[Bug]: systemd reports `[FAILED] Failed unmounting tmp.mount- Temporary Directory /tmp` if VBoxDRMClient is running at guest shutdown time

[ArrayBolt3]

Version

7.2.4

Host OS Type

Linux

Host OS name + version

Kubuntu 24.04 LTS

Host Architecture

x86

Guest OS Type

Linux

Guest Architecture

x86

Guest OS name + version

Debian 13 (trixie)

Component

Guest Additions/Other

What happened?

If a Debian Trixie guest with Guest Additions installed in it is shut down without the user manually stopping VBoxDRMClient first (i.e., by running sudo pkill VBoxDRMClient), an error message appears, [FAILED] Failed unmounting tmp.mount - Temporary Directory /tmp. appearing during a guest shutdown or reboot. I'm guessing VBoxDRMClient doesn't get stopped before the system attempts to unmount /tmp during shutdown, and I can see using lsof that VBoxDRMClient has open files in /tmp.

VBox.log

How can we reproduce this?

1. Download the Debian 13 netinstaller from https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-13.1.0-amd64-netinst.iso
2. Create a new VM with the following settings:

• VM Name: d13-plain
• OS: Linux
• OS Distribution: Debian
• OS Version: Debian 13 Trixie (64-bit)
• Uncheck "Proceed with Unattended Installation"
• Set base memory to 2048 MB
• Set number of CPUs to 2
• Create a new 20 GB disk image file
• Ensure that the default graphics controller of VMSVGA is used

3. Install a basic Debian system with no desktop environment
4. In the guest, log into a TTY, then click Devices > Insert Guest Additions CD Image...
5. In the guest, sudo mount /dev/sr0 /mnt; cd /mnt; sudo ./VBoxLinuxAdditions.run; sudo reboot
6. In the guest, run pgrep VBoxDRMClient to ensure the service is running
7. In the guest, run sudo reboot
8. See that the guest reports [FAILED] Failed unmounting tmp.mount - Temporary Directory /tmp. during the shutdown process
9. In the guest, log into a TTY
10. In the guest, run sudo pkill VBoxDRMClient
11. In the guest, run sudo reboot
12. See that the guest does not report any issue with unmounting /tmp

Did you upload all of your necessary log files, screenshots, etc.?

• Yes, I've uploaded all pertinent files to this issue.