Build kernels

This page details the process of building custom kernels for Android devices. These instructions guide you through the process of selecting the right sources, building the kernel, and embedding the results into a system image built from the Android Open Source Project (AOSP).

Download sources and build tools

For recent kernels, use repo to download the sources, toolchain, and build scripts. Some kernels (for example, the Pixel 3 kernels) require sources from multiple git repositories, while others (for example, the common kernels) require only a single source. Using the repo approach ensures a correct source directory setup.

Download the sources for the appropriate branch:

mkdir android-kernel && cd android-kernel
 
repo init -u https://android.googlesource.com/kernel/manifest -b BRANCH
 
repo sync

For a list of repo branches (BRANCH) that can be used with the previous `repo init` command, see Kernel branches and their build systems.

For details on downloading and compiling kernels for Pixel devices, See Building Pixel Kernels.

Build the kernel

Build with Bazel (Kleaf)

Android 13 introduced building kernels with Bazel.

To create a distribution for the GKI kernel for the aarch64 architecture, check out an Android Common Kernel branch no earlier than Android 13 and then run the following command:

tools/bazel run //common:kernel_aarch64_dist [-- --destdir=$DIST_DIR]

Thereafter the kernel binary, modules, and corresponding images are located in the $DIST_DIR directory. If --destdir is unspecified, see output of the command for the location of the artifacts. For details, refer to the documentation on AOSP.

Build with build.sh (legacy)

For branches at or below Android 12, OR branches without Kleaf: 

build/build.sh

The kernel binary, modules, and corresponding image are located in the out/BRANCH/dist directory.

Build the vendor modules for the virtual device

Android 13 introduced building kernels with Bazel (Kleaf), replacing build.sh.

To create a distribution for the virtual_device's modules, run:

tools/bazel run //common-modules/virtual-device:virtual_device_x86_64_dist [-- --destdir=$DIST_DIR]

For more details on building Android kernels with Bazel, see. Kleaf - Building Android Kernels with Bazel.

For details on Kleaf support for individual architectures, see Kleaf support for devices and kernels.

Build the vendor modules for the virtual device with build.sh (legacy)

In Android 12 Cuttlefish and Goldfish converge, so they share the same kernel: virtual_device. To build that kernel's modules, use this build configuration:

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

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

This example shows a kernel image configuration:

BUILD_CONFIG=common/build.config.gki.x86_64 build/build.sh

This example shows a module configuration (Cuttlefish and Emulator):

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

Run the kernel

There are multiple ways to run a custom-built kernel. The following are known ways suitable for various development scenarios.

Embed into the Android image build

Copy Image.lz4-dtb to the respective kernel binary location within the AOSP tree and rebuild the boot image.

Alternatively, define the TARGET_PREBUILT_KERNEL variable while using make bootimage (or any other make command line that builds a boot image). This variable is supported by all devices as it's set up via device/common/populate-new-device.sh. For example:

export TARGET_PREBUILT_KERNEL=DIST_DIR/Image.lz4-dtb

Flash and boot kernels with fastboot

Most recent devices have a bootloader extension to streamline the process of generating and booting a boot image.

To boot the kernel without flashing:

adb reboot bootloader
fastboot boot Image.lz4-dtb

Using this method, the kernel isn't actually flashed, and won't persist across a reboot.

Run kernels on Cuttlefish

You can run kernels in the architecture of your choice on Cuttlefish devices.

To boot a Cuttlefish device with a particular set of kernel artifacts, run the cvd create command with the target kernel artifacts as parameters. The following example command uses kernel artifacts for an arm64 target from the common-android14-6.1 kernel manifest.

cvd create \
    -kernel_path=/$PATH/$TO/common-android14-6.1/out/android14-6.1/dist/Image \
    -initramfs_path=/$PATH/$TO/common-android14-6.1/out/android14-6.1/dist/initramfs.img

For more information, see Develop kernels on Cuttlefish.

Customize the kernel build

To customize the kernel builds for Kleaf builds, see Kleaf documentation.

Customize the kernel build with build.sh (legacy)

For build/build.sh, the build process and outcome can be influenced by environment variables. Most of them are optional and each kernel branch should come with a proper default configuration. The most frequently used ones are listed here. For a complete (and up-to-date) list, refer to build/build.sh.

