# EFISTUB

Related articles

- Arch boot process
- Boot loaders
- Unified Extensible Firmware Interface

The Linux Kernel (linux>=3.3) supports EFISTUB (EFI BOOT STUB) booting. This feature allows EFI firmware to load the kernel as an EFI executable. The option is enabled by default on Arch Linux kernels or can be activated by setting `CONFIG_EFI_STUB=y` in the Kernel configuration (see The EFI Boot Stub for more information).

An EFISTUB kernel can be booted directly by a UEFI motherboard or indirectly using a boot loader. The latter is recommended if you have multiple kernel/initramfs pairs and your motherboard's UEFI boot menu is not easy to use.

## Setting up EFISTUB

After creating the EFI System Partition, you must choose how it will be mounted. The simplest option is to mount it at `/boot` or bind mount it to `/boot` since this allows pacman to directly update the kernel that the EFI firmware will read. If you elect for this option, continue to #Booting EFISTUB.

Note: You can keep kernel and initramfs out of ESP if you use a boot manager which has a file system driver for the partition where they reside, e.g. rEFInd.

### Alternative ESP Mount Points

If you do not mount the EFI System Partition to `/boot`, you will need to copy your boot files to its location (referred to hereafter as `esp`).

    # mkdir -p esp/EFI/arch
    # cp /boot/vmlinuz-linux esp/EFI/arch/vmlinuz-linux
    # cp /boot/initramfs-linux.img esp/EFI/arch/initramfs-linux.img
    # cp /boot/initramfs-linux-fallback.img esp/EFI/arch/initramfs-linux-fallback.img

Note: When using an Intel CPU, you may need to copy the Microcode to the boot-entry location.

Furthermore, you will need to keep the files on the ESP up-to-date with later kernel updates. Failure to do so could result in an unbootable system. The following sections discuss several mechanisms for automating it.

#### Using systemd

Systemd features event triggered tasks. In this particular case, the ability to detect a change in path is used to sync the EFISTUB kernel and initramfs files when they are updated in `/boot`.

    /etc/systemd/system/efistub-update.path

    [Unit]
    Description=Copy EFISTUB Kernel to UEFISYS Partition

    [Path]
    PathChanged=/boot/initramfs-linux-fallback.img

    [Install]
    WantedBy=multi-user.target
    WantedBy=system-update.target

Note: This watches for changes to `initramfs-linux-fallback.img` since this is the last file built by mkinitcpio. This is to avoid a potential race condition where systemd could copy older files before they are all done being built.

    /etc/systemd/system/efistub-update.service

    [Unit]
    Description=Copy EFISTUB Kernel to UEFISYS Partition

    [Service]
    Type=oneshot
    ExecStart=/usr/bin/cp -f /boot/vmlinuz-linux esp/EFI/arch/vmlinuz-linux
    ExecStart=/usr/bin/cp -f /boot/initramfs-linux.img esp/EFI/arch/initramfs-linux.img
    ExecStart=/usr/bin/cp -f /boot/initramfs-linux-fallback.img esp/EFI/arch/initramfs-linux-fallback.img

Tip: For Secure Boot (with your own keys), you can set up the service to also sign the image (using sbsigntools):

    ExecStart=/usr/bin/sbsign --key /path/to/db.key --cert /path/to/db.crt --output esp/EFI/arch/vmlinuz-linux esp/EFI/arch/vmlinuz_linux

Then enable and start `efistub-update.path`.

#### Using incron

incron can be used to run a script syncing the EFISTUB Kernel after kernel updates.

    /usr/local/bin/efistub-update.sh

    #!/usr/bin/env bash
    /usr/bin/cp -f /boot/vmlinuz-linux esp/EFI/arch/vmlinuz-linux
    /usr/bin/cp -f /boot/initramfs-linux.img esp/EFI/arch/initramfs-linux.img
    /usr/bin/cp -f /boot/initramfs-linux-fallback.img esp/EFI/arch/initramfs-linux-fallback.img

Note: The first parameter `/boot/initramfs-linux-fallback.img` is the file to watch. The second parameter `IN_CLOSE_WRITE` is the action to watch for. The third parameter `/usr/local/bin/efistub-update.sh` is the script to execute.

    /etc/incron.d/efistub-update.conf

    /boot/initramfs-linux-fallback.img IN_CLOSE_WRITE /usr/local/bin/efistub-update.sh

In order to use this method, enable the incrond service:

    # systemctl enable incrond.service

#### Using mkinitcpio hook

Mkinitcpio can generate a hook that does not need a system level daemon to function. It spawns a background process which waits for the generation of `vm-linuz`, `initramfs-linux.img`, and `initramfs-linux-fallback.img` before copying the files.

Add `efistub-update` to the list of hooks in `/etc/mkinitcpio.conf`.

    /usr/lib/initcpio/install/efistub-update

    #!/usr/bin/env bash
    build() {
    	/root/watch.sh &
    }

    help() {
    	cat <<HELPEOF
    This hook waits for mkinitcpio to finish and copies the finished ramdisk and kernel to the ESP
    HELPEOF
    }

    /root/watch.sh

    #!/usr/bin/env bash

    while [[ -d "/proc/$PPID" ]]; do
    	sleep 1
    done

    /usr/bin/cp -f /boot/vmlinuz-linux esp/EFI/arch/vmlinuz-linux
    /usr/bin/cp -f /boot/initramfs-linux.img esp/EFI/arch/initramfs-linux.img
    /usr/bin/cp -f /boot/initramfs-linux-fallback.img esp/EFI/arch/initramfs-linux-fallback.img

    echo "Synced kernel with ESP"

#### Using mkinitcpio hook (2)

Another alternative to the above solutions, that is potentially cleaner because there are less copies and does not need a system level daemon to function. The logic is reversed, the initramfs is directly stored in the EFI partition, not copied in /boot. Then the kernel and any other additional files are copied to the ESP partition, thanks to a mkinitcpio hook.

