user permission inconsistency

[alex <alex@mellonta.net>]

Hi everybody,

I observe a strange inconsistency in user permissions (jessie up to 
date; xfce4). Id est:

user@rheya:~$ ls -la /dev/ttyACM0
crw-rw---- 1 root dialout 166, 0 Dec  8 17:25 /dev/ttyACM0

user@rheya:~$ id alex
uid=1000(user) gid=1000(user) groups=1000(user),20(dialout)

user@rheya:~$ echo "\0" > /dev/ttyACM0
bash: /dev/ttyACM0: Permission denied

although

root@rheya:~# echo "\0" > /dev/ttyACM0 && echo ok
ok


I *suspect* this is somehow related to systemd-logind (while systemd is 
not running as pid 1)

I just added user to group dialout (and astonished not to have to logout 
in order to have updated user groups list)


Any hint?

A.