Unified kernel image: Difference between revisions
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== Overview == | == Overview == | ||
Traditionally bootloaders provided the following features: | |||
* The ability to select something to boot | |||
* The ability to boot applications from a filesystem | |||
Before UEFI these features were useful as BIOS machines could only specify a drive to boot from, not individual kernels or applications. | |||
On UEFI systems using a bootloader with these features is redundant and wastes time. | |||
https://systemd.io/BOOT_LOADER_SPECIFICATION/ | With the help of an stub you can package everything needed to boot your Linux system in to a single UEFI application. This is called a 'Unified Kernel Image', defined in [https://systemd.io/BOOT_LOADER_SPECIFICATION/ systemd's Boot Loader Specification]. | ||
There are currently two main stubs you can use to boot Linux: | |||
kernel | * The kernel's [https://docs.kernel.org/admin-guide/efi-stub.html EFI Boot Stub] which is only useful for kernel developers | ||
* [https://www.freedesktop.org/software/systemd/man/systemd-stub.html systemd-stub] which can be used to package pre-built components | |||
Because systemd-stub can be used with existing components provided by your Linux distro, it's possible to create a script that builds a Unified Kernel Image yourself. | |||
== Ubuntu install guide == | == Debian and Ubuntu install guide == | ||
Step 1: Copy and paste this file to 'install.sh' somewhere on your computer. | |||
#!/bin/bash | |||
DISK=/dev/vda | |||
apt install systemd-boot binutils efibootmgr | |||
cat /proc/cmdline > /boot/cmdline.txt | |||
mkdir -p /boot/efi/EFI/unified | |||
cat <<EOF >/etc/kernel/postinst.d/zz-unified | |||
#!/bin/bash | |||
test -e /boot/splash.bmp && SPLASH='--add-section .splash=/boot/splash.bmp --change-section-vma .splash=0x40000' | |||
exec objcopy \ | |||
--add-section .osrel=/usr/lib/os-release --change-section-vma .osrel=0x20000 \ | |||
--add-section .cmdline=/boot/cmdline.txt --change-section-vma .cmdline=0x30000 \ | |||
\$SPLASH \ | |||
--add-section .linux=/boot/vmlinuz --change-section-vma .linux=0x2000000 \ | |||
--add-section .initrd=/boot/initrd.img --change-section-vma .initrd=0x3000000 \ | |||
"/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "/boot/efi/EFI/unified/bootx64.efi" | |||
EOF | |||
chmod +x /etc/kernel/postinst.d/zz-unified | |||
/etc/kernel/postinst.d/zz-unified | |||
efibootmgr -c -d $DISK -L "Linux Unified" -l "\EFI\unified\bootx64.efi" | |||
< | Step 2: Change <code>DISK=/dev/vda</code> to the disk with your UEFI partition on it. This will probably be something like <code>DISK=/dev/sda</code>. Typing <code>lsblk</code> might help you find this. | ||
Step 3: Run <code>bash install.sh</code> as root in the directory containing the install.sh file. | |||
Step 4: Reboot and enter UEFI setup. Run the new "Linux Unified" option and see if it works. If it does, set it to be the default option. | |||
Step 5: Optionally, add a splash file in <code>/boot/splash.bmp</code> and run <code>/etc/kernel/postinst.d/zz-unified</code> . You may also want to add <code>bgrt_disable</code> to the kernel arguments to hide your computer's UEFI logo. | |||
Revision as of 01:29, 24 October 2022
If you use UEFI and only boot a single Linux kernel, you don't need a bootloader like systemd-boot or GRUB. Instead you can create a Unified Kernel Image and boot that directly. This article documents how.
Overview
Traditionally bootloaders provided the following features:
- The ability to select something to boot
- The ability to boot applications from a filesystem
Before UEFI these features were useful as BIOS machines could only specify a drive to boot from, not individual kernels or applications.
On UEFI systems using a bootloader with these features is redundant and wastes time.
With the help of an stub you can package everything needed to boot your Linux system in to a single UEFI application. This is called a 'Unified Kernel Image', defined in systemd's Boot Loader Specification.
There are currently two main stubs you can use to boot Linux:
- The kernel's EFI Boot Stub which is only useful for kernel developers
- systemd-stub which can be used to package pre-built components
Because systemd-stub can be used with existing components provided by your Linux distro, it's possible to create a script that builds a Unified Kernel Image yourself.
Debian and Ubuntu install guide
Step 1: Copy and paste this file to 'install.sh' somewhere on your computer.
#!/bin/bash DISK=/dev/vda apt install systemd-boot binutils efibootmgr cat /proc/cmdline > /boot/cmdline.txt mkdir -p /boot/efi/EFI/unified cat <<EOF >/etc/kernel/postinst.d/zz-unified #!/bin/bash test -e /boot/splash.bmp && SPLASH='--add-section .splash=/boot/splash.bmp --change-section-vma .splash=0x40000' exec objcopy \ --add-section .osrel=/usr/lib/os-release --change-section-vma .osrel=0x20000 \ --add-section .cmdline=/boot/cmdline.txt --change-section-vma .cmdline=0x30000 \ \$SPLASH \ --add-section .linux=/boot/vmlinuz --change-section-vma .linux=0x2000000 \ --add-section .initrd=/boot/initrd.img --change-section-vma .initrd=0x3000000 \ "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "/boot/efi/EFI/unified/bootx64.efi" EOF chmod +x /etc/kernel/postinst.d/zz-unified /etc/kernel/postinst.d/zz-unified efibootmgr -c -d $DISK -L "Linux Unified" -l "\EFI\unified\bootx64.efi"
Step 2: Change DISK=/dev/vda to the disk with your UEFI partition on it. This will probably be something like DISK=/dev/sda. Typing lsblk might help you find this.
Step 3: Run bash install.sh as root in the directory containing the install.sh file.
Step 4: Reboot and enter UEFI setup. Run the new "Linux Unified" option and see if it works. If it does, set it to be the default option.
Step 5: Optionally, add a splash file in /boot/splash.bmp and run /etc/kernel/postinst.d/zz-unified . You may also want to add bgrt_disable to the kernel arguments to hide your computer's UEFI logo.