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

From: Daniel Borkmann <[email-masked]>
Date: Mon, 23 Oct 2017 22:52:29 +0200

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);
> +}
[...]