Build Android Kernel and load your module in Emulator

Hi Guys!!!Hope you are doing well !!!. 

Today I am going to shade some light on Android kernel and module building.

In my previous 3 post I have given details on 

• AIDL for HIDL : The new paradigm shift in Android HAL
• Practical example of AIDLs for HIDLs
• Implement the User App to call Kernel API

For above 3 post we need kernel module invstr to get build and loaded once our Emulator/device is ready.

Step 1  Download Android kernel 

Why we need to download Android Kernel separately ?

The AOSP contains only pre-built kernel binaries. When AOSP system image is build, it copies the prebuilt kernel image to the output folder. Hence we need to download and build it with our changes.

you can use android official page to download kernel  as per your choice.

I have downloaded following Android kernel . 

Android Common Kernel ---- common-android13-5.15

I have downloaded it because I am using Android 13 for my previous  3 blogs .

Note :- Different Android kernel version configured build differently

Step 2 Build the Android common kernel
Android Common Kernels is used to run on Emulator. For real hardware devices, SoC companies (MTK, Qualcomm) will provide their customized kernel and build instruction.

Android 11 introduced GKI, which separates the kernel into a Google-maintained kernel image and vendor maintained-modules, which are built separately.

List of few Android kernel branch for Emulator

Android 11

• common-android11-5.4

Android 12

• common-android12-5.4
• common-android12-5.10

Android 13

• common-android13-5.10
• common-android13-5.15

Legacy dessert kernel branches
Legacy dessert kernels were created to guarantee that new feature development didn’t interfere with merging from the Android Common Kernel

Android 10

• android-4.9-q
• android-4.14-q
• android-4.19-q

Android 11

• android-4.14-stable
• android-4.19-stable

Building 

Install libs for building kernel

sudo apt install libssl-dev libelf-dev

A common kernels are generic, customizable kernels and therefore don’t define a default configuration. 

We have to set some environment settings:

Environment variable	Description	Example
BUILD_CONFIG	Build config file from where you initialize the build environment. The location must be defined relative to the Repo root directory. Defaults to build.config. Mandatory for common kernels.	BUILD_CONFIG=common/build.config.<target>.x86_64
CC	Override compiler to be used. Falls back to the default compiler defined by build.config.	CC=clang
DIST_DIR	Base output directory for the kernel distribution.	DIST_DIR=/path/to/my/dist
OUT_DIR	Base output directory for the kernel build.	OUT_DIR=/path/to/my/out
SKIP_DEFCONFIG	Skip make defconfig	SKIP_DEFCONFIG=1
SKIP_MRPROPER	Skip make mrproper	SKIP_MRPROPER=1

Build Config for Android common kernel 

I am going to build common-android13-5.15 hence using below configuration

BUILD_CONFIG=common-modules/virtual-device/build.config.cuttlefish.x86_64 

build/build.sh

Then run below command for Emulator

BUILD_CONFIG=common-modules/virtual-device/build.config.virtual_device.x86_64

build/build.sh

After build complete, check the kernel file

/home/saurabh/kernel-common-android13-5.15/out/android13-5.15/dist/bzImage

If you run file command  for this bzImages yoy will get below output

Fast re-build

By default, Kernel is always built with mrproper target which removes all generated files + config + various backup files to create a clean build.

When developing on one or a few modules, we can skip the some initial steps and start re-build immediately:

BUILD_CONFIG=common-modules/virtual-device/build.config.virtual_device.x86_64 

LTO=none 

FAST_BUILD=1 

SKIP_MRPROPER=1

SKIP_DEFCONFIG=1

build/build.sh

Step 3 Include custom kernel in AOSP Build System

Edit the kernel make file that generally lies in your device folder. For Emulator path is

device/generic/goldfish/x86_64-kernel.mk

Step 4 Build Complete AOSP and Run Emulator
After modifying the Kernel make file , you need to rebuild AOSP completely to get Kernel image replaced.

make -j8

After successful build Run the emulator with below Command

emulator -verbose -show-kernel -selinux permissive -writable-system

Once Emulator get launched you can check your kernel version.

In next post Will explain how to load module in kernel and build.

Thanks
Saurabh
Happy Coding!!!!