From 8286a558c86d07468c1b29607c154e849cd92800 Mon Sep 17 00:00:00 2001
From: Alan Carvalho de Assis <acassis@gmail.com>
Date: Wed, 2 Nov 2022 13:40:08 -0300
Subject: [PATCH] net/tcp: Avoid starting TCP sequence number 0

---
 net/tcp/tcp_conn.c  | 10 ++++++++++
 net/tcp/tcp_seqno.c | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/net/tcp/tcp_conn.c b/net/tcp/tcp_conn.c
index ab42955afe..de09d72c13 100644
--- a/net/tcp/tcp_conn.c
+++ b/net/tcp/tcp_conn.c
@@ -1039,6 +1039,10 @@ FAR struct tcp_conn_s *tcp_alloc_accept(FAR struct net_driver_s *dev,
       conn->tcpstateflags = TCP_SYN_RCVD;
 
       tcp_initsequence(conn->sndseq);
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+      conn->rexmit_seq = tcp_getsequence(conn->sndseq);
+#endif
+
       conn->tx_unacked    = 1;
 #ifdef CONFIG_NET_TCP_WRITE_BUFFERS
       conn->expired       = 0;
@@ -1313,6 +1317,12 @@ int tcp_connect(FAR struct tcp_conn_s *conn, FAR const struct sockaddr *addr)
   conn->tcpstateflags = TCP_SYN_SENT;
   tcp_initsequence(conn->sndseq);
 
+  /* Save initial sndseq to rexmit_seq, otherwise it will be zero */
+
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+  conn->rexmit_seq = tcp_getsequence(conn->sndseq);
+#endif
+
   conn->tx_unacked = 1;    /* TCP length of the SYN is one. */
   conn->nrtx       = 0;
   conn->timeout    = true; /* Send the SYN immediately. */
diff --git a/net/tcp/tcp_seqno.c b/net/tcp/tcp_seqno.c
index 58b4c587ad..eeaa1d43c1 100644
--- a/net/tcp/tcp_seqno.c
+++ b/net/tcp/tcp_seqno.c
@@ -45,7 +45,9 @@
 
 #include <stdint.h>
 #include <debug.h>
+#include <sys/random.h>
 
+#include <nuttx/clock.h>
 #include <nuttx/net/netconfig.h>
 #include <nuttx/net/netdev.h>
 
@@ -142,6 +144,41 @@ uint32_t tcp_addsequence(FAR uint8_t *seqno, uint16_t len)
 
 void tcp_initsequence(FAR uint8_t *seqno)
 {
+  int ret;
+
+  /* If g_tcpsequence is already initialized, just copy it */
+
+  if (g_tcpsequence == 0)
+    {
+      /* Get a random TCP sequence number */
+
+      ret = getrandom(&g_tcpsequence, sizeof(uint32_t), 0);
+      if (ret < 0)
+        {
+          ret = getrandom(&g_tcpsequence, sizeof(uint32_t), GRND_RANDOM);
+        }
+
+      /* If getrandom() failed use sys ticks, use about half of allowed
+       * values
+       */
+
+      if (ret != sizeof(uint32_t))
+        {
+          g_tcpsequence = clock_systime_ticks() % 2000000000;
+        }
+      else
+        {
+          g_tcpsequence = g_tcpsequence % 2000000000;
+        }
+
+      /* If the random value is "small" increase it */
+
+      if (g_tcpsequence < 1000000000)
+        {
+          g_tcpsequence += 1000000000;
+        }
+    }
+
   tcp_setsequence(seqno, g_tcpsequence);
 }