netutils/telnetd: fix silent listen-socket loss and per-connection daemon death

[ricardgb]

Summary

Two robustness fixes for the telnet daemon (netutils/telnetd/telnetd_daemon.c). As it stands, the standard NuttX telnet console can be disabled by a single aborted connection, and on some configurations it silently serves nothing from boot.

1. Keep the listen socket out of the standard-stream range.
When the daemon starts without open standard streams — exactly what happens when nsh_telnetstart() spawns telnetd & before the console device exists (e.g. CONFIG_NSH_USBCONSOLE boards, where nsh_initialize() runs before the USB console is connected) — socket() returns a descriptor in 0..2. The accept loop's own "go silent" close(0)..close(2) then destroys the listen socket on the first iteration; every subsequent accept4() fails and the daemon serves nothing, while appearing alive in the task list. Fix: F_DUPFD_CLOEXEC the descriptor above 2 before use.

2. Survive transient connection errors instead of exiting.
Previously any single failed connection killed the whole daemon:

• a peer resetting before the accept completes (a port scanner, or a plain nc -z probe) surfaces as an accept4() error such as ECONNABORTED and took the errout path;
• the per-connection error paths after a successful accept (setsockopt, /dev/telnet open, SIOCTELNET, session-device open, session spawn) likewise exited the daemon.

One bad or aborted connection and the telnet console is dead until reboot — a trivial remote way to take out the console of any reachable NuttX device. Fix: treat these as per-connection failures (drop the connection, keep accepting). Genuinely unrecoverable accept4() errors (EBADF/ENOTSOCK/EINVAL/EOPNOTSUPP) 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.

Impact

Any board using netutils/telnetd (directly or via CONFIG_NSH_TELNET). Bug 1 affects configurations where the daemon can start before its standard streams exist (USB-console boards being the common case). Bug 2 affects everyone: a subnet nmap sweep kills every reachable NuttX telnet console.

Testing

Reproduced and verified on hardware: Raspberry Pi Pico 2 W (RP2350), composite USB CDC-ACM console + CDC-NCM network, CONFIG_NSH_USBCONSOLE, telnetd auto-started via nsh_telnetstart().

• Before (bug 1): daemon task alive but port 23 refused all connections from boot; the RTOS-aware debugger thread list showed it looping on the failed accept with the listen socket closed by its own close(0).
• Before (bug 2): a single nc -zv <addr> 23 probe permanently killed a manually started, correctly listening daemon.
• After: daemon listens from boot (within 5 s of enumeration), survives repeated nc -z probe barrages, and interactive nsh telnet sessions work.

tools/nxstyle.c passes on the modified file.

Note

These changes were developed with the assistance of an AI agent (Claude) and have been human-reviewed and tested on real hardware as described above.

[xiaoxiang781216]

why sleep

[xiaoxiang781216]

why not ignore the error and continue directly

[xiaoxiang781216]

move to previous patch if you really want to modify

[ricardgb]

Thanks @xiaoxiang781216 . Good questions on the accept-loop error handling —
I'd like to hold this one briefly before I revise it. I'm coordinating a
related change with the maintainers and want the daemon's error handling to
line up with it rather than reworking this twice. I'll follow up here with the
updates (the fatal-vs-transient split, the sleep, and folding the comment
change into the earlier commit) once that's settled. Appreciate the review.