Can I use pkexec in a python script or a .desktop file?

Question

From the following questions

• Why is gksu no longer installed by default?
• When to use pkexec vs. gksu/gksudo?

we see that gksu will no longer be supported in the long term, and it will not be installed by default from >= 13.04. Instead we should use pkexec which does its job fine for non-graphical applications but not for apllications on the GUI:

pkexec gedit

when replacing gksu in a .desktop file

EXEC=pkexec /usr/bin/gedit

or when I run a python script to run a graphical application with root permissions I get the following error:

>>>subprocess.Popen(['pkexec','gedit'])
** (gedit:3203): WARNING **: Could not open X display

How would I have to re-write my scripts or my .desktop files to support an authentication dialog and run an application as root if I should not have it depend on gksu?

Answer 1

First create a .policy action file in /usr/share/polkit-1/actions/. It's conventional to name action files in a "vendor hierarchical" way, such as com.ubuntu.pkexec.gparted.policy or org.debian.apt.policy

Then paste the following content:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC
 "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/PolicyKit/1/policyconfig.dtd">

<policyconfig>

  <action id="org.freedesktop.policykit.pkexec.run-[Short Program Name]">
    <description>Run [Full Program Name]</description>
    <message>Authentication is required to run [Full Program Name]</message>
    <defaults>
      <allow_any>no</allow_any>
      <allow_inactive>no</allow_inactive>
      <allow_active>auth_admin_keep</allow_active>
    </defaults>
    <annotate key="org.freedesktop.policykit.exec.path">[Full Program Path]</annotate>
    <annotate key="org.freedesktop.policykit.exec.allow_gui">TRUE</annotate>
  </action>

</policyconfig>

Replace [Short/Full Program Name/Path] with appropriate values, for example gedit, gedit Text Editor and /usr/bin/gedit. <action id> value does not need to match the chosen filename (and a single file can contain multiple actions), but conventionally filename is the prefix for all its actions.

After saving the file the specific program would run with X and GUI and such.

Another fix seems to be: Add the following line in /etc/pam.d/polkit-1:

session optional pam_xauth.so

Answer 2

Yet another fix for user scripts: Determine the appropriate environment variables inside your script.

You could use a snippet like the following one to do this:

getXuser() {
        user=`pinky -fw | awk '{ if ($2 == ":'$displaynum'" || $(NF) == ":'$displaynum'" ) { print $1; exit; } }'`
        if [ x"$user" = x"" ]; then
                startx=`pgrep -n startx`
                if [ x"$startx" != x"" ]; then
                        user=`ps -o user --no-headers $startx`
                fi
        fi
        if [ x"$user" = x"" ]; then
               user=$(pinky -fw | awk '{ print $1; exit; }')
        fi
        if [ x"$user" != x"" ]; then
                userhome=`getent passwd $user | cut -d: -f6`
                export XAUTHORITY=$userhome/.Xauthority
        else
                export XAUTHORITY=""
        fi
        export XUSER=$user
}


for x in /tmp/.X11-unix/*; do
   displaynum=`echo $x | sed s#/tmp/.X11-unix/X##`
   getXuser;
      if [ x"$XAUTHORITY" != x"" ]; then
        export DISPLAY=":$displaynum"
      fi
done

(based on the ACPI getXuser function)

If you find your .desktop file still not working you could try wrapping your pkexec commandline in a sh snippet, e.g.:

Exec=sh -c "pkexec --user root script_that_needs_root.sh"

The last problem is a known bug, apparently:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=690339

https://bugzilla.xfce.org/show_bug.cgi?id=9373

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=650038

https://bugzilla.gnome.org/show_bug.cgi?id=686059