Index: ifq.c
===================================================================
RCS file: /cvs/src/sys/net/ifq.c,v
retrieving revision 1.22
diff -u -p -r1.22 ifq.c
--- ifq.c	25 Jan 2018 14:04:36 -0000	1.22
+++ ifq.c	11 Dec 2018 01:29:16 -0000
@@ -357,6 +357,21 @@ ifq_dequeue(struct ifqueue *ifq)
 	return (m);
 }
 
+int
+ifq_hdatalen(struct ifqueue *ifq)
+{
+	struct mbuf *m;
+	int len = 0;
+
+	m = ifq_deq_begin(ifq);
+	if (m != NULL) {
+		len = m->m_pkthdr.len;
+		ifq_deq_commit(ifq, m);
+	}
+
+	return (len);
+}
+
 unsigned int
 ifq_purge(struct ifqueue *ifq)
 {
Index: ifq.h
===================================================================
RCS file: /cvs/src/sys/net/ifq.h,v
retrieving revision 1.20
diff -u -p -r1.20 ifq.h
--- ifq.h	4 Jan 2018 11:02:57 -0000	1.20
+++ ifq.h	11 Dec 2018 01:29:16 -0000
@@ -396,6 +396,8 @@ struct ifq_ops {
 	void			 (*ifqop_free)(unsigned int, void *);
 };
 
+extern const struct ifq_ops * const ifq_priq_ops;
+
 /*
  * Interface send queues.
  */
@@ -409,6 +411,7 @@ struct mbuf	*ifq_deq_begin(struct ifqueu
 void		 ifq_deq_commit(struct ifqueue *, struct mbuf *);
 void		 ifq_deq_rollback(struct ifqueue *, struct mbuf *);
 struct mbuf	*ifq_dequeue(struct ifqueue *);
+int		 ifq_hdatalen(struct ifqueue *);
 void		 ifq_mfreem(struct ifqueue *, struct mbuf *);
 void		 ifq_mfreeml(struct ifqueue *, struct mbuf_list *);
 unsigned int	 ifq_purge(struct ifqueue *);
@@ -422,6 +425,12 @@ void		 ifq_barrier(struct ifqueue *);
 #define	ifq_empty(_ifq)			(ifq_len(_ifq) == 0)
 #define	ifq_set_maxlen(_ifq, _l)	((_ifq)->ifq_maxlen = (_l))
 
+static inline int
+ifq_is_priq(struct ifqueue *ifq)
+{
+	return (ifq->ifq_ops == ifq_priq_ops);
+}
+
 static inline void
 ifq_set_oactive(struct ifqueue *ifq)
 {
@@ -459,8 +468,6 @@ ifq_idx(struct ifqueue *ifq, unsigned in
 }
 
 #define IFQ_ASSERT_SERIALIZED(_ifq)	KASSERT(ifq_is_serialized(_ifq))
-
-extern const struct ifq_ops * const ifq_priq_ops;
 
 /* ifiq */
 
Index: if_tun.c
===================================================================
RCS file: /cvs/src/sys/net/if_tun.c,v
retrieving revision 1.182
diff -u -p -r1.182 if_tun.c
--- if_tun.c	12 Nov 2018 06:35:37 -0000	1.182
+++ if_tun.c	11 Dec 2018 01:29:16 -0000
@@ -670,8 +670,7 @@ tun_dev_ioctl(struct tun_softc *tp, u_lo
 			tp->tun_flags &= ~TUN_ASYNC;
 		break;
 	case FIONREAD:
-		*(int *)data = ifq_empty(&tp->tun_if.if_snd) ?
-		    0 : tp->tun_if.if_mtu;
+		*(int *)data = ifq_hdatalen(&tp->tun_if.if_snd);
 		break;
 	case TIOCSPGRP:
 		tp->tun_pgid = *(int *)data;