I think I found the source of the problem.
On my traditional Ubuntu 18.04 VM, I have NoMachines NX server and client installed.
On boot, it starts up several server root process (nxserver, nxd, nexec) as well as 2 user processes (nxclient and nxnode).
Killing those processes or terminating the session with '/etc/NX/server --terminate <session>' before logging in to the graphical console allows Guest Additions to work.
These client processes start along with the nxserver at boot because I have configured the following parameters in /usr/NX/etc/server.cfg
Code: Select all
CreateDisplay 1
DisplayOwner <username>
Alternatively, I can prevent these processes from starting up by setting :
In which case, GA works fine.
Note that the reason setting .Xauthority to root ownership (with perm 600) works is because it prevents the nxclient processes from starting -- i.e. it has nothing to do directly with VBox.
I imagine that VBox GA may be getting confused over which is the physical console and which is the Nomachine console perhaps causing the problem.
Not sure if this is a bug or a feature... but it does explain what is going on...