Unified kernel image

From ArchWiki

A unified kernel image (UKI) is a single executable which can be booted directly from UEFI firmware, or automatically sourced by boot-loaders with little or no configuration.

Although Arch supported kernels themselves can be loaded by UEFI firmware, a unified image allows to incorporate all or a subset of the following:

The resulting executable, and therefore all these elements can then be easily signed for use with Secure Boot.

Note: In the entire article esp denotes the mountpoint of the EFI system partition.

Preparing a unified kernel image

mkinitcpio

Kernel command line

First, create /etc/kernel/cmdline with your kernel parameters, taking care to remove entries pointing at microcode and initramfs, e.g. 

/etc/kernel/cmdline
root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet bgrt_disable
Tip:
  • The root= parameter may be omitted if the root partition is automounted by systemd.
  • The bgrt_disable parameter tells Linux to not display the OEM logo after loading the ACPI tables.

.preset file

Next, modify /etc/mkinitcpio.d/linux.preset, or the preset that you are using, as follows, with the appropriate mount point of the EFI system partition :

  • Un-comment (i.e. remove #) the PRESET_uki= parameter for each item in PRESETS=,
  • Optionally, comment out PRESET_image= to avoid storing a redundant initramfs-*.img file,
  • Optionally, add or un-comment the --splash parameter to each PRESET_options= line for which you want to add a splash image.

Here is a working example linux.preset for the linux kernel and the Arch splash screen. 

/etc/mkinitcpio.d/linux.preset
# mkinitcpio preset file for the 'linux' package

ALL_config="/etc/mkinitcpio.conf"
ALL_kver="/boot/vmlinuz-linux"
ALL_microcode=(/boot/*-ucode.img)

PRESETS=('default' 'fallback')

#default_config="/etc/mkinitcpio.conf"
#default_image="/boot/initramfs-linux.img"
default_uki="esp/EFI/Linux/arch-linux.efi"
default_options="--splash=/usr/share/systemd/bootctl/splash-arch.bmp"

#fallback_config="/etc/mkinitcpio.conf"
#fallback_image="/boot/initramfs-linux-fallback.img"
fallback_uki="esp/EFI/Linux/arch-linux-fallback.efi"
fallback_options="-S autodetect"
Tip:
  • If all you want to do is boot from the unified kernel images, you can mount the ESP to /efi and only those need to reside on the ESP partition.
  • You can append --cmdline /etc/kernel/fallback_cmdline to fallback_options to use different a different cmdline than above for the fallback image (e.g. without quiet)
Note: PRESET_uki options were previously known as PRESET_efi_image, changed November 2022, with older option deprecated but working for now

Signing the UKIs for Secure Boot

By using a mkinitcpio post hook, the generated unified kernel images can be signed for Secure Boot. Create the following file and make it executable: 

/etc/initcpio/post/uki-sbsign
#!/usr/bin/env bash

uki="$3"
[[ -n "$uki" ]] || exit 0

keypairs=(/path/to/db.key /path/to/db.crt)

for (( i=0; i<${#keypairs[@]}; i+=2 )); do
    key="${keypairs[$i]}" cert="${keypairs[(( i + 1 ))]}"
    if ! sbverify --cert "$cert" "$uki" &>/dev/null; then
        sbsign --key "$key" --cert "$cert" --output "$uki" "$uki"
    fi
done

Replace /path/to/db.key and /path/to/db.crt with the paths to the key pair you want to use for signing the image.

Building the UKIs

Finally, make sure that the directory for the UKIs exists and regenerate the initramfs. For example, for the linux preset: 

# mkdir -p esp/EFI/Linux
# mkinitcpio -p linux

Optionally, remove any leftover initramfs-*.img from /boot or /efi.

kernel-install

You can use systemd's kernel-install(8) script to automatically install kernels in the UKI format to the esp both for custom kernels and for kernel packages (installed using Pacman) by switching Pacman hooks from mkinitcpio to kernel-install.

kernel-install is not an initramfs generator, but it is a framework where packages can hook into the installation/generation of kernels of the system, through its "plugin" system. During its execution it will call the proper initramfs generator of the system (i.e.: mkinitcpio). The plugins are involved in kernel image/initramfs generation, signing, installation, etc. Packages that care about doing something during kernel installation can be notified by installing their own "plugin" for kernel-install. (The "plugins" are located in /usr/lib/kernel/install.d/.)

There are configuration options like "layout" available that affect where and how the kernel is installed when kernel-install is getting called.

mkinitcpio ships with a kernel-install plugin that generate the appropriate image (for layout=uki an UKI image). Other programs, such as sbctl, also ship with a kernel-install plugin.

To setup kernel-install to produce UKIs:

  • Set the kernel-install layout to 'uki'. e.g.: 
    # echo "layout=uki" >> /etc/kernel/install.conf
  • Mask the direct kernel installation Pacman hooks:
    # ln -s /dev/null /etc/pacman.d/hooks/60-mkinitcpio-remove.hook
# ln -s /dev/null /etc/pacman.d/hooks/90-mkinitcpio-install.hook
  • Create a Pacman hook for kernel-install. You can use pacman-hook-kernel-installAUR.
  • Remove and reinstall the kernel packages that you use.

dracut

See dracut#Unified kernel image and dracut#Generate a new initramfs on kernel upgrade.

sbctl

Install the sbctl package. Store the kernel command line in /etc/kernel/cmdline. Use the sbctl bundle command with the --save parameter to create a bundle and have it be regenerated by a Pacman hook at appropriate times: 

# sbctl bundle --save esp/archlinux.efi

To create more EFI binaries for other kernels and initramfs images, repeat the above command with parameters --kernel-img and --initramfs, see sbctl(8) § EFI BINARY COMMANDS. The EFI binaries can be regenerated at any time with sbctl generate-bundles.

ukify

Install the systemd-ukify package. Since ukify cannot generate an initramfs on its own, if required, it must be generated using dracut, mkinitcpio or booster.

