docs: reworder/rewrite BLS to read more like a specification

[thirdparty/systemd.git] / docs / BOOT_LOADER_SPECIFICATION.md

---
title: Boot Loader Specification
category: Booting
layout: default
SPDX-License-Identifier: LGPL-2.1-or-later
---

# The Boot Loader Specification

This document defines a set of file formats and naming conventions that allow
the boot loader configuration to be shared between multiple operating systems
and boot loaders installed on one device.

Operating systems cooperatively manage a boot loader configuration directory
that contains drop-in files, making multi-boot scenarios easy to support. Boot
menu items are defined via a simple format that can be understood by different
boot loader implementations, operating systems, and userspace programs. The
same scheme can be used to prepare OS media for cases where the firmware
includes a boot loader.

## Target audience

The target audience for this specification is:

* Boot loader developers, to write a boot loader that directly reads its
  configuration from these files
* Firmware developers, to add generic boot loading support directly to the
  firmware itself
* OS installer developers, to create appropriate partitions and set up the
  initial drop-in directory
* Distribution developers, to create appropriate configuration snippets when
  installing or updating kernel packages
* UI developers, to implement user interfaces that list and select among the
  available boot options

## The boot partition

Everything described below is located on a file system that will be called
`$BOOT`. The boot loader or user-space programs reading the boot loader
configuration should pick `$BOOT` according to the following rules:

* On disks with an MBR partition table:

  * A partition with the type ID of 0xEA shall be used as `$BOOT`.

* On disks with GPT (GUID Partition Table)

  * If an Extended Boot Loader Partition (XBOOTLDR partition for short) — a
    partition with GPT type GUID of `bc13c2ff-59e6-4262-a352-b275fd6f7172` —
    is found, it should be used as `$BOOT`.

  * Otherwise, the EFI System Partition (ESP for short) — a partition with
    GPT type GUID of `c12a7328-f81f-11d2-ba4b-00a0c93ec93b` — should
    be used as `$BOOT`.

An installer for the operating system should use this logic when selecting or
creating partitions:

  * If `$BOOT` is not found, a new suitably sized partition (let's say 500MB)
    should be created and used as `$BOOT`, matching the characteristics
    described above. On disks with GPT, just an ESP partition without a
    XBOOTLDR partition should be created.

  * If the OS is installed on a disk with GPT and the ESP partition is found
    but is too small, a new suitably sized (let's say 500MB) XBOOTLDR partition
    shall be created and used as `$BOOT`.

This placeholder file system shall be determined during _installation time_,
and an fstab entry may be created. It should be mounted to either `/boot/` or
`/efi/`. Additional locations like `/boot/efi/` (with `/boot/` being a separate
file system) might be supported by implementations. This is not recommended
because the mounting of `$BOOT` is then dependent on and requires the mounting
of the intermediate file system.

**Note:** _`$BOOT` is **shared** among all OS installations of a system.
Instead of maintaining one `$BOOT` per installed OS (as `/boot/` was
traditionally handled), all installed OSes use the same place for boot-time
configuration._

For systems where the firmware is able to read file systems directly, `$BOOT`
must be a file system readable by the firmware. For other systems and generic
installation and live media, `$BOOT` must be a VFAT (16 or 32) file system.
Applications accessing `$BOOT` should hence not assume that fancier file system
features such as symlinks, hardlinks, access control or case sensitivity are
supported.

## Boot loader entries

This specification defines two types of boot loader entries. The first type is
text based, very simple, and suitable for a variety of firmware, architecture
and image types ("Type #1"). The second type is specific to EFI, but allows
single-file images that embed all metadata in the kernel binary itself, which
is useful to cryptographically sign them as one file for the purpose of
SecureBoot ("Type #2").

Not all boot loader entries will apply to all systems. For example, Type #1
entries that use the `efi` key and all Type #2 entries only apply to EFI
systems. Entries using the `architecture` key might specify an architecture that
doesn't match the local one. Boot loaders should ignore all entries that don't
match the local platform and what the boot loader can support, and hide them
from the user. Only entries matching the feature set of boot loader and system
shall be considered and displayed. This allows image builders to put together
images that transparently support multiple different architectures.