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.gki.aarch64
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

Custom kernel config for local builds

In Android 14 and above, you may use defconfig fragments to customize kernel configs. see Kleaf documentation on defconfig fragments.

Custom kernel config for local builds with build configs (legacy)

In Android 13 and below, see the following.

If you need to switch a kernel configuration option regularly, for example, when working on a feature, or if you need an option to be set for development purposes, you can achieve that flexibility by maintaining a local modification or copy of the build config.

Set the variable POST_DEFCONFIG_CMDS to a statement that is evaluated right after the usual make defconfig step is done. As the build.config files are sourced into the build environment, functions defined in build.config can be called as part of the post-defconfig commands.

A common example is disabling link time optimization (LTO) for crosshatch kernels during development. While LTO is beneficial for released kernels, the overhead at build time can be significant. The following snippet added to the local build.config disables LTO persistently when using build/build.sh.

POST_DEFCONFIG_CMDS="check_defconfig && update_debug_config"
function update_debug_config() {
    ${KERNEL_DIR}/scripts/config --file ${OUT_DIR}/.config \
         -d LTO \
         -d LTO_CLANG \
         -d CFI \
         -d CFI_PERMISSIVE \
         -d CFI_CLANG
    (cd ${OUT_DIR} && \
     make O=${OUT_DIR} $archsubarch CC=${CC} CROSS_COMPILE=${CROSS_COMPILE} olddefconfig)
}

Identify kernel versions

You can identify the correct version to build from two sources: the AOSP tree and the system image.

Kernel version from AOSP tree

The AOSP tree contains prebuilt kernel versions. The git log reveals the correct version as part of the commit message:

cd $AOSP/device/VENDOR/NAME
git log --max-count=1

If the kernel version isn't listed in the git log, obtain it from the system image, as described below.

Kernel version from system image

To determine the kernel version used in a system image, run the following command against the kernel file:

file kernel

For Image.lz4-dtb files, run:

grep -a 'Linux version' Image.lz4-dtb

Build a boot image

It's possible to build a boot image using the kernel build environment.

Build a boot image for devices with init_boot

For devices with the init_boot partition, the boot image is built along with the kernel. The initramfs image is not embedded in the boot image.

For example, with Kleaf, you may build the GKI boot image with:

tools/bazel run //common:kernel_aarch64_dist [-- --destdir=$DIST_DIR]

With build/build.sh (legacy), you may build the GKI boot image with:

BUILD_CONFIG=common/build.config.gki.aarch64 build/build.sh

The GKI boot image is located in $DIST_DIR.

Build a boot image for devices without init_boot (legacy)

For devices without the init_boot partition, you need a ramdisk binary, which you can obtain by downloading a GKI boot image and unpacking it. Any GKI boot image from the associated Android release will work.

tools/mkbootimg/unpack_bootimg.py --boot_img=boot-5.4-gz.img
mv $KERNEL_ROOT/out/ramdisk gki-ramdisk.lz4

The target folder is the top-level directory of the kernel tree (the current working directory).

If you're developing with the latest AOSP release branch, you can instead download the ramdisk-recovery.img build artifact from an aosp_arm64 build on ci.android.com and use that as your ramdisk binary.

When you have a ramdisk binary and have copied it to gki-ramdisk.lz4 in the root directory of the kernel build, you can generate a boot image by executing:

BUILD_BOOT_IMG=1 SKIP_VENDOR_BOOT=1 KERNEL_BINARY=Image GKI_RAMDISK_PREBUILT_BINARY=gki-ramdisk.lz4 BUILD_CONFIG=common/build.config.gki.aarch64 build/build.sh

If you're working with x86-based architecture, replace Image with bzImage, and aarch64 with x86_64:

BUILD_BOOT_IMG=1 SKIP_VENDOR_BOOT=1 KERNEL_BINARY=bzImage GKI_RAMDISK_PREBUILT_BINARY=gki-ramdisk.lz4 BUILD_CONFIG=common/build.config.gki.x86_64 build/build.sh

That file is located in the artifact directory $KERNEL_ROOT/out/$KERNEL_VERSION/dist.

The boot image is located at out/<kernel branch>/dist/boot.img.