
+
+       mutex_lock(&bpf_event_mutex);
+
+       if (!event->prog)
+               return;

Ditto.

+
+       old_array = rcu_dereference_protected(event->tp_event->prog_array,
+                                             
lockdep_is_held(&bpf_event_mutex));
+
+       ret = bpf_prog_array_copy(old_array, event->prog, NULL, &new_array);
+       if (ret < 0) {
+               bpf_prog_array_delete_safe(old_array, event->prog);
+       } else {
+               rcu_assign_pointer(event->tp_event->prog_array, new_array);
+               bpf_prog_array_free(old_array);
+       }
+
+       bpf_prog_put(event->prog);
+       event->prog = NULL;
+
+       mutex_unlock(&bpf_event_mutex);
+}

[...]