From 8dc7f6d79e63b26b5890fcee318334f196d0a595 Mon Sep 17 00:00:00 2001
From: Simon Piriou <spiriou31@gmail.com>
Date: Sun, 14 May 2017 09:08:32 -0600
Subject: [PATCH] tcp: wait for 3-Way Handshare before accept() returns

---
 net/tcp/Kconfig     |  7 +++++++
 net/tcp/tcp_input.c | 20 ++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/net/tcp/Kconfig b/net/tcp/Kconfig
index fac41794dc..1660b080ee 100644
--- a/net/tcp/Kconfig
+++ b/net/tcp/Kconfig
@@ -156,6 +156,13 @@ config NET_TCPBACKLOG
 		Incoming connections pend in a backlog until accept() is called.
 		The size of the backlog is selected when listen() is called.
 
+config NET_ACCEPT_ON_ACK
+	bool "accept() returns on TCP 3-Way Handshake completion"
+	default n
+	---help---
+		accept() returns after 3-Way Handshake is complete (SYN, SYN/ACK, ACK).
+		Without this flag it returns after SYN packet is received.
+
 config NET_TCP_SPLIT
 	bool "Enable packet splitting"
 	default n
diff --git a/net/tcp/tcp_input.c b/net/tcp/tcp_input.c
index 1918bd3492..22e11be7ae 100644
--- a/net/tcp/tcp_input.c
+++ b/net/tcp/tcp_input.c
@@ -185,6 +185,8 @@ static void tcp_input(FAR struct net_driver_s *dev, unsigned int iplen)
                */
 
               conn->crefs = 1;
+
+#ifndef CONFIG_NET_ACCEPT_ON_ACK
               if (tcp_accept_connection(dev, conn, tmp16) != OK)
                 {
                   /* No, then we have to give the connection back and drop the packet */
@@ -193,6 +195,7 @@ static void tcp_input(FAR struct net_driver_s *dev, unsigned int iplen)
                   tcp_free(conn);
                   conn = NULL;
                 }
+#endif
             }
 
           if (!conn)
@@ -464,6 +467,23 @@ found:
           {
             conn->tcpstateflags = TCP_ESTABLISHED;
 
+#ifdef CONFIG_NET_ACCEPT_ON_ACK
+            if (tcp_accept_connection(dev, conn, tcp->destport) != OK)
+              {
+                /* No more listener for current port.  We can free conn here
+                 * because it has not been shared with upper layers yet as
+                 * handshake is not complete
+                 */
+
+                nerr("Listen canceled while waiting for ACK on port %d\n",
+                     tcp->destport);
+                conn->crefs = 0;
+                tcp_free(conn);
+                conn = NULL;
+                goto drop;
+              }
+#endif
+
 #ifdef CONFIG_NET_TCP_WRITE_BUFFERS
             conn->isn           = tcp_getsequence(tcp->ackno);
             tcp_setsequence(conn->sndseq, conn->isn);