LightDM Greeter Uses User's Background

Symptom:

In a display manager for the X Window System, specifically LightDM, the main process spawns an X-Server and a session child for the LightDM special user, which runs one of the LightDM greeters, which asks you for your loginID and password. If that's satisfactory, the LightDM user will log out, a new session child will be started, and it will start a new X-Server and the user's preferred session (XFCE in my case).

As the first step in starting each session, /etc/X11/xdm/Xsetup shows a background image (or solid color) on the root window, which in my case has a custom logo. However, the LightDM greeter has a feature (that's not a bug, it's a feature!) that it prefills the loginID of the last user to log in, and in the common case that other users rarely log in, the previous user only has to give his password. Starting in lightdm-1.28.0-3.1.x86_64 or xfce4-session-4.14.0-1.1.x86_64 (about 2019-08-27) this feature has been extended to show the previous user's own background. Without the custom logo. Growl!

The first time after the update that the user logs in, PolicyKit pops a dialog box saying Privileges required to set own greeter properties; it wants the user's password. This can produce calls to the helpdesk. Growl.

How to fix:

The fix is very simple but has to be done by (or for) each user: prevent the LightDM special user from reading the user's background image file. Then it will leave alone the background it started with, provided by Xsetup.

• If the user has a personal background image, e.g. a photo or artwork, change the mode to 600 (e.g. chmod u=rw my-cat.jpeg).

• If public artwork is used, or if there is a reason that the user's image has to be readable by other users, he should make a copy, change its permissions, and configure that as the background image.

Credit for this fix to the unnamed author of Set LightDM Wallpaper that is Independent of the User's Wallpaper, published 2012-07-02 in an Ubuntu/Mint context. He also shows a less brutal (but I suspect, more fragile) method using d-bus to reconfigure the background settings.

There's got to be a better way to influence the greeter's behavior, but I can't find a relevant configuration file.