ja:ユニファイドカーネルイメージ pt:Unified kernel image zh-hans:统一内核映像 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. It is the combination of a UEFI boot stub program like systemd-stub(7), a Linux kernel image, an initramfs, and further resources in a single UEFI PE file.

This file, and therefore all these elements can then easily be signed for use with Secure Boot.

Preparing a unified kernel image

There are several ways to generate a UKI image and install it to the proper place (the esp/Linux directory). Currently several tools compete for doing this functionality, so choose one of the following based on your needs and your likings.

mkinitcpio

mkinitcpio will assemble the UKI itself unless systemd-ukify is installed. In which case, UKI creation will be offloaded to ukify unless this is explicitly disabled with the --no-ukify option. Note: when offloading to ukify mkinitcpio forcibly supplies it a configuration file from {/etc,/usr/lib}/kernel/uki.conf.

Kernel command line

mkinitcpio supports reading kernel parameters from command line files in the /etc/cmdline.d directory. Mkinitcpio will concatenate the contents of all files with a .conf extension in this directory and use them to generate the kernel command line. Any lines in the command line file that start with a # character are treated as comments and ignored by mkinitcpio. Take care to remove entries pointing to microcode and initramfs.

For example:

root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw

/etc/cmdline.d/root.conf

# enable apparmor
lsm=landlock,lockdown,yama,integrity,apparmor,bpf audit=1 audit_backlog_limit=256

/etc/cmdline.d/security.conf

Alternatively, /etc/kernel/cmdline can be used to configure the kernel command line.

For example:

root=UUID=0a3407de-014b-458b-b5c1-848e92a327a3 rw quiet bgrt_disable

/etc/kernel/cmdline

.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.

# mkinitcpio preset file for the 'linux' package

#ALL_config="/etc/mkinitcpio.conf"
ALL_kver="/boot/vmlinuz-linux"

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"

/etc/mkinitcpio.d/linux.preset

pacman hook

Updates to systemd-stub (part of systemd), microcode (both intel-ucode and amd-ucode), and linux kernel will automatically trigger a UKI rebuild. But you may want to review other pacman hooks in the /etc/pacman.d/hooks/ directory.

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

Kernel-install is part of systemd and requires systemd-ukify to build unified kernel images. Make sure kernel-install is properly set up.

To generate UKIs, install systemd-ukify and set the kernel-install layout to uki:

layout=uki

/etc/kernel/install.conf

Any configuration for #ukify must be done in /etc/kernel/uki.conf in order to be used by kernel-install, e.g.

[UKI]
Splash=/usr/share/systemd/bootctl/splash-arch.bmp

/etc/kernel/uki.conf

Alternatively, for mkinitcpio to generate the UKI, set it as the default uki_generator:

layout=uki
uki_generator=mkinitcpio

/etc/kernel/install.conf

In that case, systemd-ukify is not necessary. You can also set a different initrd_generator, see kernel-install(8).

Reinstall the kernel packages that you use in order for the change to take effect.

dracut

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

ukify

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

A minimal working example can look something like this:

# ukify build --linux=/boot/vmlinuz-linux \
              --initrd=/boot/initramfs-linux.img \
              --cmdline="quiet rw"

For further information, see ukify(1).

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 initramfs, as follows:

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

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

$ align="$(objdump -p /usr/lib/systemd/boot/efi/linuxx64.efi.stub  awk '{ if ($1 == "SectionAlignment"){print $2} }')"
$ align=$((16#$align))
$ 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}')"
$ osrel_offs=$((osrel_offs + "$align" - osrel_offs % "$align"))
$ cmdline_offs=$((osrel_offs + $(stat -Lc%s "/usr/lib/os-release")))
$ cmdline_offs=$((cmdline_offs + "$align" - cmdline_offs % "$align"))
$ splash_offs=$((cmdline_offs + $(stat -Lc%s "/etc/kernel/cmdline")))
$ splash_offs=$((splash_offs + "$align" - splash_offs % "$align"))
$ initramfs_offs=$((splash_offs + $(stat -Lc%s "/usr/share/systemd/bootctl/splash-arch.bmp")))
$ initramfs_offs=$((initramfs_offs + "$align" - initramfs_offs % "$align"))
$ linux_offs=$((initramfs_offs + $(stat -Lc%s "initramfs-file")))
$ linux_offs=$((linux_offs + "$align" - linux_offs % "$align"))

$ 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 $splash_offs) \
    --add-section .initrd="initramfs-file" \
    --change-section-vma .initrd=$(printf 0x%x $initramfs_offs) \
    --add-section .linux="vmlinuz-file" \
    --change-section-vma .linux=$(printf 0x%x $linux_offs) \
    "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "linux.efi"

A few things to note:

• The offsets are dynamically calculated so no sections overlap, as recommended in https://github.com/systemd/systemd/commit/0fa2cac4f0cdefaf1addd7f1fe0fd8113db9360b#commitcomment-76747223.

• The sections are aligned to what the SectionAlignment field of the PE stub indicates (usually 0x1000).

• The kernel image must be in the last section, to prevent in-place decompression from overwriting the sections that follow, as stated in https://github.com/systemd/systemd/commit/0fa2cac4f0cdefaf1addd7f1fe0fd8113db9360b#commitcomment-84868898.

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

# cp linux.efi esp/EFI/Linux/

To create a UKI with xen, see https://xenbits.xen.org/docs/unstable/misc/efi.html#unified-xen-kernel-image.

Signing the UKIs for Secure Boot

sbctl

sbctl provides a kernel-install script, a mkinitcpio post-hook, and pacman hooks to sign updated binaries.

mkinitcpio

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:

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

key=/path/to/db.key
cert=/path/to/db.crt

if ! sbverify --cert "$cert" "$uki" &>/dev/null; then
    sbsign "$uki" --key "$key" --cert "$cert" --output "$uki"
fi

/etc/initcpio/post/uki-sbsign

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.

ukify

To automatically sign UKIs using systemd-sbsign(1), set the following in your configuration file (you can copy and edit the template from /usr/lib/kernel/uki.conf):

[UKI]
SecureBootSigningTool=systemd-sbsign
SignKernel=true
SecureBootPrivateKey=/etc/kernel/secure-boot-private-key.pem
SecureBootCertificate=/etc/kernel/secure-boot-certificate.pem

/etc/kernel/uki.conf

Booting

Limine

Limine does not automatically detect unified kernel images (UKIs). However, limine.conf can be manually configured to load them.

Example 1: Booting a UKI from the default EFI system partition

If a UKI file is stored in esp/EFI/Linux/, add the following configuration to limine.conf:

/Arch Linux
  protocol: efi
  path: boot():/EFI/Linux/arch-linux.efi

limine.conf

Example 2: Booting a UKI from another partition

If a UKI file is located on a different FAT32 partition, use uuid(PARTUUID) with the PARTUUID instead:

/Arch Linux
  protocol: efi
  path: uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx):/EFI/Linux/arch-linux.efi

limine.conf

For more details about supported paths and configuration options, see the Limine Paths documentation.

systemd-boot

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

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:

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

esp/EFI/refind/refind.conf

Keep in mind that no kernel parameters from esp/EFI/refind_linux.conf will be passed when booting this way. If the UKI was generated without a .cmdline section, specify the kernel parameters in the menu entry with an options line.

GRUB

GRUB can chainload UKIs as described in GRUB#Chainloading a unified kernel image.

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.

See also

• All Systems Go! talk on UKIs

• Unified kernel image specification

• mkinitcpio v31 and UEFI stubs