[Fixed] Attention when upgrading: only black screen with cursor instead of gdm

[josch]

Hi,

when upgrading to the latest packages from Debian unstable and rebooting, you will only see a black screen with a cursor on the upper left corner instead of gdm coming up with the login prompt.

I was unable to track down the problem so far. Help is welcome.

But you should probably hold off on upgrading your system until this is fixed.

Update 1: Pressing ctrl+alt+F5 (or F2, F3, F4, …) gets you to a textual login prompt. You can start sway from there or continue working on the text terminal. This is how you’ll be able to fix this situation once a solution is known without resorting to booting from an sd-card.

Update 2: found the culprit. To work around the issue, downgrade libmozjs-140-0 to 140.6.0-1 Details in this bugreport: I Challenge Thee.

So to fix this, log in to another tty (ctrl+alt+F5 for example), put an older snapshot timestamp, like: debian:/ 2026-01-15 20:27:01 - snapshot.debian.org and then downgrade: sudo apt install libmozjs-140-0=140.6.0-1

[blipblub]

Thanks for reporting! So now the bug is fixed, and it is safe to upgrade?

[josch]

Yes. Upgrading to the latest packages from Debian unstable will fix this issue.

I verified this yesterday myself.

[josch]

When I tried to apply the fix by switching to a different text terminal by pressing ctrl+alt+f5, the terminal would become unresponsive to keyboard input after a few dozen seconds. Can somebody confirm?

If fixing this via another tty does not work and because there is no graphics support in u-boot so that you could choose the single user mode rescue shell in the boot menu, you have the following options:

• connect to serial via uart and log into your system that way
• if you have an ssh server configured, log in via ssh after booting
• flash a rescue system to sd-card, boot that and mount and chroot your system on eMMC/NVMe from the rescue system

To make the last option easier, I started writing a new script. I put an initial version here:

Testers wanted for new tool reform-rescue-shell (maybe interesting for users with the gdm issue)

Hi, while the issue with gdm/mozjs/gcj is fixed in unstable, it might be tricky to apply the fix without being able to login. Since we still don’t have display support in u-boot, we cannot just boot Debian into single user mode to fix problems like this. You can switch to another tty but when I tried that, the tty froze after a short time (maybe because gdm tries to restart?). If you don’t have a serial uart adapter and you didn’t set up ssh access, then upgrading your system to a fixed version…

I welcome tests and feedback for this script. Thank you!

[computergripe]

I have found since upgrading to the most recent version of libmozjs that the black screen with the cursor still shows up for a moment or two before the graphical login starts; I’m curious if anyone else who has fixed this problem is seeing something similar?

[elb]

My Pocket Reform has shown the cursor for a second or two when using gdm since I first installed it. I think gdm is just slow.

[computergripe]

This is good to know, thank you!

[O.o]

I just did an apt upgrade and gdm3 is now broken in the same way - no login screen, only a blinking cursor. The systemd status says GdmLocalDisplayFactory: maximum number of X display failures reached: check X server log for errors or some such.

So I switched to lightdm. gdm3 has been super-slow for me for months anyway, and this is the last straw.