[Bug 1938692] [NEW] systemd-cryptsetup-generator generated unit for root filesystem fails with keyfile

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I'm using Ubuntu 20.10 on a Raspberry Pi 4b.

/etc/crypttab:
# <target name> <source device>         <key file>      <options>
crypt_root UUID=12989868-19fe-4834-90a8-f1bf13977745    /dev/disk/by-label/wopr_keys:/crypt_root_key  luks,discard,keyscript=/lib/cryptsetup/scripts/passdev,initramfs,x-initrd.attach
crypt_swap /dev/disk/by-id/scsi-SSamsung_Portable_SSD_T5_4B1C18654321-part2    /dev/urandom  swap,discard,cipher=aes-xts-plain64,size=256,plain

/etc/fstab
LABEL=system-boot       /boot/firmware  vfat    defaults        0       1
/dev/mapper/crypt_root  /               btrfs   defaults,ssd,compress=zstd,discard=async,relatime,x-initrd.mount,subvol=@     0 0
/dev/mapper/crypt_root  /home           btrfs   defaults,ssd,compress=zstd,discard=async,relatime,x-initrd.mount,subvol=@home 0 0
/dev/mapper/crypt_swap  none            swap    defaults        0       0

/boot/firmware/cmdline.txt
dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 cryptdevice=LABEL=crypt_root:crypt_root root=/dev/mapper/crypt_root rootflags=subvol=@ rootfstype=btrfs rootwait fixrtc zswap.enabled=1

On the Raspberry Pi, it seems that cryptsetup-initramfs is responsible
for unlocking devices in the initramfs. Things worked perfectly before I
started using a keyfile on a USB Drive to unlock my root device.

The issue is that /lib/cryptsetup/scripts/passdev expects the keyfile to
be in a particular format, and systemd-cryptsetup-generator expects a
slightly different format. So cryptsetup-initramfs successfully unlocks
the root filesystem and mounts it, but systemd's auto-generated unit
attempts to mount the (in its view) incorrectly formatted keyfile device
and fails. As a result the entire boot fails. In the rescue shell, I can
see that all filesystems have mounted correctly.

I can think of a couple of solutions:
1. I disabled systemd luks unit generation by passing luks=no in the kernel command line. This also disables other crypttab entries from unlocking, so my swap partition mount fails.
2. I tried disabling systemd unit generation just for my root device with x-initrd.attach,noauto,nofail,initramfs options. This resulted in the exact same error. I also tried adding noauto to the fstab entry for my root device and this did not work.
3. Arch Linux has a separate crypttab.initramfs file that is only loaded into the initramfs. Could this be implemented somehow?

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: bot-comment cryptsetup initramfs luks systemd
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systemd-cryptsetup-generator generated unit for root filesystem fails with keyfile
https://bugs.launchpad.net/bugs/1938692
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