diff --git a/sys/netinet/ip_mroute.h b/sys/netinet/ip_mroute.h
--- a/sys/netinet/ip_mroute.h
+++ b/sys/netinet/ip_mroute.h
@@ -265,6 +265,7 @@
     u_long		v_bytes_in;	/* # bytes in on interface	     */
     u_long		v_bytes_out;	/* # bytes out on interface	     */
 #ifdef _KERNEL
+#define	MROUTE_VIF_SYSCTL_LEN	__offsetof(struct vif, v_spin)
     struct mtx		v_spin;		/* Spin mutex for pkt stats          */
     char		v_spin_name[32];
 #endif
diff --git a/sys/netinet/ip_mroute.c b/sys/netinet/ip_mroute.c
--- a/sys/netinet/ip_mroute.c
+++ b/sys/netinet/ip_mroute.c
@@ -2727,18 +2727,23 @@
 static int
 sysctl_viflist(SYSCTL_HANDLER_ARGS)
 {
-	int error;
+	int error, i;
 
 	if (req->newptr)
 		return (EPERM);
 	if (V_viftable == NULL)		/* XXX unlocked */
 		return (0);
-	error = sysctl_wire_old_buffer(req, sizeof(*V_viftable) * MAXVIFS);
+	error = sysctl_wire_old_buffer(req, MROUTE_VIF_SYSCTL_LEN * MAXVIFS);
 	if (error)
 		return (error);
 
 	MRW_RLOCK();
-	error = SYSCTL_OUT(req, V_viftable, sizeof(*V_viftable) * MAXVIFS);
+	/* Copy out user-visible portion of vif entry. */
+	for (i = 0; i < MAXVIFS; i++) {
+		error = SYSCTL_OUT(req, &V_viftable[i], MROUTE_VIF_SYSCTL_LEN);
+		if (error)
+			break;
+	}
 	MRW_RUNLOCK();
 	return (error);
 }