EFI stub kernel

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The (U)EFI firmware present in many computers can function as bootloader, allowing systems to boot without needing an additional software bootloader. This article shows how to configure and install an unsigned kernel in the EFI System Partition (ESP) of a computer running in EFI mode with secureboot turned off and how it shares the ESP with another OS for multiboot.

Kernel configuration

In order to boot directly from UEFI, the kernel needs to have these options enabled:

Enable CONFIG_EFI and CONFIG_EFI_STUB support. It is also a good idea to enable CONFIG_EFI_VARS so that the system firmware can be manipulated via efibootmgr. CONFIG_EFI_MIXED as proposed by Gentoo Handbook is not suitable for the EFI boot stub.

KERNEL Enable EFI stub support
Processor type and features  --->
    [*] EFI runtime service support 
    [*]   EFI stub support
    [ ]     EFI mixed-mode support

Firmware Drivers  --->
    EFI (Extensible Firmware Interface) Support  --->
        <*> EFI Variable Support via sysfs

UEFI does not pass kernel parameters to the kernel during normal boot, so they must be hard-coded them via CONFIG_CMDLINE values. In order to boot a root partition located at /dev/sda2 input the following text in the kernel configuration:

KERNEL Enable built-in kernel parameters
Processor type and features  --->
    [*] Built-in kernel command line

Using root=PARTUUID= might be preferable. To find out use blkid:

The partition's PARTUUID is distinct from the filesystem's UUID.
root #blkid | grep sda2
/dev/sda2: UUID="d1e0c1e0-3a40-42c5-8931-cfa2c7deae32" TYPE="ext4" PARTUUID="adf55784-15d9-4ca3-bb3f-56de0b35d88d"
KERNEL Enable built-in kernel parameters - alternative using PATUUID
Processor type and features  --->
    [*] Built-in kernel command line


If an ESP does not exist, one needs to be created. See EFI System Partition. Still in the kernel directory, build the kernel and install the modules:

root #cd /usr/src/linux
root #make -jN
root #make modules_install

Have the ESP mounted at /boot:

root #mount /dev/sda1 /boot

Run the install command:

root #make install
sh ./arch/x86/boot/install.sh 4.9.16-gentoo arch/x86/boot/bzImage \
        System.map "/boot"
user $tree -L 2 /boot
 ├── config-4.9.16-gentoo
 ├── EFI
 ├── System.map-4.9.16-gentoo
 └── vmlinuz-4.9.16-gentoo

Move vmlinuz to the right place adding the .efi suffix and delete the copies of config and System.map:

root #mkdir -p /boot/EFI/Gentoo
root #mv /boot/vmlinuz-4.9.16-gentoo /boot/EFI/Gentoo/vmlinuz-4.9.16-gentoo.efi
root #rm /boot/config-4.9.16-gentoo /boot/System.map-4.9.16-gentoo
user $tree -L 3 /boot
 └── EFI
     └── Gentoo
         └── vmlinuz-4.9.16-gentoo.efi

Alternatively the fallback directory /boot/EFI/Boot could be used additionally to or instead of /boot/EFI/Gentoo.

Next, tell the UEFI firmware that a boot entry called "Gentoo" is to be created, which has the freshly compiled EFI stub kernel:

root #efibootmgr --create --disk /dev/sda --part 1 --label "Gentoo" --loader '\efi\gentoo\vmlinuz-4.9.16-gentoo.efi'

Optionally, additional kernels can be installed in EFI/Gentoo and made known to the UEFI firmware. This is especially useful when wanting to test more kernels or to dual-boot with another operating system. These will be shown in the boot selection prompt, normally after a keyboard hotkey is pressed at the right time during system initialization. The latest added entry always gets highest boot priority, so it will be default. If the hotkey combination is unknown, search for official documentation from the computer manufacture. This information is usually not difficult to find.

In order to modify UEFI boot entries, the system needs to be booted in EFI mode. This means that the EFI stub image or some EFI capable live media must have been booted as described above.
EFI uses \ (backslash) as path separators.

Optional: Initramfs

In case an (optional) initramfs is needed, it also needs to be copied to the same place.

root #mv /boot/initramfs-4.9.16-gentoo.img /boot/EFI/Gentoo/initramfs.img
root #efibootmgr --create --part 1 --label "Gentoo" --loader '\efi\gentoo\vmlinuz-4.9.16-gentoo.efi' -u 'initrd=\efi\gentoo\initramfs.img'
It should be noted that EFI stub kernels can not load an initramfs if it is specified in CONFIG_CMDLINE. Check out efibootmgr, CONFIG_INITRAMFS_SOURCE, or separate bootloaders like (gummiboot or GRUB2) if an initramfs is a must-have.

The EFI stub can load an initramfs and pass it to the kernel, but this requires passing a parameter from the EFI bootloader (ie. initrd=\efi\boot\myinitrd.img). This parameter cannot be built into CONFIG_CMDLINE as outlined above. This may defeat the purpose of using EFI stub since many users prefer it as a way to avoid the necessity of a bootloader and/or dealing with efibootmgr. Another possibility is to embed the initramfs directly into the kernel image. In order to do this:

  • Generate the initramfs using a preferred method (Genkernel, Dracut, etc.).
  • Ensure the image is not compressed (just a cpio archive). Genkernel and Dracut both have command line switches for this.
  • Rename the initramfs with a .cpio extension, the kernel requires this in order to embed a cpio archive. The kernel image (with the cpio archive within it) will still be compressed as usual.
  • Ensure that the initramfs has the correct read-permissions for the user building the kernel. Most users build the kernel as the root user and may skip this step.
    • Be aware that Dracut makes the initramfs image readable only by the root user, so if the kernel was built as any other user adjust its permissions as needed. Failing to do so (and ignoring the kernel build warnings) will result in a kernel with no embedded initramfs which will leave the system potentially unbootable.
KERNEL Build in the initramfs CPIO archive
General setup  --->
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support
    (/boot/initramfs-3.12.21-gentoo-r1.cpio) Initramfs source file(s)

However, if passing an initramfs as a boot parameter is still desired, the only way it can be done is by using efibootmgr (GRUB2 will not work). An external initramfs has some advantages over building the initramfs directly into the kernel (see the kernel help on on the CONFIG_INITRAMFS_SOURCE variable for details on its limitations). In fact, both a built-in and external initramfs can be used simultaneously. Create and install the initramfs with Genkernel, Dracut, by hand, etc. Install the kernel as outlined in the Installation section below. Then:

This guide was created with the x64 architecture in mind. According the specification, UEFI is rich enough to cover a range of contemporary processor designs.
UEFI image's name is composed by the machine type defined in the PE32+ format [1] [2]. For example, a UEFI OS loader for the x86 architecture is bootIA32.efi

See also

External resources