[systemd-devel] How to make an encrypted disk mentioned in /etc/crypttab "optional"?

[Aaron Rainbolt]

Good morning/evening, and thanks for your time.

I'm attempting to create a Fedora-based immutable distro (not based on 
Silverblue) that stores user data in an encrypted /home partition. The 
goal is to have something that behaves somewhat similar to Chrome OS. 
One feature I'm attempting to implement is a "guest mode", whereby a 
user can sign into the system without providing any password, but if 
they do so they don't gain access to the system's owner's data and 
virtually anything they do is erased upon shutdown.

In order to do this, I have two /home directories - one is part of the 
(immutable) root filesystem, which can only be written to thanks to a 
Dracut-created ramdisk overlay. The other is stored in an encrypted 
partition. I'm using a crypttab line like this to prepare the encrypted 
partition:

     firebook-home LABEL=firebook-crypt none luks,discard,nofail

And I'm using an fstab line like this to mount it:

     /dev/mapper/firebook-home /home ext4 defaults,nofail 0 0

Note that I've marked both of these with "nofail" - the goal is that the 
user will be prompted for their password by systemd upon boot, but if 
they do not provide the password (by intentionally providing a wrong 
password three times), the encrypted drive should not be mounted and the 
system should boot normally using the ephemeral home directory provided 
by the root filesystem + ramdisk overlay.

This seems to be *almost* working, however if I intentionally provide a 
wrong password to the password prompt a few times, it doesn't actually 
"give up" on getting a password from me. What it does instead is it 
stops asking me for the password at boot, but then rather than starting 
GNOME it just leaves me at a console screen. If I am able to get GDM to 
appear somehow, I can't sign in.

What I end up doing is switching to a TTY, signing in, and then 
elevating to root to troubleshoot. Once I've elevated to root, I get a 
`wall` message informing me that the system is *still waiting* for a 
password and that I need to run `systemd-tty-ask-password-agent` (I 
think?) to provide it. If I go ahead and do this, then restart GDM, I'm 
able to sign in after that. (I could be wrong about what command it's 
asking me to run, but I think it was `systemd-tty-ask-password-agent`.)

 From my research, it looks like systemd is refusing to ever truly "give 
up" on getting the password for the encrypted /home directory, despite 
the use of `nofail` in the fstab and crypttab files. I'm not finding any 
documentation on how to get systemd to "give up" on getting the 
password. For my particular use case, I'd like systemd to just forget 
that the encrypted drive exists at all if the wrong password is given. 
If the user wants to mount the encrypted drive after that, they should 
either reboot or use cryptsetup manually.

Is there any way to make systemd "give up" on getting a password?

Thanks for your help!

Aaron