Access another user's D-Bus session

Question

Let's assume this kind of situation: we have one user logged in, executing some application through sudo or su. This user has got a dbus-daemon running.
However, when an application running with root privileges tries to access D-Bus, it just spawns another dbus-daemon, owned by root user. That's not a desired situation.

Is there a way to gain access to D-Bus session of user who ran the application through sudo or su?

Answer 1

If you're on a systemd distro, the relatively new machinectl shell command can do the work of su/sudo, and it will also set session variables like XDG_RUNTIME_DIR and DBUS_SESSION_BUS_ADDRESS. So for example, if I want to run systemctl --user as user test, the normal approach will fail:

$ sudo --user=test systemctl --user
Failed to connect to bus: No such file or directory

But this way works:

$ sudo machinectl shell --uid=test .host -- /usr/bin/systemctl --user

If you need to "reach back" into the user session that invoked a sudo script, you could use the SUDO_USER/SUDO_UID to hack something together.