Bootloaders

From Alpine Linux

By default Alpine uses Syslinux as bootloader. This page shows the basic steps you need to perform, if you for any reason want to switch bootloaders or apply some manual configuration.


Installing Syslinux

If you want to switch from another bootloader back to Syslinux, or if you for some reason want to install Syslinux manually, the following steps are required.

Install the syslinux package: 

apk add syslinux

If you're using GPT partitions, install the GPT MBR onto the drive you want to install the bootloader on (in this case /dev/sda): 

dd bs=440 count=1 conv=notrunc if=/usr/share/syslinux/gptmbr.bin of=/dev/sda

Or if you're using DOS partitions, install the DOS MBR instead: 

dd bs=440 count=1 conv=notrunc if=/usr/share/syslinux/mbr.bin of=/dev/sda


Next install the required Syslinux binaries. Despite being called extlinux, Syslinux supports booting from FAT12/16/32, NTFS, ext2/3/4, Btrfs, XFS, and UFS/FFS filesystems. 

extlinux --install /boot

The configuration file is located in /boot/extlinux.conf. Alpine ships with a script called update-extlinux which automatically (re)generates this file, for example on updates to Syslinux. The settings for this script can be found in /etc/update-extlinux.conf, including the option to disable automatic overwriting of /boot/extlinux.conf. You can also place additional menu entries in the /etc/update-extlinux.d/ directory, e.g. for dual booting.


EFI

Todo: Work in progress. This should at least get you started.


Assuming /mnt is a FAT32 partition of type EF00 and /boot belongs to the rootfs created after running setup-disk: 

mkdir -p /mnt/EFI/syslinux
cp /usr/share/syslinux/efi64/* /mnt/EFI/syslinux/
cp /boot/extlinux.conf /mnt/EFI/syslinux/syslinux.cfg
cp /boot/vmlinuz* /mnt/
cp /boot/initramfs* /mnt/

You may need to modify /mnt/EFI/syslinux/syslinux.cfg to change the paths to absolute paths (just add a / in front of the vmlinuz/initramfs entries), or copy the files to /mnt/EFI/syslinux instead (XXX: untested).

GRUB

To install GRUB in BIOS mode, (optionally) remove the Syslinux package and install the required GRUB packages: 

apk del syslinux
apk add grub grub-bios

For EFI, install Grub's EFI package instead. Note that /boot has to be an EFI compatible filesystem like FAT32. 

apk add grub-efi

Next install the MBR and GRUB binaries to disk for BIOS mode: 

grub-install /dev/vda

For EFI mode: 

grub-install --target=x86_64-efi --efi-directory=/boot

GRUB ships with an automatic config generator, including some automatic detection of other operating systems installed on the device: 

grub-mkconfig -o /boot/grub/grub.cfg

This script can be configured via the /etc/default/grub file. See [1] for a list of available options.

systemd-boot

Todo: Work in progress. It is mostly done in Linux basics but doesn't cover topics like creating entries for other Operating Systems.


Systemd-boot is the simple EFI bootloader part of the systemd project. It is also available in the Alpine Linux testing repository as a standalone component.

Installing

To install the systemd-boot, enable the testing repository, and install the systemd-boot package. Then make sure the boot and EFI partitions are mounted as read-write and install the bootloader with bootctl. 

# apk add systemd-boot
# bootctl install

The bootctl binary also has support for checking if the bootloader is installed. 

# bootctl is-installed
yes

Configuring

Example configuration files for alpine are available on /usr/share/systemd/bootctl as loader.conf and alpine.conf.

The bootloader (not the boot entries) can be configured via the loader.conf file, an example is available in /usr/share/systemd/bootctl/loader.conf. 

# cp /usr/share/systemd/bootctl/loader.conf /boot/loader
# vi /boot/loader/loader.conf

systemd-boot makes use of configuration files in /boot/loader/entries to list entries for different OSes it can boot into. An example file for Alpine Linux is present on /usr/share/systemd/bootctl/alpine.conf. 

# cp /usr/share/systemd/bootctl/alpine.conf /boot/loader/entries
# vi /boot/loader/entries/alpine.conf

The example file has a skeleton structure that requires minimal modifications to make them work. You need to modify the example file to fit your oeprating system, among them:

  • Replace root=UUID=XXXX with the UUID of the root partition.
  • Replace rootfstype with the filesystem used for the root filesystem.
  • Add boot options required/recommended for booting your operating system (E.g: cryptdm=, cryptroot=, modules=, etc).
  • (If you don't use the default linux-lts kernel) Replace vmlinuz-lts and initramfs-lts with the ones that are used.
  • (If you use an Intel CPU and need Intel microcode) add initrd /intel-ucode.img before the initrd call for the initramfs

All the options available are documented in the systemd boot loader specification under Technical Details.

Unified Kernel Images

Systemd-boot also accepts unified kernel images, as outlined in their document. They are a combination of various pieces that are usually separate when normally booting, like the kernel, the initramfs and the kernel commandline.