Note that the `$BOOT` partition is not supposed to be exclusive territory of
this specification. This specification only defines semantics of the `/loader/`
directory inside the file system (see below), but it doesn't intend to define
ownership of the whole file system exclusively. Boot loaders, firmware, and
other software implementing this specification may choose to place other files
and directories in the same file system. For example, boot loaders that
implement this specification might install their own boot code on the same
partition; on systems where `$BOOT` is the ESP this is a particularly common
setup. Implementations of this specification must be able to operate correctly
if files or directories other than `/loader/` are found in the top level
directory. Implementations that add their own files or directories to the file
systems should use well-named directories, to make name collisions between
multiple users of the file system unlikely.

### Type #1 Boot Loader Specification Entries

This specification defines two directories below `$BOOT`:

* `$BOOT/loader/` is the directory containing all files needed for Type #1
  entries

* `$BOOT/loader/entries/` is the directory containing the drop-in
  snippets. This directory contains one `.conf` file for each boot menu item.

**Note:** _In all cases the `/loader/` directory should be located directly in
the root of the file system. Specifically, if `$BOOT` is the ESP, then
`/loader/` directory should be located directly in the root directory of the
ESP, and not in the `/EFI/` subdirectory._

Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or
more configuration snippets with the suffix `.conf`, one for each boot menu
item. The file name of the file is used for identification of the boot item but
shall never be presented to the user in the UI. The file name may be chosen
freely but should be unique enough to avoid clashes between OS installations.
More specifically, it is suggested to include the machine ID (`/etc/machine-id`
or the D-Bus machine ID for OSes that lack `/etc/machine-id`), the kernel
version (as returned by `uname -r`) and an OS identifier (the `ID=` field of
`/etc/os-release`).

Example: `$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.

In order to maximize compatibility with file system implementations and
restricted boot loader environments, and to minimize conflicting character use
with other programs, file names shall be chosen from a restricted character
set: ASCII upper and lower case characters, digits, "+", "-", "_" and ".".
Also, the file names should have a length of at least one and at most 255
characters (including the file name suffix).

These configuration snippets shall be UNIX-style text files (i.e. lines
separated by a single newline character), in the UTF-8 encoding. The
configuration snippets are loosely inspired by Grub1's configuration syntax.
Lines beginning with "#" are used for comments and shall be ignored. The first
word of a line is used as key and is separated by one or more spaces from the
value.

#### Type #1 Boot Loader Entry Keys

The following keys are recognized:

* `title` is a human-readable title for this menu item to be displayed in the
  boot menu. It is a good idea to initialize this from the `PRETTY_NAME=` of
  `/etc/os-release`. This name should be descriptive and does not have to be
  unique. If a boot loader discovers two entries with the same title it should
  show more than just the raw title in the UI, for example by appending the
  `version` field. This field is optional.

  Example: `title Fedora 18 (Spherical Cow)`

* `version` is a human-readable version for this menu item. This is usually the
  kernel version and is intended for use by OSes to install multiple kernel
  versions with the same `title` field. This field shall be in a syntax that is
  useful for Debian-style version sorts, so that the boot loader UI can
  determine the newest version easily and show it first or preselect it
  automatically. This field is optional.

  Example: `version 3.7.2-201.fc18.x86_64`

* `machine-id` is the machine ID of the OS. This can be used by boot loaders
  and applications to filter out boot entries, for example to show only a
  single newest kernel per OS, to group items by OS, or to filter out the
  currently booted OS when showing only other installed operating systems.
  This ID shall be formatted as 32 lower case hexadecimal characters
  (i.e. without any UUID formatting). This key is optional.

  Example: `machine-id 4098b3f648d74c13b1f04ccfba7798e8`

* `sort-key` is a short string used for sorting entries on display. This should
  typically be initialized from the `IMAGE_ID=` or `ID=` fields of
  `/etc/os-release`, possibly with an additional suffix. This field is
  optional. If set, it is used as primary sorting key for the entries on
  display (lexicographically increasing). It does not have to be unique (and
  usually is not). If non-unique the the `machine-id` (lexicographically
  increasing) and `version` (lexicographically decreasing, i.e. newest version
  first) fields described above are used as secondary/ternary sorting keys. If
  this field is not set entries are typically sorted by the `.conf` file name
  of the entry.

