Index: sys/socketvar.h
===================================================================
RCS file: /cvsroot/src/sys/sys/socketvar.h,v
retrieving revision 1.158
diff -u -u -r1.158 socketvar.h
--- sys/socketvar.h	1 Aug 2018 23:35:32 -0000	1.158
+++ sys/socketvar.h	16 Dec 2018 16:32:50 -0000
@@ -68,6 +68,7 @@
 #include <sys/queue.h>
 #include <sys/mutex.h>
 #include <sys/condvar.h>
+#include <sys/common_limits.h>
 
 #if !defined(_KERNEL)
 struct uio;
@@ -412,6 +413,21 @@
 	return lmin(sb->sb_hiwat - sb->sb_cc, sb->sb_mbmax - sb->sb_mbcnt);
 }
 
+static __inline u_long
+sbspace_oob(const struct sockbuf *sb)
+{
+	u_long hiwat = sb->sb_hiwat;
+
+	if (hiwat < ULONG_MAX - 1024)
+		hiwat += 1024;
+
+	KASSERT(solocked(sb->sb_so));
+
+	if (hiwat <= sb->sb_cc || sb->sb_mbmax <= sb->sb_mbcnt)
+		return 0;
+	return lmin(hiwat - sb->sb_cc, sb->sb_mbmax - sb->sb_mbcnt);
+}
+
 /* do we have to send all at once on a socket? */
 static __inline int
 sosendallatonce(const struct socket *so)
Index: netinet/dccp_usrreq.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/dccp_usrreq.c,v
retrieving revision 1.20
diff -u -u -r1.20 dccp_usrreq.c
--- netinet/dccp_usrreq.c	14 Sep 2018 05:09:51 -0000	1.20
+++ netinet/dccp_usrreq.c	16 Dec 2018 16:32:51 -0000
@@ -2157,7 +2157,7 @@
 		}
 	}
 
-	if (sbspace(&so->so_snd) < -512) {
+	if (sbspace_oob(&so->so_snd) == 0) {
 		INP_UNLOCK(inp);
 		error = ENOBUFS;
 		goto release;
Index: netinet/tcp_usrreq.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_usrreq.c,v
retrieving revision 1.221
diff -u -u -r1.221 tcp_usrreq.c
--- netinet/tcp_usrreq.c	24 Nov 2018 17:05:54 -0000	1.221
+++ netinet/tcp_usrreq.c	16 Dec 2018 16:32:51 -0000
@@ -1154,7 +1154,7 @@
 	ostate = tcp_debug_capture(tp, PRU_SENDOOB);
 
 	s = splsoftnet();
-	if (sbspace(&so->so_snd) < -512) {
+	if (sbspace_oob(&so->so_snd) == 0) {
 		m_freem(m);
 		splx(s);
 		return ENOBUFS;