To generate a unified kernel image the objcopy binary can be used: 

# objcopy \
	--add-section .osrel="/etc/os-release" --change-section-vma .osrel=0x20000 \
	--add-section .cmdline="/proc/cmdline" --change-section-vma .cmdline=0x30000 \
	--add-section .linux="$(bootctl -x)/vmlinuz-lts" --change-section-vma .linux=0x40000 \
	--add-section .initrd="$(bootctl -x)/initramfs-lts" --change-section-vma .initrd=0x3000000 \
	/usr/lib/systemd/boot/efi/linuxx64.efi.stub $(bootctl -p)/EFI/Linux/linux.efi

The above can be signed, with the bootloader, for use in Secure Boot.

Users that require CPU microcode (like intel-ucode) can include it by concatenating the CPU microcode .img file before the normal initramfs into a single .img file. 

# cat $(bootctl -x)/cpu_microcode.img $(bootctl -x)/initramfs-lts > /tmp/initramfs-lts

Then use that in the .initrd section of the objcopy call.

Listing Information and Status

bootctl can also list the entries available, which one is default and a wealth of information from them. (Example from Alpine Linux developer Leo, which inculdes an entry and a Unified Kernel Image) 

# bootctl list
Boot Loader Entries:
        title: Alpine Linux
           id: alpine
       source: /boot/loader/entries/alpine.conf
        linux: /vmlinuz-lts
       initrd: /intel-ucode.img
               /initramfs-lts
      options: root=UUID=1437f4dd-7efd-4a4e-8fac-797b548076d1 cryptdm=pool cryptroot=/dev/sda2 modules=ext4,xfs rootfstype=ext4 psi=1 quiet

        title: Alpine Linux edge (default)
           id: alpine-3.12_alpha20200122
       source: /boot/EFI/Linux/linux.efi
        linux: EFI/Linux/linux.efi
      options: initrd=\intel-ucode.img initrd=\initramfs-lts root=UUID=1437f4dd-7efd-4a4e-8fac-797b548076d1 cryptdm=pool cryptroot=/dev/sda2 modules=ext4,xfs rootfstype=ext4 psi=1 quiet

        title: Reboot Into Firmware Interface
           id: auto-reboot-to-firmware-setup
       source: /sys/firmware/efi/efivars/LoaderEntries-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f

And it can also list general status of the system, including information on the current bootloader, the system firmware and options (like if Secure Boot is enabled). 

# bootctl status
System:
     Firmware: UEFI 2.40 (American Megatrends 5.11)
  Secure Boot: enabled
   Setup Mode: user

Current Boot Loader:
      Product: systemd-boot 243
     Features: ✓ Boot counting
               ✓ Menu timeout control
               ✓ One-shot menu timeout control
               ✓ Default entry control
               ✓ One-shot entry control
               ✓ Support for XBOOTLDR partition
               ✓ Support for passing random seed to OS
         Stub: systemd-stub 243
          ESP: /dev/disk/by-partuuid/00e7649e-bf1f-481b-87cd-7228af8c118f
         File: └─/EFI/systemd/systemd-bootx64.efi

Random Seed:
 Passed to OS: no
 System Token: set
       Exists: yes

Available Boot Loaders on ESP:
          ESP: /boot (/dev/disk/by-partuuid/00e7649e-bf1f-481b-87cd-7228af8c118f)
         File: └─/EFI/systemd/systemd-bootx64.efi (systemd-boot 243)
         File: └─/EFI/BOOT/BOOTX64.EFI (systemd-boot 243)

Boot Loaders Listed in EFI Variables:
        Title: Linux Boot Manager
           ID: 0x0007
       Status: active, boot-order
    Partition: /dev/disk/by-partuuid/00e7649e-bf1f-481b-87cd-7228af8c118f
         File: └─/EFI/systemd/systemd-bootx64.efi

        Title: Windows Boot Manager
           ID: 0x0000
       Status: active
    Partition: /dev/disk/by-partuuid/fd36bbae-be4f-409a-b1ce-14771e091067
         File: └─/EFI/Microsoft/Boot/bootmgfw.efi

Boot Loader Entries:
        $BOOT: /boot (/dev/disk/by-partuuid/00e7649e-bf1f-481b-87cd-7228af8c118f)

Default Boot Loader Entry:
        title: Alpine Linux edge
           id: alpine-3.12_alpha20200122
       source: /boot/EFI/Linux/linux.efi
        linux: EFI/Linux/linux.efi
      options: initrd=\intel-ucode.img initrd=\initramfs-lts root=UUID=1437f4dd-7efd-4a4e-8fac-797b548076d1 cryptdm=pool cryptroot=/dev/sda2 modules=ext4,xfs rootfstype=ext4 psi=1 quiet

Updating

When the systemd-boot package is updated the bootloader installed needs to be updated as well: 

# bootctl update

Removing

If for any reason you wish to not use systemd-boot anymore then the bootctl binary also provides a command for easily removing the files that were installed with the install command: 

# bootctl remove
# apk del systemd-boot

External Links

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