Index: head/sys/kern/uipc_sockbuf.c
===================================================================
--- head/sys/kern/uipc_sockbuf.c
+++ head/sys/kern/uipc_sockbuf.c
@@ -955,21 +955,14 @@
 	return (retval);
 }
 
-int
+void
 sbappendcontrol_locked(struct sockbuf *sb, struct mbuf *m0,
     struct mbuf *control)
 {
-	struct mbuf *m, *n, *mlast;
-	int space;
+	struct mbuf *m, *mlast;
 
-	SOCKBUF_LOCK_ASSERT(sb);
-
-	space = m_length(control, &n) + m_length(m0, NULL);
-
-	if (space > sbspace(sb))
-		return (0);
 	m_clrprotoflags(m0);
-	n->m_next = m0;			/* concatenate data to control */
+	m_last(control)->m_next = m0;
 
 	SBLASTRECORDCHK(sb);
 
@@ -983,18 +976,15 @@
 	SBLASTMBUFCHK(sb);
 
 	SBLASTRECORDCHK(sb);
-	return (1);
 }
 
-int
+void
 sbappendcontrol(struct sockbuf *sb, struct mbuf *m0, struct mbuf *control)
 {
-	int retval;
 
 	SOCKBUF_LOCK(sb);
-	retval = sbappendcontrol_locked(sb, m0, control);
+	sbappendcontrol_locked(sb, m0, control);
 	SOCKBUF_UNLOCK(sb);
-	return (retval);
 }
 
 /*
Index: head/sys/kern/uipc_usrreq.c
===================================================================
--- head/sys/kern/uipc_usrreq.c
+++ head/sys/kern/uipc_usrreq.c
@@ -1174,16 +1174,22 @@
 			unp2->unp_flags &= ~UNP_WANTCRED;
 			control = unp_addsockcred(td, control);
 		}
+
 		/*
-		 * Send to paired receive port, and then reduce send buffer
-		 * hiwater marks to maintain backpressure.  Wake up readers.
+		 * Send to paired receive port and wake up readers.  Don't
+		 * check for space available in the receive buffer if we're
+		 * attaching ancillary data; Unix domain sockets only check
+		 * for space in the sending sockbuf, and that check is
+		 * performed one level up the stack.  At that level we cannot
+		 * precisely account for the amount of buffer space used
+		 * (e.g., because control messages are not yet internalized).
 		 */
 		switch (so->so_type) {
 		case SOCK_STREAM:
 			if (control != NULL) {
-				if (sbappendcontrol_locked(&so2->so_rcv, m,
-				    control))
-					control = NULL;
+				sbappendcontrol_locked(&so2->so_rcv, m,
+				    control);
+				control = NULL;
 			} else
 				sbappend_locked(&so2->so_rcv, m, flags);
 			break;
@@ -1192,14 +1198,8 @@
 			const struct sockaddr *from;
 
 			from = &sun_noname;
-			/*
-			 * Don't check for space available in so2->so_rcv.
-			 * Unix domain sockets only check for space in the
-			 * sending sockbuf, and that check is performed one
-			 * level up the stack.
-			 */
 			if (sbappendaddr_nospacecheck_locked(&so2->so_rcv,
-				from, m, control))
+			    from, m, control))
 				control = NULL;
 			break;
 			}
Index: head/sys/sys/sockbuf.h
===================================================================
--- head/sys/sys/sockbuf.h
+++ head/sys/sys/sockbuf.h
@@ -139,9 +139,9 @@
 	    struct mbuf *m0, struct mbuf *control);
 int	sbappendaddr_nospacecheck_locked(struct sockbuf *sb,
 	    const struct sockaddr *asa, struct mbuf *m0, struct mbuf *control);
-int	sbappendcontrol(struct sockbuf *sb, struct mbuf *m0,
+void	sbappendcontrol(struct sockbuf *sb, struct mbuf *m0,
 	    struct mbuf *control);
-int	sbappendcontrol_locked(struct sockbuf *sb, struct mbuf *m0,
+void	sbappendcontrol_locked(struct sockbuf *sb, struct mbuf *m0,
 	    struct mbuf *control);
 void	sbappendrecord(struct sockbuf *sb, struct mbuf *m0);
 void	sbappendrecord_locked(struct sockbuf *sb, struct mbuf *m0);