how to re-enable input on a serial console

Question

I have an old router which runs Linux and I'd like to have a serial console on. This used to work fine, but after a firmware update, I can now only get output on the console and cannot give any input back to the device. During boot there is a line that says "Console input is disabled" that wasn't there before.

Without changing the firmware, how do I re-enable input on the serial console?

... my attempts so far ...

I can log in over the LAN and load any files (and cross compile tools if need be), and there is a small non-volatile user flash partition, so any solution requiring me to write a quick program using ioctl or whatnot is fine. The embedded Linux has the system files like /sys/bus and /sys/devices/platform/serial8250.0 and so on in case that is useful. The files /dev/console, /dev/ttyS0, /dev/tty all have read and write permissions. Running getty -L 115200 ttyS0 makes a login prompt appear on the serial console, but I can't get it to accept any input. Adding the line

::respawn:-/bin/sh

to inittab makes busybox give a shell prompt ... but again, the serial console won't take input.

dmesg reports

Kernel command line: root=/dev/ram rw init=/init console=ttyS0,115200

and stty gives

speed 38400 baud; line=0;  

along with a bunch of unimportant looking settings, stty -a -F /dev/ttyS0 gives

speed 115200 baud; stty: /dev/ttyS0  
line = 0;  

(and a whole bunch of settings that don't look important, but I'll type them up if people need them.)

I'm at a loss here, so I'm not sure what information is needed.
Any ideas for solving this puzzle? I have a feeling there is something simple I forgot to try.

Answer 1

The busybox example inittab looks a little different,

# Example of how to put a getty on a serial line (for a terminal)
#
#::respawn:/sbin/getty -L ttyS0 9600 vt100
#::respawn:/sbin/getty -L ttyS1 9600 vt100

could you give this a try?