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

On 10/23/17 1:52 PM, Daniel Borkmann wrote:

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

quoted

      __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.

Thanks for catching this! Will fix this and the below one as well in the 
new revision.

quoted

+
+    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.

quoted

+
+    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);
+}

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