Not sure if the original reporter had the exact same problem as me, but I can add some analysis and confirm the conclusion "overlayroot actually depends on busybox"
Note: My case, as well as the original report refers to bookworm, however I see no changes in either klibc, overlayroot or the Debian dependencies that would prevent the exact same problem from happening on trixie. In my case - debootstrap bookworm - apt install --no-install-recommends grub-pc overlayroot linux-image-amd64 - in /etc/overlayroot.local.conf set overlayroot_cfgdisk=/dev/xvdb - in [/dev/xvdb]/overlayroot.conf set overlayroot=/dev/xvdb the system booted, but overlayroot did not activate. I eventually traced the failure down to line 474 of /usr/share/initramfs-tools/scripts/init-bottom/overlayroot where trying to mount the cfgdisk using | mount -o ro "$cfgdev" "$mp" fails with the error message | mount [-r] [-w] [-o options] [-t type] [-f] [-i] [-n] device directory This error message comes from klibc's mount command. klibc's utils get included in the initramfs as a fallback if busybox is NOT installed (which is the case with the above setup). The error message is a bit misleading: The actual cause of the error is that the overlayroot script does not pass the filesystem type to the mount command via the -t option. klibc's mount implementation _requires_ the caller to specify the filesystem type. (I reported the misleading error message as https://lists.zytor.com/archives/klibc/2025-December/004856.html ) In this respect klibc's mount differs from busybox, where -t is optional and busybox will iterate through the supported filesystems until one works. In the overlayroot script, mounting of the backing device in line 764 also calls mount without the -t option. This is where the non-cfgdisk case would fail with klibc. The conclusion is that overlayroot can not work with klibc's mount implementation. In other words: overlayroot needs busybox to work and therefore should declare it as dependency. Just like the original reporter, I can confirm that after installing busybox, rebuilding the initramfs and rebooting, the overlayroot worked.
