December 26, 2008

Linux & Automounting of removable USB disks: Xfce take

Automounting of the removable USB devices seems to be a recurring problem for many Linux users. It's caused by the opaque interaction between ConsoleKit, PolicyKit and the desktop environment. Even if the desktop environment supports the volume management (and I believe all major Linux desktops do: GNOME, KDE, XFCE), missing just one link in the fragile chain of interactions between the desktop components will break automounting, and possibly other things such as sound in PulseAudio.

I'll take Xfce desktop as an example since I've had some troubles with it automounting the USB flash sticks. The prerequisites for working automounting in Xfce are: running dbus daemon (it's a must for the recent desktops anyway), running consolekit daemon, running Thunar (check for "thunar --daemon" process), and configured Thunar volume management (check "Enable Volume Management" option in the Advanced tab of the File Manager section in the settings).

But even having all that running doesn't guarantee the success. Enter the infamous " no <-- (action, result )" error message, which seems to be perfectly in line with a long-time Unix tradition of the cryptic error messages. What's worse, the error window doesn't elaborate on what component has failed and why.

Common answer to that is the addition of the following snippet to /etc/PolicyKit/PolicyKit.conf:
<match action="">
<return result="yes">

However this is actually wrong - this fix is akin to removing the password from some user account if the user is unable to login instead of providing them with the new password.

Another symptom of this problem is the lack of PolicyKit authorizations in the polkit-auth output (check at the end of this post for a "good" authorization list):
$ polkit-auth

After the long investigation I found that my X session wasn't registered with ConsoleKit:
$ ck-list-sessions 
uid = '501'
realname = ''
seat = 'Seat1'
session-type = ''
active = FALSE
x11-display = ''
x11-display-device = ''
display-device = '/dev/tty1'
remote-host-name = ''
is-local = TRUE
on-since = '2008-12-23T19:04:59Z'
idle-since-hint = '2008-12-23T19:05:33Z'

See, only my tty1 console session is there, no sign of my X session started from that console. Here's how a registered X session should look:
$ ck-list-sessions 
uid = '501'
realname = ''
seat = 'Seat1'
session-type = ''
active = TRUE
x11-display = ':0'
x11-display-device = '/dev/tty7'
display-device = ''
remote-host-name = ''
is-local = TRUE
on-since = '2008-12-26T09:28:52Z'

Why my Xorg session wasn't registered?
Here I must confess that I don't use GDM or any other display manager, so I (correctly) assumed that it was the cause. Apparently GDM and KDM take care of registering with the ConsoleKit, and /usr/bin/startxfce4 script used to start Xfce from the console fails to do that.

After some more Googling and experimentation I found this XDM bug which gave me a hint - /usr/bin/ck-xinit-session should be executed during the Xorg startup, preferably immediately after X server is launched.

Here's my solution to this: create .config/xfce4/xinitrc in your home directory, make it executable and invoke /usr/bin/ck-xinit-session before Xfce xinitrc:

echo -e '#!/bin/sh\n\n' > ~/.config/xfce4/xinitrc
echo 'exec /usr/bin/ck-xinit-session sh /etc/xdg/xfce4/xinitrc' >> ~/.config/xfce4/xinitrc
chmod u+x ~/.config/xfce4/xinitrc

After starting Xfce, I checked the session list and my X session was there. Bingo! After inserting a guinea USB stick and waiting a few seconds, I was greeted with a Thunar window with the USB stick content. So automounting finally worked, too, and without enabling it for everyone and everywhere with that PolicyKit.conf hack.

Here's a "working" output from polkit-auth for the reference:

I hope this guide will be useful for those diehards who don't use GDM/KDM. As a bonus, this technique fixes the sound in PulseAudio for me.


Nicholas Ring said...

Good work Figvam.
Thanks for taking the time to describe the infrastructure behind the automounting process. It was a significant help in resolving this irritating problem.

My distro of choice (arch linux) has a different process for launching my desktop environment so I needed to go about it in a similar but slightly different way to what you described. By using ck-launch-session to launch xfce in my existing ~/.xinitrc

exec ck-launch-session startxfce4

Apparently GDM/KDM would manage the ConsoleKit registration process for me automatically. But I'm happy to stick with XDM for now.

Anonymous said...

Wow thanks,
I'm also a archlinux user. I had the same problem and adding the following line in my xinirc solved it:

exec ck-launch-session openbox-session

instead of just
exec openbox-session

Happy computing!

Anonymous said...

Arch user here; thanks for your highly informative post. Like Nicholas, doing "exec ck-launch-session startxfce4" instead of just "exec startxfce4" solved it for me. Now my Lumix camera connects/automounts via USB!

Thanks again.

figvam said...

Strangely "exec ck-launch-session startxfce4" from the command line wouldn't work for me in Fedora.

By the way, apparently there's no need to copy xinitrc from /etc/xdg/xfce4. Just put this line in ~/.config/xfce4/xinitrc and make it u+x:

"exec /usr/bin/ck-xinit-session sh /etc/xdg/xfce4/xinitrc"

I'll amend my post accordingly.