Introduction to System-as-Root (SAR)

The Android ecosystem constantly evolves, bringing significant changes to how the operating system is structured on devices. One such pivotal change is the adoption of System-as-Root (SAR). Introduced in Android 9 Pie and mandated for all devices launching with Android 10 and later, SAR fundamentally alters the partition layout and the boot process. Traditionally, Android devices had a separate ramdisk.img responsible for initializing the system, distinct from the system.img. With SAR, the root filesystem is now directly embedded within the system.img, and the ramdisk is merged into the boot.img (or more recently, init_boot.img on devices with Android 12+).

This architectural shift provides several benefits, including improved boot-time integrity checks and the ability to update the entire system partition atomically via A/B seamless updates. However, for enthusiasts and developers accustomed to flashing custom ROMs, kernels, and rooting, SAR introduces a new set of challenges and methodologies. Understanding the nuances of SAR is crucial for successfully modifying your device without bricking it.

Understanding the SAR Partition Layout

In a System-as-Root setup, the device’s storage layout is significantly re-architected. The most notable change is the introduction of the super partition, which acts as a container for several dynamic partitions:

• system: Contains the core Android OS, including the root filesystem.
• vendor: Holds device-specific hardware abstraction layers (HALs) and vendor binaries.
• product: Contains OEM-specific applications and features.
• odm: Original design manufacturer customizations (less common on modern SAR).

These dynamic partitions are not fixed in size but are allocated space within the larger super partition. This allows for more flexible storage management and facilitates A/B updates, where `system_a`, `vendor_a`, `product_a` and `system_b`, `vendor_b`, `product_b` pairs exist, allowing one set to be updated while the other is active.

For the boot process, SAR devices typically use either a combined boot.img (for older SAR devices) or a separate init_boot.img (for Android 12+). The `init_boot.img` contains the kernel and the initial ramdisk (which is now minimal and mostly handles mounting the `system` partition). The actual root filesystem is located within the `system` partition itself.

Key Differences from Non-SAR:

• No separate ramdisk.img; it’s integrated into boot.img/init_boot.img.
• system partition is the root partition (`/`).
• Dynamic partitions within the super container, managed by `Logical Partition Manager (LPM)`.

Prerequisites and Essential Tools

Before embarking on flashing custom images, ensure you have the following:

• Android SDK Platform-Tools: This includes adb and fastboot, essential for communicating with your device. Ensure they are up-to-date and added to your system’s PATH.
• Unlocked Bootloader: This is paramount. Flashing custom images requires an unlocked bootloader. Be warned: unlocking the bootloader will factory reset your device and void your warranty. The command is typically fastboot flashing unlock or fastboot oem unlock.
• Stock Firmware: Always have the complete stock firmware package for your device. This is your lifeline in case anything goes wrong. You’ll need to extract specific images like init_boot.img (or boot.img) from it.
• Custom Images/Kernels: Download the custom ROM (e.g., a GSI), kernel, or recovery image you intend to flash, ensuring it’s compatible with your device and SAR.

Step-by-Step Guide to Flashing SAR Devices

Step 1: Preparing Your Device and Firmware

1. Enable Developer Options and USB Debugging: Go to Settings > About Phone, tap ‘Build number’ seven times. Then, in Developer Options, enable ‘USB debugging’ and ‘OEM unlocking’.
2. Reboot to Bootloader: Connect your device to your PC. Open a terminal or command prompt and type:

   adb reboot bootloader

   Alternatively, power off your device and use a key combination (e.g., Volume Down + Power) to enter fastboot mode.

3. Extract Stock Images: From your downloaded stock firmware package, extract the crucial images. For Android 12+ SAR devices, focus on init_boot.img. For older SAR devices, look for boot.img. You might also need super.img, or individual dynamic partition images like system.img, vendor.img, etc., if flashing a full factory image without using `fastboot update`.

Step 2: Patching for Root Access (Magisk Example)

Rooting a SAR device primarily involves patching the init_boot.img (or boot.img). This allows Magisk to inject its necessary components at the earliest stage of the boot process.

1. Transfer init_boot.img: Copy your device’s stock init_boot.img to your device’s internal storage.
2. Patch with Magisk: Install the Magisk app on your device. Open Magisk, select
   
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