  Example: `sort-key fedora`

* `linux` is the Linux kernel to spawn and as a path relative to `$BOOT`. It
  is recommended that every distribution creates a machine id and version
  specific subdirectory below `$BOOT` and places its kernels and initial RAM
  disk images there.

  Example: `linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux`

* `initrd` is the initrd to use when executing the kernel. This also
  shall be a path relative to `$BOOT`. This key is optional. This key may
  appear more than once in which case all specified images are used, in the
  order they are listed.

  Example: `initrd 6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`

* `efi` refers to an arbitrary EFI program. This also takes a path relative to
  `$BOOT`. If this key is set, and the system is not an EFI system this entry
  should be hidden.

* `options` shall contain kernel parameters to pass to the Linux kernel to
  spawn. This key is optional and may appear more than once in which case all
  specified parameters are used in the order they are listed.

  Example: `options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet`

* `devicetree` refers to the binary device tree to use when executing the
  kernel. This also shall be a path relative to `$BOOT`. This key is
  optional.

  Example: `devicetree 6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`

* `devicetree-overlay` refers to a list of device tree overlays that should be
  applied by the boot loader. Multiple overlays are separated by spaces and
  applied in the same order as they are listed. This key is optional but
  depends on the `devicetree` key.

  Example: `devicetree-overlay /6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_A.dtbo /6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_B.dtbo`

* `architecture` refers to the architecture this entry is for. The argument
  should be an architecture identifier, using the architecture vocabulary
  defined by the EFI specification (i.e. `IA32`, `x64`, `IA64`, `ARM`, `AA64`,
  …). If specified and it does not match the local system architecture this
  entry should be hidden. The comparison should be done case-insensitively.

  Example: `architecture aa64`

Each configuration drop-in snippet must include at least a `linux` or an `efi`
key. Here is an example for a complete drop-in file:

    # /boot/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf
    title        Fedora 19 (Rawhide)
    sort-key     fedora
    machine-id   6a9857a393724b7a981ebb5b8495b9ea
    version      3.8.0-2.fc19.x86_64
    options      root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet
    architecture x64
    linux        /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
    initrd       /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd

On EFI systems all Linux kernel images should be EFI images. In order to
increase compatibility with EFI systems it is highly recommended only to
install EFI kernel images, even on non-EFI systems, if that's applicable and
supported on the specific architecture.

Conversely, in order to increase compatibility it is recommended to install
generic kernel images that make few assumptions about the firmware they run on,
i.e. it is a good idea that both images shipped as UEFI PE images and those
which are not don't make unnecessary assumption on the underlying firmware,
i.e. don't hard depend on legacy BIOS calls or UEFI boot services.

Note that these configuration snippets may only reference kernels (and EFI
programs) that reside on the same file system as the configuration snippets,
i.e. everything referenced must be contained in the same file system. This is
by design, as referencing other partitions or devices would require a
non-trivial language for denoting device paths. If kernels/initrds are to be
read from other partitions/disks the boot loader can do this in its own native
configuration, using its own specific device path language, and this is out of
focus for this specification. More specifically, on non-EFI systems
configuration snippets following this specification cannot be used to spawn
other operating systems (such as Windows).

### Standard-conformance Marker File

Unfortunately, there are implementations of boot loading infrastructure that
are also using the `/loader/entries/` directory, but installing files that do
not follow this specification. In order to minimize confusion, a boot loader
implementation may place the file `/loader/entries.srel` next to the
`/loader/entries/` directory containing the ASCII string `type1` (followed by a
UNIX newline). Tools that need to determine whether an existing directory
implements the semantics described here may check for this file and contents:
if it exists and contains the mentioned string, it shall assume a
standards-compliant implementation is in place. If it exists but contains a
different string it shall assume other semantics are implemented. If the file
does not exist, no assumptions should be made.

