2.4. Cross-compiling the Linux Kernel

2.4.1. References

• Bootlin slides, chapter Linux kernel introduction
• Mainline kernel: https://www.kernel.org/

2.4.2. Goals

• Cross-compile a kernel for an emulated target
• Use the Debian packaged cross-compiler
• Make a kernel configuration

2.4.3. Kernel make arguments for cross-compilation

• Possible parameters passed to make for cross compilation:

Option	Description	Example
ARCH=	indicate the architecture	ARCH=arm
CROSS_COMPILE=	cross compiler prefix	CROSS_COMPILE=arm-linux-
INSTALL_MOD_PATH=	path to install modules	INSTALL_MOD_PATH=../staging

• Pre-defined configuration files available for a lot of embedded boards.
• Architecture and vendor specific configurations enabled.

2.4.4. Steps

1. Install packaged ARM cross-compiler toolchain from distribution:

user@host: sudo apt install gcc-arm-linux-gnueabihf

2. Get the kernel source:

user@host: cd ${DL_PATH}
user@host: wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.0.6.tar.xz

3. Extract the source tarball on your host:

user@host: mkdir ${LINUX_PATH}
user@host: tar -C ${LINUX_PATH} -xvf linux-5.0.6.tar.xz
user@host: cd ${LINUX_PATH}/linux-5.0.6/

4. Set a default configuration for vexpress, a motherboard containing an ARM Cortex-A9:

user@host: ARCH=arm make vexpress_defconfig

5. Optionally: Start kernel configuration menu:

user@host: ARCH=arm make menuconfig

6. Start kernel build process, (note: the number of jobs for make should equal 2*cpus + 1):

user@host: ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make -j 9

7. Test if the kernel boots in an emulated environment:

user@host: qemu-system-arm -M vexpress-a9 -nographic -no-reboot \
                -kernel arch/arm/boot/zImage \
                -dtb arch/arm/boot/dts/vexpress-v2p-ca9.dtb \
                -append "panic=5 console=ttyAMA0"

8. The kernel should panic

2.4.5. Assignments

• Cross compile init-hello-world with arm-linux-gnueabihf-gcc and test init= in qemu

2.4.6. Questions

• Show the differences between the default and your kernel configuration.
• What is the dtb file being passed to the qemu?

2.4.7. Staging Installation Steps (Preparing the root file system)

1. Cross-compile the kernel modules:

user@host: ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make -j 9 modules

2. Copy the kernel to staging directory:

user@host: mkdir ${STAGING_PATH}/boot
user@host: cp arch/arm/boot/zImage ${STAGING_PATH}/boot/zImage

3. Copy the device tree to staging directory:

user@host: cp arch/arm/boot/dts/vexpress-v2p-ca9.dtb \
                    ${STAGING_PATH}/boot/vexpress-v2p-ca9.dtb

4. Install kernel modules to staging directory:

user@host: ARCH=arm INSTALL_MOD_PATH=${STAGING_PATH} make modules_install

2.4.8. Assignments

• Clean the Busybox build
• Enable the following applets in the Busybox menu: modprobe, lsmod
• Cross compile Busybox
• Install Busybox to the the staging directory
• Create a compressed cpio archive and boot in qemu
• Load a kernel module using modprobe and check with lsmod

2.4.9. Optional Assignments

• Cross compile toybox, create a new compressed cpio archive and boot in qemu. Compare the differences between toybox and busybox.
• Configure the kernel in a way to make the resulting binary as small as possible (non used funtionality can be removed from the config, e.g. ipv6, wireless networking, exotic protocols, file systems, …)