netutils/telnetd: survive transient connection errors instead of exiting

Any single failed connection killed the whole daemon:

- A peer that resets before the accept completes (a port scanner, or a
  simple "nc -z" probe) surfaces as an accept4() error such as
  ECONNABORTED, which took the errout path and closed the listener.
- The per-connection error paths after a successful accept (setsockopt,
  opening /dev/telnet, SIOCTELNET, opening the session device, spawning
  the session) likewise exited the daemon instead of dropping the one
  connection.

Either way, one bad or aborted connection and the telnet console is
dead until reboot - a trivial remote way to take out the NuttX console
on any reachable network.  Verified on hardware: one "nc -zv" probe
permanently killed the listener.

Treat these as per-connection failures: drop the connection and keep
accepting.  Genuinely unrecoverable accept4() errors (EBADF, ENOTSOCK,
EINVAL, EOPNOTSUPP - a bad listen socket) still exit loudly, and a
short pause on repeated transient failures avoids busy-spinning while
an interface is down.  Daemon setup errors (socket/bind/listen) exit
as before.

Assisted-by: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Ricard Rosson <ricard@groundbits.com>

netutils/telnetd/telnetd_daemon.c

@@ -126,12 +126,11 @@ int telnetd_daemon(FAR const struct telnetd_config_s *config)
     }
 
   /* If the daemon was started without standard streams (e.g. spawned by
-   * nsh_telnetstart() before a USB console device exists), socket() may
-   * have returned a descriptor in 0..2.  The "go silent"
-   * close(0)..close(2) at the top of the accept loop below would then
-   * destroy the listen socket: every subsequent accept4() fails and the
-   * daemon serves nothing.  Move the descriptor above the
-   * standard-stream range.
+   * nsh_telnetstart() before a USB console exists), socket() may have
+   * returned a descriptor in 0..2.  The "go silent" close(0)..close(2)
+   * at the top of the accept loop below would then destroy the listen
+   * socket: every subsequent accept4() fails and the daemon serves
+   * nothing.  Move the descriptor above the standard-stream range.
    */
 
   if (listensd <= 2)
@@ -224,17 +223,33 @@ int telnetd_daemon(FAR const struct telnetd_config_s *config)
       acceptsd = accept4(listensd, &addr.generic, &addrlen, SOCK_CLOEXEC);
       if (acceptsd < 0)
         {
-          /* Just continue if a signal was received */
+          int err = errno;
 
-          if (errno == EINTR)
+          /* A bad listen socket can never recover - exiting loudly beats
+           * spinning forever while serving nothing.
+           */
+
+          if (err == EBADF || err == ENOTSOCK || err == EINVAL ||
+              err == EOPNOTSUPP)
             {
-              continue;
-            }
-          else
-            {
-              nerr("ERROR: accept failed: %d\n", errno);
+              nerr("ERROR: accept failed fatally: %d\n", err);
               goto errout_with_socket;
             }
+
+          /* Everything else is transient: a peer that reset before the
+           * accept completed (a port scan, nc -z, ECONNABORTED), a
+           * signal, or the interface bouncing.  None of those may take
+           * the daemon down; the pause keeps a persistent transient
+           * (interface down) from busy-spinning.
+           */
+
+          nerr("ERROR: accept failed: %d\n", err);
+          if (err != EINTR)
+            {
+              usleep(100 * 1000);
+            }
+
+          continue;
         }
 
 #ifdef CONFIG_NET_SOLINGER
@@ -247,8 +262,14 @@ int telnetd_daemon(FAR const struct telnetd_config_s *config)
       if (setsockopt(acceptsd, SOL_SOCKET, SO_LINGER,
                      &ling, sizeof(struct linger)) < 0)
         {
+          /* Per-connection failure (e.g. the peer already reset the
+           * connection - a port scan does this).  Drop the connection and
+           * keep accepting; exiting here lets any probe kill the daemon.
+           */
+
           nerr("ERROR: setsockopt failed: %d\n", errno);
-          goto errout_with_acceptsd;
+          close(acceptsd);
+          continue;
         }
 #endif
 
@@ -258,7 +279,8 @@ int telnetd_daemon(FAR const struct telnetd_config_s *config)
       if (drvrfd < 0)
         {
           nerr("ERROR: open(/dev/telnet) failed: %d\n", errno);
-          goto errout_with_acceptsd;
+          close(acceptsd);
+          continue;
         }
 
       /* Create a character device to "wrap" the accepted socket descriptor */
@@ -271,9 +293,10 @@ int telnetd_daemon(FAR const struct telnetd_config_s *config)
       if (ioctl(drvrfd, SIOCTELNET,
                 (unsigned long)((uintptr_t)&session)) < 0)
         {
-          nerr("ERROR: open(/dev/telnet) failed: %d\n", errno);
+          nerr("ERROR: SIOCTELNET failed: %d\n", errno);
           close(drvrfd);
-          goto errout_with_acceptsd;
+          close(acceptsd);
+          continue;
         }
 
       close(drvrfd);
@@ -284,8 +307,12 @@ int telnetd_daemon(FAR const struct telnetd_config_s *config)
       drvrfd = open(session.ts_devpath, O_RDWR);
       if (drvrfd < 0)
         {
+          /* The socket now belongs to the telnet driver instance; just
+           * keep accepting.
+           */
+
           nerr("ERROR: Failed to open %s: %d\n", session.ts_devpath, errno);
-          goto errout_with_socket;
+          continue;
         }
 
       /* Use this driver as stdin, stdout, and stderror */
@@ -341,16 +368,12 @@ int telnetd_daemon(FAR const struct telnetd_config_s *config)
           if (ret > 0)
             {
               nerr("ERROR: Failed start the telnet session: %d\n", ret);
-              errno = ret;
-              goto errout_with_socket;
+              continue;
             }
         }
 #endif
     }
 
-errout_with_acceptsd:
-  close(acceptsd);
-
 errout_with_socket:
   close(listensd);
 errout: