EFISTUB.md

Why

We don't want extra stuff

What

We want to boot Linux without installing a boot loader

How

1. We need to create an entry in the UEFI NVRAM entries list.
2. We can use the bcfg tool for that. But we need a unified kernel image(something with .efi).
3. We use the objcopy utility to do generate the unified kernel image.
4. Finally, we need to setup Pacman hooks to have the unified kernel image every time we upgrade linux.

Steps

1. Install binutils package, as we need the objcopy utility

   pacman -S binutils
   

2. Get PARTUUID using lsblk command

   lsblk --output=PATH,PARTUUID
   

3. Create a file with your kernel command line parameters

   cat <<EOF > kernel-command-line.txt
   root=PARTUUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx rw
   EOF
   

4. Create and place linux.efi at /boot/EFI/Linux/linux.efi

   objcopy \
       --add-section .osrel="/usr/lib/os-release" --change-section-vma .osrel=0x20000 \
       --add-section .cmdline="kernel-command-line.txt" --change-section-vma .cmdline=0x30000 \
       --add-section .splash="/usr/share/systemd/bootctl/splash-arch.bmp" --change-section-vma .splash=0x40000 \
       --add-section .linux="/boot/vmlinuz-linux" --change-section-vma .linux=0x2000000 \
       --add-section .initrd="/boot/initramfs-linux.img" --change-section-vma .initrd=0x3000000 \
       "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "linux.efi"
   mkdir -p /boot/EFI/Linux/
   cp linux.efi /boot/EFI/Linux/
   

5. Use the bcfg command in UEFI Shell v2 (i.e., from the Arch Linux live iso).

   bcfg boot add 0 fs2:\EFI\Linux\linux.efi "Arch Linux"
   

6. Create the hook file /etc/pacman.d/hooks/90-update-efistub.hook with content

   [Trigger]
   Operation = Install
   Operation = Upgrade
   Type = Path
   Target = usr/src/*/dkms.conf
   Target = usr/lib/modules/*/build/include/
   Target = usr/lib/modules/*/modules.alias
   
   [Action]
   Description = Updating linux efistub...
   When = PostTransaction
   Depends = linux
   Depends = binutils
   Depends = util-linux
   Exec = /etc/pacman.d/scripts/efistub-update
   NeedsTargets
   

7. Also create the corresponding script /etc/pacman.d/scripts/efistub-update with content

   #!/bin/bash -e
   
   SCRIPT_TMPDIR=$(mktemp -d)
   ROOT_PARTITION=$(findmnt -n -o SOURCE --target /)
   ROOT_PARTUUID=$(lsblk --output=PARTUUID $ROOT_PARTITION --noheadings)
   
   trap "{ rm -rf $SCRIPT_TMPDIR; }" EXIT HUP INT QUIT TERM STOP PWR
   
   pushd "$SCRIPT_TMPDIR" > /dev/null
   
   cat <<EOF > kernel-command-line.txt
   root=PARTUUID=$ROOT_PARTUUID rw
   EOF
   
   objcopy \
       --add-section .osrel="/usr/lib/os-release" --change-section-vma .osrel=0x20000 \
       --add-section .cmdline="kernel-command-line.txt" --change-section-vma .cmdline=0x30000 \
       --add-section .splash="/usr/share/systemd/bootctl/splash-arch.bmp" --change-section-vma .splash=0x40000 \
       --add-section .linux="/boot/vmlinuz-linux" --change-section-vma .linux=0x2000000 \
       --add-section .initrd="/boot/initramfs-linux.img" --change-section-vma .initrd=0x3000000 \
       "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "linux.efi"
   rm /boot/EFI/Linux/linux.efi
   cp linux.efi /boot/EFI/Linux/
   
   popd > /dev/null
   

8. Make the script executable

   chmod +x /etc/pacman.d/scripts/efistub-update