Customizing the Boot Process

Most Linux systems use a special boot image to control the system boot process after BIOS or UEFI hands over control of the hardware to the operating system. This boot image is a compressed cpio initial RAM disk, and it’s called the initrd. The Linux kernel loads the initrd into the RAM and executes init or, if present, linuxrc.

Depending on the image type, KIWI NG creates the boot image automatically during the create step. To create the initrd, KIWI NG uses a tool called dracut. dracut-generated initrd archives can be extended with custom modules to add functionality which is not natively provided by dracut itself. In the scope of KIWI NG, the following dracut modules are used:

kiwi-dump: Serves as an image installer. It provides the required implementation to install a KIWI NG image on a selectable target. This module is required if one of the attributes in the image type definition installiso, installstick or installpxe is set to true.
kiwi-dump-reboot: Serves to boot the system into the installed image after installation is completed.
kiwi-live: Boots up a KIWI NG live image. This module is required if the iso image type is selected.
kiwi-overlay: Allows to boot disk images with the attribute overlayroot set to true. A disk like that has its root partition compressed and readonly. The disk boots up using overlayfs for the root filesystem with a separate partition on the same disk for persistent data.
kiwi-repart: Resizes an OEM disk image after installation on the target disk to meet the size limits configured in the oemconfig section of the image description. The module takes over the tasks of repartitioning the disk, resizing RAID, LVM, LUKS and other layers as well as resizing the system filesystems.
kiwi-lib: Provides common functions used by dracut modules.

Note

Using Custom Boot Image Support

In addition to the standard dracut-based creation of the boot image, KIWI NG supports the use of custom boot images for the image types oem and pxe. The use of a custom boot image is enabled by setting the following attribute in the image description:

<type ... initrd_system="kiwi"/>

Along with this setting, you must provide a reference to a boot image description in the boot attribute as follows:

  <type ... boot="netboot/suse-tumbleweed"/>

While KIWI NG supports this approach, it is recommended using dracut instead.
Keep also in mind that although KIWI NG supports creation of custom boot
images, KIWI NG does not include any official boot image descriptions. You
can find an OEM boot description example at
https://build.opensuse.org/package/show/Virtualization:Appliances:Images:Testing_x86:tumbleweed/custom-oem-boot-description
and an PXE boot description example at
https://build.opensuse.org/package/show/Virtualization:Appliances:Images:Testing_x86:tumbleweed/custom-pxe-boot-description

The custom boot image descriptions makes it possible to completely customize the behavior of the initrd. This concept is mostly used in PXE environments that are usually heavily customized and require a specific boot and deployment workflow.

Boot Image Hook-Scripts

The dracut initrd system uses systemd to implement a predefined workflow of services documented in the bootup man page:

http://man7.org/linux/man-pages/man7/dracut.bootup.7.html

To hook in a custom boot script to this workflow, it is necessary to provide a dracut module that dracut picks when KIWI NG calls it. The module files can be provided either as a package or as part of the overlay directory in the image description.

The following example shows how to include a custom hook script before the system rootfs is mounted.

Create a subdirectory for the dracut module:

$ mkdir -p root/usr/lib/dracut/modules.d/90my-module

Register the dracut module in the configuration file:

$ vi root/etc/dracut.conf.d/90-my-module.conf

add_dracutmodules+=" my-module "

Create the hook script:

$ touch root/usr/lib/dracut/modules.d/90my-module/my-script.sh

Create a module setup file in root/usr/lib/dracut/modules.d/90my-module/module-setup.sh containing the following:

#!/bin/bash

# called by dracut
check() {
    # check module integrity
}

# called by dracut
depends() {
    # return list of modules depending on this one
}

# called by dracut
installkernel() {
    # load required kernel modules when needed
    instmods _kernel_module_list_
}

# called by dracut
install() {
    declare moddir=${moddir}
    inst_multiple _tools_my_module_script_needs_

    inst_hook pre-mount 30 "${moddir}/my-script.sh"
}

Note

Declaring Extra Tools for Hook Scripts

The install() function called by dracut can define extra tools required by the specified hook script. The inst_multiple command and its parameters instruct dracut to include these extra tools and items into the initrd.

The specified tools and items can be files. Normally, they are executables and libraries required by the hook script.

Each file must be included in the KIWI NG description either in a package, archive, or in the root tree of the image description directory.
The parameters of the inst_multiple command are space separated.
Each parameter can be a single executable name if it exists in /bin, /sbin, /usr/bin, or /usr/sbin` directories.
Otherwise, a full path to the file is required. This normally applies for libraries and other special files.

When KIWI NG calls dracut, the 90my-module is installed into the generated initrd. At boot time, systemd calls the scripts as part of the dracut-pre-mount.service.

The dracut system offers many other possibilities to customize the initrd than shown in the example above. For more information, visit the dracut project page.

