Introduction to Android A/B Updates and Immutability

Android’s A/B (seamless) update system has revolutionized how operating system updates are delivered, significantly enhancing reliability and user experience. By maintaining two identical system partitions (A and B), updates can be applied to the inactive slot in the background. If an update fails, the device can simply revert to the previous, working slot. This system, however, introduces a formidable challenge for advanced users and developers: achieving persistent, system-level modifications. The core issue lies in the read-only nature of the active system partition and the way updates overwrite the inactive one. Traditional methods of modifying /system often fail to survive reboots or updates.

This article dives deep into leveraging OverlayFS to inject persistent changes into an A/B partitioned Android system. OverlayFS provides a flexible mechanism to create a writable layer over a read-only filesystem, allowing modifications to appear permanent without altering the underlying immutable partitions. This technique is crucial for custom ROM developers, security researchers, and enthusiasts looking to deeply customize their Android devices beyond typical root access.

The Mechanics of Android A/B Updates

Android A/B updates operate on a simple yet effective principle. A device typically has two sets of partitions: system_a, vendor_a, boot_a, and system_b, vendor_b, boot_b. At any given time, one set is