What permission does the "lock" group in /etc/group grant?

Question

I'm running Fedora.

$ ls -l /dev/ttyACM0 
crw-rw----. 1 root dialout 166, 0 Jul  4 10:50 /dev/ttyACM0   

I was not able to access /dev/tty/ACM0 (dynamic usb serial with an Arduino on the other end), even though I added myself to the dialout group. I had to either chmod a+rw /dev/tty/ACM0 or sudo my programming command (arduino or avrdude).

I've recently "solved" a permission problem by cargo culting. The script /usr/bin/arduino, installed by my Fedora, includes a check and warning if the user is not a member of groups dialout and lock.

Adding myself to lock made it work. Why? I'm trying to understand the implications of this hidden ownership, and whether it applies to all the other flavors of Linux I use, but lock group is hard to google.

The command sudo find / -group lock turns up only one result: /usr/sbin/lockdev

Answer 1

Groups and users belong to processes. They are usually only applied at login, and then propagate to child processes. Editing /etc/group will not change the group of any existing process. And will not effect any new processes, unless a privileged process reads this file and adds the group (login, X11 display manager: X11 login system, newgrp, etc)

The lock thing was probably a coincidence. You probably logged out and back in again.

To find out what files have group lock.

find / -mount -group lock -print | less