### Type #2 EFI Unified Kernel Images

A unified kernel image is a single EFI PE executable combining an EFI stub
loader, a kernel image, an initramfs image, and the kernel command line. See
the description of the `--uefi` option in
[dracut(8)](http://man7.org/linux/man-pages/man8/dracut.8.html). Such unified
images are installed in the`$BOOT/EFI/Linux/` directory and must have the
extension `.efi`. Support for images of this type is of course specific to
systems with EFI firmware. Ignore this section if you work on systems not
supporting EFI.

Type #2 file names should be chosen from the same restricted character set as
Type #1 described above (but with the file name suffix of `.efi` instead of
`.conf`).

Images of this type have the advantage that all metadata and payload that makes
up the boot entry is contained in a single PE file that can be signed
cryptographically as one for the purpose of EFI SecureBoot.

A valid unified kernel image must contain two PE sections:

* `.cmdline` section with the kernel command line,
* `.osrel` section with an embedded copy of the
  [os-release](https://www.freedesktop.org/software/systemd/man/os-release.html)
  file describing the image.

The `PRETTY_NAME=` and `VERSION_ID=` fields in the embedded `os-release` file
are used the same as `title` and `version` in the Type #1 entries. The
`.cmdline` section is used instead of the `options` field. `linux` and `initrd`
fields are not necessary, and there is no counterpart for the `machine-id`
field.

On EFI, any such images shall be added to the list of valid boot entries.

### Additional notes

Note that these configurations snippets do not need to be the only
configuration source for a boot loader. It may extend this list of entries with
additional items from other configuration files (for example its own native
configuration files) or automatically detected other entries without explicit
configuration.

To make this explicitly clear: this specification is designed with "free"
operating systems in mind, starting Windows or macOS is out of focus with these
configuration snippets, use boot-loader specific solutions for that. In the
text above, if we say "OS" we hence imply "free", i.e. primarily Linux (though
this could be easily be extended to the BSDs and whatnot).

Note that all paths used in the configuration snippets use a Unix-style "/" as
path separator. This needs to be converted to an EFI-style "\\" separator in
EFI boot loaders.


## Locating boot entries

A _boot loader_ locates `$BOOT`, then simply reads all files
`$BOOT/loader/entries/*.conf`, and populates its boot menu with this. On EFI,
it then extends this with any unified kernel images found in
`$BOOT/EFI/Linux/*.efi`. It may also add additional entries, for example a
"Reboot into firmware" option. Optionally it may sort the menu based on the
`sort-key`, `machine-id` and `version` fields, and possibly others. It uses the
file name to identify specific items, for example in case it supports storing
away default entry information somewhere. A boot loader should generally not
modify these files.

For "Boot Loader Specification Entries" (Type #1), the _kernel package
installer_ installs the kernel and initrd images to `$BOOT` (it is recommended
to place these files in a vendor and OS and installation specific directory)
and then generates a configuration snippet for it, placing this in
`$BOOT/loader/entries/xyz.conf`, with "xyz" as concatenation of machine id and
version information (see above). The files created by a kernel package are tied
to the kernel package and should be removed along with it.

For "EFI Unified Kernel Images" (Type #2), the vendor or kernel package
installer should create the combined image and drop it into
`$BOOT/EFI/Linux/`. This file is also tied to the kernel package and should be
removed along with it.

A _UI application_ intended to show available boot options shall operate
similarly to a boot loader, but might apply additional filters, for example by
filtering the booted OS via the machine ID, or by suppressing all but the
newest kernel versions.

An _OS installer_ picks the right place for `$BOOT` as defined above (possibly
creating a partition and file system for it) and creates the `/loader/entries/`
directory in it. It then installs an appropriate boot loader that can read
these snippets. Finally, it installs one or more kernel packages.


## Additional discussion

### Why is there a need for this specification?

This specification brings the following advantages:

* Installation of new boot entries is more robust, as no explicit rewriting of
  configuration files is required.

* It allows an out-of-the-box boot experience on any platform without the need
  of traditional firmware mechanisms (e.g. BIOS calls, UEFI Boot Services).

