Proposal: Support for UKI (Unified Kernel Images) synchronization during Btrfs backup and restore

Title Proposal

Proposal: Support for UKI (Unified Kernel Images) synchronization during Btrfs backup and restore

Description

With the increasing adoption of UKIs (Unified Kernel Images) and systemd-boot, there is a growing challenge for system restore utilities like Timeshift.

In a typical Btrfs setup, Timeshift creates snapshots of the root subvolume (@). However, the EFI System Partition (ESP), where the UKIs reside, is usually a separate FAT32 partition and is not included in the Btrfs snapshot.

The Problem

When a user restores a snapshot from a previous state:

1. The root filesystem (/) is reverted to an older version (including old kernel modules).
2. The ESP remains in its current state (with a newer UKI/Kernel).
3. Result: The system fails to boot or modules fail to load because the restored /lib/modules/ do not match the newer kernel in the ESP.

Proposed Solution

I have developed and tested a hook-based approach that ensures atomic synchronization between the root snapshot and the UKI. The logic is as follows:

1. Pre-Backup Hook: Just before the snapshot is created, the current functional UKI is copied from the ESP to a directory inside the root (e.g., /etc/timeshift/uki-backup/). This ensures the UKI is "captured" inside the Btrfs snapshot.
2. Post-Restore Hook: Immediately after a restoration, the script takes the restored UKI from /etc/timeshift/uki-backup/ and copies it back to the physical ESP partition, ensuring the boot binary matches the restored modules.

Reference Implementation

I have published a working implementation with robust scripts that handle:

• Dynamic detection of /boot vs /efi mount points.
• Safe remounting of the ESP (handling RO/RW states).
• Integration with timeshift-autosnap and native Timeshift restore hooks.

You can review the logic here: https://github.com/jairogaleano/timeshift-uki-hooks

Goal

I believe this logic (or a native version of it) would be a valuable addition to Timeshift to support modern Linux boot standards. At the very least, providing an official or documented way to handle these "out-of-snapshot" boot files would greatly benefit the community.

I'm open to discussing how this could be integrated into the core or if there's interest in a pull request regarding hook documentation.

No repository files, tests, or entry points are named. Start by reviewing the linked timeshift-uki-hooks implementation and the repository's Btrfs backup and restore entry points; done means an agreed core or documented hook approach for synchronizing the ESP UKI with the restored root.