bpf: support BPF_PROG_QUERY for BPF_FLOW_DISSECTOR attach_type

target_fd is target namespace. If there is a flow dissector BPF program
attached to that namespace, its (single) id is returned.

v5:
* drop net ref right after rcu unlock (Daniel Borkmann)

v4:
* add missing put_net (Jann Horn)

v3:
* add missing inline to skb_flow_dissector_prog_query static def
  (kbuild test robot <lkp@intel.com>)

v2:
* don't sleep in rcu critical section (Jakub Kicinski)
* check input prog_cnt (exit early)

Cc: Jann Horn <jannh@google.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/skbuff.h

@@ -1258,11 +1258,19 @@ void skb_flow_dissector_init(struct flow_dissector *flow_dissector,
 			     unsigned int key_count);
 
 #ifdef CONFIG_NET
+int skb_flow_dissector_prog_query(const union bpf_attr *attr,
+				  union bpf_attr __user *uattr);
 int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
 				       struct bpf_prog *prog);
 
 int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr);
 #else
+static inline int skb_flow_dissector_prog_query(const union bpf_attr *attr,
+						union bpf_attr __user *uattr)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
 						     struct bpf_prog *prog)
 {

kernel/bpf/syscall.c

@@ -2009,6 +2009,8 @@ static int bpf_prog_query(const union bpf_attr *attr,
 		break;
 	case BPF_LIRC_MODE2:
 		return lirc_prog_query(attr, uattr);
+	case BPF_FLOW_DISSECTOR:
+		return skb_flow_dissector_prog_query(attr, uattr);
 	default:
 		return -EINVAL;
 	}

net/core/flow_dissector.c

@@ -65,6 +65,45 @@ void skb_flow_dissector_init(struct flow_dissector *flow_dissector,
 }
 EXPORT_SYMBOL(skb_flow_dissector_init);
 
+int skb_flow_dissector_prog_query(const union bpf_attr *attr,
+				  union bpf_attr __user *uattr)
+{
+	__u32 __user *prog_ids = u64_to_user_ptr(attr->query.prog_ids);
+	u32 prog_id, prog_cnt = 0, flags = 0;
+	struct bpf_prog *attached;
+	struct net *net;
+
+	if (attr->query.query_flags)
+		return -EINVAL;
+
+	net = get_net_ns_by_fd(attr->query.target_fd);
+	if (IS_ERR(net))
+		return PTR_ERR(net);
+
+	rcu_read_lock();
+	attached = rcu_dereference(net->flow_dissector_prog);
+	if (attached) {
+		prog_cnt = 1;
+		prog_id = attached->aux->id;
+	}
+	rcu_read_unlock();
+
+	put_net(net);
+
+	if (copy_to_user(&uattr->query.attach_flags, &flags, sizeof(flags)))
+		return -EFAULT;
+	if (copy_to_user(&uattr->query.prog_cnt, &prog_cnt, sizeof(prog_cnt)))
+		return -EFAULT;
+
+	if (!attr->query.prog_cnt || !prog_ids || !prog_cnt)
+		return 0;
+
+	if (copy_to_user(prog_ids, &prog_id, sizeof(u32)))
+		return -EFAULT;
+
+	return 0;
+}
+
 int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
 				       struct bpf_prog *prog)
 {