* It improves dual-boot scenarios. Without cooperation, multiple Linux
  installations tend to fight over which boot loader becomes the primary one in
  possession of the MBR or the boot partition, and only that one installation
  can then update the boot loader configuration. Other Linux installs have to
  be manually configured to never touch the MBR and instead install a
  chain-loaded boot loader in their own partition headers. In this new scheme
  all installations share a loader directory and no manual configuration has to
  take place. All participants implicitly cooperate due to removal of name
  collisions and can install/remove their own boot menu entries without
  interfering with the entries of other installed operating systems.

* Drop-in directories are now pretty ubiquitous on Linux as an easy way to
  extend configuration without having to edit, regenerate or manipulate
  configuration files. For the sake of uniformity, we should do the same for
  the boot menu.

* Userspace code can sanely parse boot loader configuration which is essential
  with modern firmware which does not necessarily initialize USB keyboards
  during boot, which makes boot menus hard to reach for the user. If userspace
  code can parse the boot loader configuration too, UI can be written that
  select a boot menu item to boot into before rebooting the machine, thus not
  requiring interactivity during early boot.

* To unify and thus simplify configuration of the various boot loaders, which
  makes configuration of the boot loading process easier for users,
  administrators, and developers alike.

* For boot loaders with configuration _scripts_ such as grub2, adopting this
  spec allows for mostly static scripts that are generated only once at first
  installation, but then do not need to be updated anymore as that is done via
  drop-in files exclusively.

### Why not simply rely on the EFI boot menu logic?

EFI is not ubiquitous, especially not in embedded systems. But even on systems
with EFI, which provides a boot options logic that can offer similar
functionality, this specfication is still needed for the following reasons:

* The various EFI implementations implement the boot order/boot item logic to
  different levels. Some firmware implementations do not offer a boot menu at
  all and instead unconditionally follow the EFI boot order, booting the first
  item that is working.

* If the firmware setup is used to reset data, usually all EFI boot entries
  are lost, making the system entirely unbootable, as the firmware setups
  generally do not offer a UI to define additional boot items. By placing the
  menu item information on disk, it is always available, even if the firmware
  configuration is lost.

* Harddisk images should be movable between machines and be bootable without
  requiring firmare configuration. This also requires that the list
  of boot options is defined on disk, and not in EFI variables alone.

* EFI is not universal yet (especially on non-x86 platforms), this
  specification is useful both for EFI and non-EFI boot loaders.

* Many EFI systems disable USB support during early boot to optimize boot
  times, thus making keyboard input unavailable in the EFI menu. It is thus
  useful if the OS UI has a standardized way to discover available boot options
  which can be booted to.

### Out of Focus

There are a couple of items that are out of focus for this specification:

* If userspace can figure out the available boot options, then this is only
  useful so much: we'd still need to come up with a way how userspace could
  communicate to the boot loader the default boot loader entry temporarily or
  persistently. Defining a common scheme for this is certainly a good idea, but
  out of focus for this specification.

* This specification is just about "Free" Operating systems. Hooking in other
  operating systems (like Windows and macOS) into the boot menu is a different
  story and should probably happen outside of this specification. For example,
  boot loaders might choose to detect other available OSes dynamically at
  runtime without explicit configuration (like `systemd-boot` does it), or via
  native configuration (for example via explicit Grub2 configuration generated
  once at installation).

* This specification leaves undefined what to do about systems which are
  upgraded from an OS that does not implement this specification. As the
  previous boot loader logic was largely handled by in distribution-specific
  ways we probably should leave the upgrade path (and whether there actually is
  one) to the distributions. The simplest solution might be to simply continue
  with the old scheme for old installations and use this new scheme only for
  new installations.


## Links

[GUID Partition Table](https://en.wikipedia.org/wiki/GUID_Partition_Table)<br>
[Boot Loader Interface](BOOT_LOADER_INTERFACE.md)<br>
[Discoverable Partitions Specification](DISCOVERABLE_PARTITIONS.md)<br>
[`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)<br>
[`bootctl(1)`](https://www.freedesktop.org/software/systemd/man/bootctl.html)<br>
[`systemd-gpt-auto-generator(8)`](https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html)