Make failed TUN socket operation non-fatal

It might be desirable to run the UE without superuser rights. More
concretely, it might run without NET_ADMIN. In this case, various
corresponding operations on the TUN socket fd might fail. In this
commit,

- improve error reports to make them more clear
- do not automatically exit(1), but return from the function to ensure
  continuity of the softmodem

openair1/SIMULATION/ETH_TRANSPORT/netlink_init.c

@@ -144,7 +144,7 @@ int netlink_init_tun(char *ifprefix, int num_if, int id) {//for UE, id = 1, 2, .
 
     if (nas_sock_fd[i] == -1) {
       LOG_E(PDCP, "TUN: Error opening socket %s (%d:%s)\n",ifname,errno, strerror(errno));
-      exit(1);
+      return 0;
     }
 
     LOG_I(PDCP, "TUN: Opened socket %s with fd nas_sock_fd[%d]=%d\n",

openair2/LAYER2/nr_pdcp/nr_pdcp_oai_api.c

@@ -406,8 +406,7 @@ static void reblock_tun_socket(void)
   f = fcntl(nas_sock_fd[0], F_GETFL, 0);
   f &= ~(O_NONBLOCK);
   if (fcntl(nas_sock_fd[0], F_SETFL, f) == -1) {
-    LOG_E(PDCP, "reblock_tun_socket failed\n");
-    exit(1);
+    LOG_E(PDCP, "fcntl(F_SETFL) failed on fd %d: errno %d, %s\n", nas_sock_fd[0], errno, strerror(errno));
   }
 }
 
@@ -425,8 +424,8 @@ static void *enb_tun_read_thread(void *_)
   while (1) {
     len = read(nas_sock_fd[0], &rx_buf, NL_MAX_PAYLOAD);
     if (len == -1) {
-      LOG_E(PDCP, "%s:%d:%s: fatal\n", __FILE__, __LINE__, __FUNCTION__);
-      exit(1);
+      LOG_E(PDCP, "could not read(): errno %d %s\n", errno, strerror(errno));
+      return NULL;
     }
 
     LOG_D(PDCP, "%s(): nas_sock_fd read returns len %d\n", __func__, len);
@@ -483,8 +482,8 @@ static void *ue_tun_read_thread(void *_)
   while (1) {
     len = read(nas_sock_fd[0], &rx_buf, NL_MAX_PAYLOAD);
     if (len == -1) {
-      LOG_E(PDCP, "%s:%d:%s: fatal\n", __FILE__, __LINE__, __FUNCTION__);
-      exit(1);
+      LOG_E(PDCP, "error: cannot read() from fd %d: errno %d, %s\n", nas_sock_fd[0], errno, strerror(errno));
+      return NULL; /* exit thread */
     }
 
     LOG_D(PDCP, "%s(): nas_sock_fd read returns len %d\n", __func__, len);