Introduction: Unlocking Android 15 Developer Preview Devices

Rooting Android Developer Preview builds has always presented unique challenges, primarily due to heightened security measures like verified boot, dm-verity, and SELinux enforcing policies. With Android 15 DP, these mechanisms are more robust than ever, making direct patching of the stock boot image insufficient for many rooting methods. This expert guide delves into the intricate process of building a custom permissive kernel and integrating it into a boot image specifically for Android 15 DP, enabling advanced modifications and system-level access.

A permissive SELinux policy is often a prerequisite for many root solutions (like Magisk) to function correctly on modified systems, as it relaxes restrictions that would otherwise block operations required for root access. Building a custom boot image allows us to embed a kernel compiled with these permissive settings, bypassing the default enforcing state and other security checks. This is a critical step for anyone looking to truly unlock their Android 15 DP device for development or customization.

Prerequisites: Setting Up Your Build Environment

Before embarking on this journey, ensure you have a robust Linux-based build environment. Ubuntu LTS (20.04 or 22.04) is highly recommended. You’ll need substantial disk space (at least 200GB) and a powerful CPU with plenty of RAM (16GB+).

Essential Tools and Dependencies

First, install the necessary packages:

sudo apt update
sudo apt install git-core gnupg flex bison build-essential zip curl zlib1g-dev gcc-multilib g++-multilib libc6-dev-i386 libncurses5 lib32ncurses5-dev x11proto-core-dev libx11-dev lib32z1-dev libgl1-mesa-dev libxml2-utils xsltproc schedtool libssl-dev libsdl1.2-dev rsync ccache android-sdk-platform-tools-common

Next, set up `repo` for syncing AOSP sources:

mkdir ~/bin
PATH=~/bin:$PATH
curl https://storage.googleapis.com/git-repo-downloads/repo > ~/bin/repo
chmod a+x ~/bin/repo

Obtaining Device-Specific Kernel Source

For Android 15 DP, you’ll need the corresponding kernel source. Google typically releases device-specific kernel sources for Pixel devices. Navigate to the AOSP kernel project or the specific device’s kernel repository. For this guide, we’ll assume a generic Pixel device and use the AOSP kernel manifest. Initialize and sync a minimal AOSP tree:

mkdir android15-dp-kernel
cd android15-dp-kernel
repo init -u https://android.googlesource.com/platform/manifest -b android-15.0.0_r1 --depth=1
repo sync -j$(nproc) platform/build platform/bionic platform/external/toolchain-utils kernel/prebuilts/build-tools kernel/prebuilts/rust kernel/prebuilts/clang kernel/configs

This will fetch the necessary build tools and a minimal set of AOSP components, including the prebuilt Clang toolchain. Then, locate your device’s kernel source. For Pixel devices, this is often found under `kernel/google/devices/`. For example, for a hypothetical Pixel device:

git clone https://android.googlesource.com/kernel/google/gs201.git kernel/google/gs201

Adjust the repository URL and path based on your specific device’s kernel source.

Modifying the Kernel Configuration for Permissive SELinux

Once you have the kernel source, navigate into its directory. The exact path will vary, but for a Pixel device, it might be `kernel/google/gs201` or similar.

Locating and Editing the .config

Identify the correct defconfig for your device. This is usually found in `arch/arm64/configs/`. For Pixel 6/7/8 devices, it might be `gki_defconfig` or a device-specific variant. Copy it to `.config`:

cd kernel/google/gs201 # Replace with your kernel source path
export ARCH=arm64
export CROSS_COMPILE=<path_to_aosp_prebuilts>/bin/aarch64-linux-android-
export PATH=<path_to_aosp_prebuilts>/bin:$PATH

make O=out <your_device>_defconfig # e.g., make O=out gki_defconfig
cp out/.config .