Boot Image Parameters

A dracut generated initrd in a KIWI NG image build process includes one or more of the KIWI NG provided dracut modules. The following list documents the available kernel boot parameters for these modules:

rd.kiwi.term: Exports the TERM variable into the initrd environment. If the default value for the terminal emulation is not correct, rd.kiwi.term can be used to overwrite the default. The environment is also passed to the systemd unit that calls dialog based programs in KIWI NG dracut code, which means that the TERM setting applies there too.
rd.kiwi.debug: Activates the debug log file for the KIWI NG part of the boot process in /run/initramfs/log/boot.kiwi.
rd.kiwi.install.pxe: Instructs an OEM installation image to lookup the system image on a remote location specified in rd.kiwi.install.image.
rd.kiwi.install.image=URI: Specifies the remote location of the system image in a PXE based OEM installation.
rd.kiwi.install.pass.bootparam: Instructs an OEM installation image to pass an additional boot parameters to the kernel used to boot the installed image. This can be used, for example, to pass on first boot configuration for a PXE image. Note that options starting with rd.kiwi are not passed to avoid side effects.
rd.kiwi.oem.maxdisk=size[KMGT]: Specifies the maximum disk size an unattended OEM installation uses for image deployment. Unattended OEM deployments default to deploying on /dev/sda (or more precisely, the first device that is not filtered out by oem-device-filter). With RAID controllers, you may have big JBOD disks along with a 480G RAID1 configured for OS deployment. With rd.kiwi.oem.maxdisk=500G, the deployment is performed on the RAID disk.
rd.kiwi.oem.force_resize: Forces the disk resize process on an OEM disk image. If set, no sanity check for unpartitioned/free space is performed and also an eventually configured <oem-resize-once> configuration from the image description will not be taken into account. The disk resize will be started which includes re-partition as well as all steps to resize the block layers up to the filesystem holding the data. As rd.kiwi.oem.force_resize bypasses all sanity checks to detect if such a resize process is needed or not, it can happen that all program calls of the resize process ends without any effect if the disk is already properly resized. It’s also important to understand that the partition UUIDs will change on every resize which might be an unwanted side effect of a forced resize.
rd.kiwi.oem.installdevice: Configures the disk device to use in an OEM installation. This overwrites or resets any other OEM device-specific settings, such as oem-device-filter, oem-unattended-id or rd.kiwi.oem.maxdisk, and continues the installation on the given device. The device must exist and must be a block special.

Note

Non interactive mode activated by rd.kiwi.oem.installdevice

When setting rd.kiwi.oem.installdevice explicitly through the kernel command line, KIWI NG uses the device without prompting for confirmation.

rd.live.overlay.size: Specifies the size for the tmpfs filesystem of a live ISO image that is used for the overlayfs mount process. If the write area of the overlayfs mount uses this tmpfs, any new data written during the runtime of the system is written in this space. The default value is 50%, meaning half of the available RAM space can be used for writing new data.
rd.live.overlay.persistent: Instructs a live ISO image to prepare a persistent write partition.
rd.live.overlay.cowfs: Specifies which filesystem of a live ISO image to use for storing data on the persistent write partition.
rd.live.cowfile.mbsize: Specifies the size of the COW file in MB. When using tools like live-grub-stick, the live ISO image is copied as a file on the target device, and a GRUB loopback setup is created there to boot the live system from the file. In this case, the persistent write setup that normally creates an extra write partition on the target will fail in most situations, because the target has no free and unpartitioned space available. To prevent this from happening, a COW file (live_system.cow) of a partition is created alongside the live ISO image file. The default size of the COW file is 500MB.
rd.live.cowfile.path: Effectively used in isoscan loop mounted live systems. For details on this type of live system refer to Deploy ISO Image as File on a FAT32 Formated USB Stick. Specifies the path of the COW file below the /run/initramfs/isoscan loop mount point. If not specified the cowfile is placed at /run/initramfs/isoscan/live_system.cow.
rd.live.dir: Specifies a directory that contains the live OS root directory. Default is LiveOS.
rd.live.squashimg: Specifies the name of the squashfs image file which contains the OS root. Default is squashfs.img.
rd.kiwi.allow_plymouth: By default kiwi stops plymouth if present and active in the initrd. Setting rd.kiwi.allow_plymouth will keep plymouth active in the initrd including all effects that might have to the available consoles.

Boot Debugging

If the boot process encounters a fatal error, the default behavior is to stop the boot process without any possibility to interact with the system. To prevent this, activate dracut’s builtin debug mode in combination with the KIWI NG debug mode as follows:

rd.debug rd.kiwi.debug

This must be set at the kernel command line. With these parameters activated, the system enters a limited shell environment when a fatal error occurs during boot. The shell provides a basic set of tools, and it can be used for inspection using the following command:

less /run/initramfs/log/boot.kiwi