Edit the file `/etc/mkinitcpio.d/linux.preset` :

    /etc/mkinitcpio.d/linux.preset

    # mkinitcpio preset file for the 'linux' package

    # Directory to copy the kernel, the initramfs...
    ESP_DIR="/boot/efi/EFI/arch"

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

    PRESETS=('default' 'fallback')

    #default_config="/etc/mkinitcpio.conf"
    default_image="${ESP_DIR}/initramfs-linux.img"
    default_options="-A esp-update-linux"

    #fallback_config="/etc/mkinitcpio.conf"
    fallback_image="${ESP_DIR}/initramfs-linux-fallback.img"
    fallback_options="-S autodetect"

Then create the file `/usr/lib/initcpio/install/esp-update-linux` which need to be executable :

    /usr/lib/initcpio/install/esp-update-linux

    # Directory to copy the kernel, the initramfs...
    ESP_DIR="/boot/efi/EFI/arch"

    build() {
    	cp /boot/vmlinuz-linux "${ESP_DIR}/vmlinuz-linux.efi"
    	# If ucode is used uncomment this line
    	#cp /boot/intel-ucode.img "${ESP_DIR}/"
    }

    help() {
    	cat <<HELPEOF
    This hook copies the kernel to the ESP partition
    HELPEOF
    }

To test that, just run:

    rm /boot/initramfs-linux-fallback.img
    rm /boot/initramfs-linux.img
    mkinitcpio -p linux

## Booting EFISTUB

Warning: Linux Kernel EFISTUB initramfs path should be relative to the EFI System Partition's root. For example, if the initramfs is located in `$esp/EFI/arch/initramfs-linux.img`, the corresponding UEFI formatted line should be `initrd=/EFI/arch/initramfs-linux.img` or `initrd=\EFI\arch\initramfs-linux.img`. In the following examples we will assume that everything is in `$esp/`.

### Using a boot manager

There are several UEFI boot managers which can provide additional options or simplify the process of UEFI booting - especially if you have multiple kernels/operating systems. See Boot loaders for more information.

### Using UEFI Shell

It is possible to launch an EFISTUB kernel from UEFI Shell as if it is a normal UEFI application. In this case the kernel parameters are passed as normal parameters to the launched EFISTUB kernel file.

    > fs0:
    > /vmlinuz-linux root=PARTUUID=3518bb68-d01e-45c9-b973-0b5d918aae96 rw initrd=/initramfs-linux.img

To avoid needing to remember all of your kernel parameters every time, you can save the executable command to a shell script such as `archlinux.nsh` on your UEFI System Partition, then run it with:

    > fs0:
    > archlinux

### Using UEFI directly

UEFI is designed to remove the need for an intermediate bootloader such as GRUB. If your motherboard has a good UEFI implementation, it is possible to embed the kernel parameters within a UEFI boot entry and for the motherboard to boot Arch directly. You can use efibootmgr or UEFI Shell v2 to modify your motherboard's boot entries.

#### efibootmgr

The command looks like

    # efibootmgr -d /dev/sdX -p Y -c -L "Arch Linux" -l /vmlinuz-linux -u "root=/dev/sdBZ rw initrd=/initramfs-linux.img"

Where `/dev/sdX` and `Y` are the disk and partition where the ESP is located. Change the `root=` parameter to reflect your Linux root (disk UUIDs can also be used). Note that the `-u` argument in double quotes is just the list of kernel parameters, so you may need to add additional parameters e.g. for suspend to disk or microcode.

It is a good idea to then run

    # efibootmgr -v

to verify that the resulting entry is correct.

Warning: Some kernel and `efibootmgr` version combinations might refuse to create new boot entries. This could be due to lack of free space in the NVRAM. You can try deleting any EFI dump files

    # rm /sys/firmware/efi/efivars/dump-*

Or, as a last resort, boot with the `efi_no_storage_paranoia` kernel parameter. You can also try to downgrade your efibootmgr install to version 0.11.0 if you have it available in your cache. This version works with Linux version 4.0.6. See the bug discussion FS#34641 for more information.

To set the boot order, run:

    # efibootmgr -o XXXX,XXXX

where XXXX is the number that appears in the output of `efibootmgr` command against each entry.

Tip: Save the command for creating your boot entry in a shell script somewhere, which makes it easier to modify (when changing kernel parameters, for example).

More info about efibootmgr at UEFI#efibootmgr. Forum post https://bbs.archlinux.org/viewtopic.php?pid=1090040#p1090040 .

#### UEFI Shell

Some UEFI implementations make it difficult to modify the NVRAM successfully using efibootmgr. If efibootmgr cannot successfully create an entry, you can use the bcfg command in UEFI Shell v2.

To add an entry for your kernel, use:

    Shell> bcfg boot add N fsV:\vmlinuz-linux "Arch Linux"

where `N` is the priority (0 is top priority) and `V` is the volume number representing your EFI partition. If you don't know which volume number you should use, use the map command to list file systems, and the ls command to list their contents:

    Shell> map
    Shell> ls fs0:

To add the bare minimum necessary kernel options, use:

    Shell> bcfg boot -opt N "root=/dev/sdX# rw initrd=\initramfs-linux.img"

where `N` is the priority number you selected in the first step, and `/dev/sdX#` represents your root partition.

Category:

- Boot loaders