Now, modify the `.config` file to set SELinux to permissive by default and disable `dm-verity` if necessary. Open the `.config` file with your preferred text editor:

nano .config

Search for `CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE` and change its value to `”permissive”`:

CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE="permissive"

Also, locate `CONFIG_DM_VERITY` and ensure it’s either unset or commented out (`# CONFIG_DM_VERITY is not set`) if you plan extensive system modifications. While setting SELinux to permissive is the primary goal, disabling `dm-verity` can prevent boot loops on heavily modified partitions.

Building the Custom Kernel and Boot Image

Compiling the Kernel

With the `.config` adjusted, it’s time to build the kernel. Ensure your `CROSS_COMPILE` and `ARCH` environment variables are correctly set, pointing to the AOSP prebuilt toolchain.

# Assuming you are in the kernel source root, e.g., kernel/google/gs201
export PATH="$(pwd)/../prebuilts/clang/host/linux-x86/clang-r498227b/bin:$(pwd)/../prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/bin:$PATH" # Adjust toolchain path
export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-android-
export CLANG_TRIPLE=aarch64-linux-gnu-
export LLVM=1
export LLVM_IAS=1

make O=out -j$(nproc)

This command will compile your kernel. Upon successful compilation, you will find the `Image.gz` (or `Image`) and `dtb.img` (device tree blob, if applicable) in the `out/arch/arm64/boot/` directory.

Creating the Custom Boot Image

You’ll need `mkbootimg` to pack your new kernel, along with an existing ramdisk, into a flashable `boot.img`. You can usually extract the ramdisk from your device’s stock `boot.img` using tools like `Magiskboot` or `AOSP dump_image.py` script.

1. Extract Stock Boot Image: Obtain your device’s stock `boot.img` (e.g., from the factory image for Android 15 DP). Use `AIK-TWRP` or a similar tool to unpack it and get the `ramdisk.img`.

   # Example using AIK-TWRP (assuming it's in your PATH)
   unpackimg --input boot.img
   

   Alternatively, if you have the full AOSP source synced, you can build a generic ramdisk, though it’s safer to reuse the device’s original.

2. Build `mkbootimg` (if not already present): If you synced AOSP, `mkbootimg` should be available in `out/host/linux-x86/bin/`. Otherwise, you might need to build it from AOSP `system/core/mkbootimg` source.

3. Assemble the new `boot.img`:

   # Assume kernel Image and dtb are in 'out/arch/arm64/boot/'
   # Assume ramdisk.img is extracted to current directory
   
   # Adjust cmdline, base, pagesize, board, etc., to match your device's stock boot.img parameters
   # You can get these by unpacking your original boot.img
   MKBOOTIMG_PATH="<path_to_aosp_root>/out/host/linux-x86/bin/mkbootimg"
   
   $MKBOOTIMG_PATH  
   --kernel out/arch/arm64/boot/Image.gz  
   --ramdisk ramdisk.img  
   --dtb out/arch/arm64/boot/dts/google/gs201-v2.dtb  # Adjust DTB path if different
   --cmdline "androidboot.hardware=gs201 console=ttyS0,300000 buildvariant=userdebug androidboot.selinux=permissive"  
   --base 0x40000000  
   --pagesize 4096  
   --board ""  
   --os_version 15.0.0  
   --os_patch_level 2024-03-01  # Adjust to current DP patch level
   --header_version 4  # Or 3, depending on device
   -o custom_boot.img
   

   Important: The `–cmdline`, `–base`, `–pagesize`, `–board`, `–os_version`, `–os_patch_level`, and `–header_version` parameters *must* match those of your device’s original `boot.img`. Incorrect values can lead to boot loops. Unpack your original `boot.img` to get these details.

Flashing and Verification

Once `custom_boot.img` is created, you can flash it to your device using `fastboot`. Ensure your device is in fastboot mode.

fastboot flash boot custom_boot.img
fastboot reboot

After the device reboots, verify SELinux status using a terminal emulator on the device or `adb shell`:

adb shell su -c "getenforce"

It should return `Permissive`. You can also check the kernel version and build date to confirm your custom kernel is running. With a permissive kernel, you are now ready to proceed with installing Magisk or other root solutions, leveraging the relaxed security context for full system control on your Android 15 DP device.