[1.20.0] getty: setsid: Operation not permitted (dissected)

[Tomasz Czapiewski]

On Tue, 24 Apr 2012, Denys Vlasenko wrote:

> On Tue, Apr 24, 2012 at 11:27 AM, Tomasz Czapiewski <xeros at irc.pl> wrote:
>> dmesg:
>> Mon Apr 23 23:16:57 2012: process '/sbin/getty -L ttyS0 115200 vt102' (pid
>> 909) exited. Scheduling for restart.
>>
>> I'm also getting "getty: setsid: Operation not permitted" when trying to run
>> above command from shell.
>
> It means that setsid fails. We used to just assume it always succeeds.
> Which was wrong. For one, successful setsid drops ctty,
> but failing one does not.
>
> setsid fails if getty is started in a way so that it is a process leader,
> but not a session leader. For one, this is what happens
> when you run a single command (not a pipe) from interactive shell:
> it is made a process leader by the shell. That's why getty
> from interactive shell doesn't work (and probably is not expected to).
>
> How do you start your getty? From init? Which init - busybox's?

Yes, it's started from init, which is a symlink to the same busybox 
binary.
/etc/inittab:
::sysinit:/etc/init.d/rcS
ttyS0::respawn:/sbin/getty -L ttyS0 115200 vt102
::ctrlaltdel:/sbin/reboot
null::shutdown:/bin/umount -a -r
(changing order of ttyS0 and 115200 arguments doesn't seem to make any 
difference)

In the middle of /etc/init.d/rcS there's:
stty -F /dev/ttyS0 brkint

After end of rcS, getty is supposed to get respawned and take control over 
serial port.

Such configuration worked fine under previous BusyBox versions.

Regards,
Tom