Partial output of remote telnet to local console

[mfld-fr]

While testing the fix to the select() weakness (see #208), I can observe a problem that looks closely linked to the one reported by @Mellvik in issue #133 about the console management.

I am using busybox telnet 127.0.0.1 2323 on the host and QEMU for ELKS target, with only ktcp and telnetd daemons. Sometimes part of the output of the remote command is directed to the ELKS console in QEMU, and not to the remote telnet client, even if not logged in !

In remote telnet client:

mfld@home /home/mfld/elks $ busybox telnet 127.0.0.1 2323
Welcome to the ELKS telnet server

login: root
# cd /bin
# 
# ls -l

-xr-x  1 root     root         8396  May 13 2018  banner
-rwxr-xr-x  1 root     root          648  May 13 2018  basename

And in local console (in QEMU):

ELKS 0.2.0

login: Telnetserver:accept connection
Commands entered by remote terminal:
># cd /bin
>ls -l
>-rwxr-xr-x  1 root    root        50652  May 13 2018  ash
-rwxr>

Here, the beginning of the ls -l command output goes to the local console, then the remaining goes to the remote telnet client.

[Mellvik]

I can confirm that I have seen this behavior a number of times during recent testing.

[mfld-fr]

The observed bad behaviour is coming from a poor redirection implementation in telnetd. I revised that implementation with a cleaner code, but I detected two new problems on the master side of the pseudo-terminal.

First is that select() does not work, and second is that read() blocks when at least one character is available but when requesting more characters than available (not compliant to read manual page).

I also had to disable getty on virtual consoles 2 and 3 to get enough memory space for ktcp, telnetd, sh and its children, but that latest is not really a bug, rather a known limitation.

[mfld-fr]

Server and PTY master side fixed. But the current implementation of telnetd is still poor : no login, no IAC handling, not able to restart another session, etc.