A minimal working example can look something like this: 

# /usr/lib/systemd/ukify --cmdline"kernel commandline options" /usr/lib/modules/A.B.C-name/vmlinuz /path/to/initramfs

Then, copy the resulting file to the EFI System Partition: 

# cp filename.efi esp/EFI/Linux/
Tip:
  • To skip having copy over the resulting EFI executable to the EFI System Partition, use the --output=esp/EFI/Linux/filename.efi command line option to ukify.
  • When specifying the --cmdline option, one can specify a file name to read the kernel parameters from (e.g. /etc/kernel/cmdline by adding the @ symbol before the file name, like --cmdline=@/path/to/cmdline.

An example for automatic UKI building with a systemd service for normal kernel image with intel ucode and /efi mounted ESP: 

/etc/ukify.conf
KERNEL=/boot/vmlinuz-linux
UCODE=/boot/intel-ucode.img
INITRD=/boot/initramfs-linux.img
CMDLINE=/etc/kernel/cmdline
SPLASH=/usr/share/systemd/bootctl/splash-arch.bmp
EFI=/efi/EFI/Linux/arch-linux.efi
/usr/lib/systemd/ukify ${KERNEL} ${UCODE} ${INITRD} --cmdline @${CMDLINE} \
    --splash ${SPLASH} --output ${EFI}
/etc/systemd/system/run_ukify.service
[Unit]
Description=Run systemd ukify
[Service]
Type=oneshot
ExecStart=/usr/bin/bash -c "source /etc/ukify.conf"
/etc/systemd/system/run_ukify.path
[Unit]
Description=Run systemd ukify
[Path]
PathChanged=/boot/initramfs-linux.img
PathChanged=/boot/intel-ucode.img
Unit=run_ukify.service
[Install]
WantedBy=multi-user.target

Then enable run_ukify.path.

Manually

Put the kernel command line you want to use in a file, and create the bundle file using objcopy(1).

For microcode, first concatenate the microcode file and your initrd, as follows: 

$ cat esp/cpu_manufacturer-ucode.img esp/initramfs-linux.img > /tmp/combined_initrd.img

When building the unified kernel image, pass in /tmp/combined_initrd.img as the initrd. This file can be removed afterwards. 

$ osrel_offs=$(objdump -h "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" | awk 'NF==7 {size=strtonum("0x"$3); offset=strtonum("0x"$4)} END {print size + offset}')
$ cmdline_offs=$((osrel_offs + $(stat -Lc%s "/usr/lib/os-release")))
$ splash_offs=$((cmdline_offs + $(stat -Lc%s "/etc/kernel/cmdline")))
$ linux_offs=$((splash_offs + $(stat -Lc%s "/usr/share/systemd/bootctl/splash-arch.bmp")))
$ initrd_offs=$((linux_offs + $(stat -Lc%s "vmlinuz-file")))
$ objcopy \
    --add-section .osrel="/usr/lib/os-release" --change-section-vma .osrel=$(printf 0x%x $osrel_offs) \
    --add-section .cmdline="/etc/kernel/cmdline" \
    --change-section-vma .cmdline=$(printf 0x%x $cmdline_offs) \
    --add-section .splash="/usr/share/systemd/bootctl/splash-arch.bmp" \
    --change-section-vma .splash=$(printf 0x%x $splash_offs) \
    --add-section .linux="vmlinuz-file" \
    --change-section-vma .linux=$(printf 0x%x $linux_offs) \
    --add-section .initrd="initrd-file" \
    --change-section-vma .initrd=$(printf 0x%x $initrd_offs) \
    "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "linux.efi"

The offsets are simply calculated so no sections overlap, as recommended in [1].

After creating the image, copy it to the EFI system partition: 

# cp linux.efi esp/EFI/Linux/

Booting

systemd-boot

systemd-boot searches in esp/EFI/Linux/ for unified kernel images, and there is no further configuration needed. See sd-boot(7) § FILES

rEFInd

rEFInd will autodetect unified kernel images on your EFI system partition, and is capable of loading them. They can also be manually specified in refind.conf, by default located at: 

esp/EFI/refind/refind.conf
menuentry "Arch Linux" {
    icon \EFI\refind\icons\os_arch.png
    ostype Linux
    loader \EFI\Linux\arch-linux.efi
}

Recall that no kernel parameters from esp/EFI/refind_linux.conf will be passed when booting this way.

Directly from UEFI

efibootmgr can be used to create a UEFI boot entry for the .efi file: 

# efibootmgr --create --disk /dev/sdX --part partition_number --label "Arch Linux" --loader 'EFI\Linux\arch-linux.efi' --unicode

See efibootmgr(8) for an explanation of the options.

Note: If options is present in a boot entry and Secure Boot is disabled, the value of options will override any .cmdline string embedded in the EFI image that is specified by efi or linux (see #Preparing a unified kernel image). With Secure Boot, however, options (and any edits made to the kernel command line in the bootloader UI) will be ignored, and only the embedded .cmdline will be used.

See also

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