Teach initrd to find root partition on the same device as the boot or ESP one

[travier]

Describe the enhancement

When booting up, we want the initrd to pick the right root partition from the ones available on potentially multiple disks. The challenge is on the first boot as we generally start from a generic image with generic UUIDs and do not have a clear root UUID on the kernel command line.

Thus we want to implement some logic in the initrd to make sure that we would pick the root partition from the same disk as the partition that was used to boot, either in BIOS or EFI mode.

For the EFI mode, this logic is similar (or exactly like) systemd-gpt-auto-generator .

We also want this logic to use it for every boot in the UKI case, were we can not update the kernel command line to set a root=$UUID argument and where we want to instead rely on PART UUID from the Discoverable Partition Specification.

See also:

• "Alternative" option in Pre-bind bootloader to bootfs at installation time coreos-installer#798
• Current implementation in rdcore: add bind-boot command coreos-installer#671

This should help with: https://issues.redhat.com/browse/OCPBUGS-61102

System details

N/A

Additional information

No response

[jlebon]

• "Alternative" option in Pre-bind bootloader to bootfs at installation time coreos-installer#798
• Current implementation in rdcore: add bind-boot command coreos-installer#671

To clarify, that RFE and implementation are not alternatives, but complementary. Those are about making sure GRUB picks the right bootfs (on every boot).

This RFE is about making sure the initramfs picks the right rootfs (on first boot only normally since we inject root=UUID=... for subsequent boots, but on every boot for UKI).

[travier]

Also related to https://gitlab.com/fedora/bootc/tracker/-/issues/38.