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

Browse Source 
I tried not to introduce any semantic changes, but to reorder the whole
text to be more usable as a reference specification: more sections are
created and the discussion and justifications are moved to the end.
Also, "BIOS" is changed to "firmware" in various places, and other parts
of the text that made sense when this was originally written are now dated
are adjusted. I separated and extended the examples a bit.

The abstract at the top ("TL;DR: Currently there’s no common boot scheme…")
is dropped. It didn't seem to fit anywhere.
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Zbigniew Jędrzejewski-Szmek 2022-05-23 14:22:40 +02:00
parent 65df0ce39a
commit 7bb36d2d7a
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docs/BOOT_LOADER_SPECIFICATION.md
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@ -7,120 +7,63 @@ SPDX-License-Identifier: LGPL-2.1-or-later
# The Boot Loader Specification # The Boot Loader Specification
_TL;DR: Currently there's no common boot scheme across architectures and This document defines a set of file formats and naming conventions that allow
platforms for open-source operating systems. There's also little cooperation the boot loader configuration to be shared between multiple operating systems
between multiple distributions in dual-boot (or triple, … multi-boot) and boot loaders installed on one device.
setups. We'd like to improve this situation by getting everybody to commit to a
single boot configuration format that is based on drop-in files, and thus is
robust, simple, works without rewriting configuration files and is free of
namespace clashes._
The Boot Loader Specification defines a scheme how different operating systems Operating systems cooperatively manage a boot loader configuration directory
can cooperatively manage a boot loader configuration directory, that accepts that contains drop-in files, making multi-boot scenarios easy to support. Boot
drop-in files for boot menu items that are defined in a format that is shared menu items are defined via a simple format that can be understood by different
between various boot loader implementations, operating systems, and userspace boot loader implementations, operating systems, and userspace programs. The
programs. The same scheme can be used to prepare OS media for cases where the same scheme can be used to prepare OS media for cases where the firmware
firmware includes a boot loader. The target audience for this specification is: 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 * Boot loader developers, to write a boot loader that directly reads its
configuration at runtime from these drop-in snippets configuration from these files
* Firmware developers, to add generic boot loading support directly to the * Firmware developers, to add generic boot loading support directly to the
firmware itself firmware itself
* Distribution and Core OS developers, in order to create these snippets at * OS installer developers, to create appropriate partitions and set up the
OS/kernel package installation time initial drop-in directory
* UI developers, for implementing a user interface that discovers the available * Distribution developers, to create appropriate configuration snippets when
boot options installing or updating kernel packages
* OS Installer developers, to prepare their installation media and for setting * UI developers, to implement user interfaces that list and select among the
up the initial drop-in directory available boot options
## Why is there a need for this specification? ## The boot partition
Of course, without this specification things already work mostly fine. But here's why we think this specification is needed: Everything described below is located on a file system that will be called
`$BOOT`. The boot loader or user-space programs reading the boot loader
* To make the boot more robust, as no explicit rewriting of configuration files configuration should pick `$BOOT` according to the following rules:
is required any more
* To allow an out of the box boot experience on any platform without the need
of traditional firmware mechanisms (e.g. BIOS calls, UEFI Boot Services)
* To improve dual-boot scenarios. Currently, multiple Linux installations tend
to fight over which boot loader becomes the primary one in possession of the
MBR, and only that one installation can then update the boot loader
configuration of it freely. 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 as all
installations share a loader directory no manual configuration has to take
place, and all participants implicitly cooperate due to removal of name
collisions and can install/remove their own boot menu entries at free will,
without interfering with the entries of other installed operating systems.
* Drop-in directories are otherwise 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
extending the boot menu.
* Userspace code can sanely parse boot loader configuration which is essential
with modern BIOSes which do not necessarily initialize USB keyboards anymore
during boot, which makes boot menus hard to reach for the user. If userspace
code can parse the boot loader configuration, too, this allows for UIs that
can 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 around,
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. If you have an EFI
system, it provides a boot options logic that can offer similar
functionality. Here's why we think that it is not enough for our uses:
* 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 all 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, regardless if the BIOS
setup data is lost.
* Harddisk images should be movable between machines and be bootable without
requiring explicit EFI variables to be set. 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.
## Technical Details
Everything described below is located on a placeholder file system `$BOOT`. The
installer program should pick `$BOOT` according to the following rules:
* On disks with an MBR partition table: * On disks with an MBR partition table:
* If the OS is installed on a disk with an MBR partition table, and a
partition with the type id of 0xEA already exists it should be used as * A partition with the type ID of 0xEA shall be used as `$BOOT`.
`$BOOT`.
* Otherwise, if the OS is installed on a disk with an MBR partition table, a
new partition with type id of 0xEA shall be created, of a suitable size
(let's say 500MB), and it should be used as `$BOOT`.
* On disks with GPT (GUID Partition Table) * On disks with GPT (GUID Partition Table)
* If the OS is installed on a disk with GPT, and an Extended Boot Loader
Partition (or XBOOTLDR partition for short), i.e. a partition with GPT type * If an Extended Boot Loader Partition (XBOOTLDR partition for short) — a
GUID of `bc13c2ff-59e6-4262-a352-b275fd6f7172`, already exists, it should partition with GPT type GUID of `bc13c2ff-59e6-4262-a352-b275fd6f7172` —
is found, it should be used as `$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 `$BOOT`. be used as `$BOOT`.
* Otherwise, if the OS is installed on a disk with GPT, and an EFI System
Partition (or ESP for short), i.e. a partition with GPT type UID of An installer for the operating system should use this logic when selecting or
`c12a7328-f81f-11d2-ba4b-00a0c93ec93b` already exists and is large enough creating partitions:
(let's say 250MB) and otherwise qualifies, it should be used as `$BOOT`.
* Otherwise, if the OS is installed on a disk with GPT, and if the ESP * If `$BOOT` is not found, a new suitably sized partition (let's say 500MB)
already exists but is too small, a new suitably sized (let's say 500MB) should be created and used as `$BOOT`, matching the characteristics
XBOOTLDR partition shall be created and used as `$BOOT`. described above. On disks with GPT, just an ESP partition without a
* Otherwise, if the OS is installed on a disk with GPT, and no ESP exists XBOOTLDR partition should be created.
yet, a new suitably sized (let's say 500MB) ESP should be created and used
as `$BOOT`. * If the OS is installed on a disk with GPT and the ESP partition is found
but is too small, a new suitably sized (let's say 500MB) XBOOTLDR partition
shall be created and used as `$BOOT`.
This placeholder file system shall be determined during _installation time_, 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 and an fstab entry may be created. It should be mounted to either `/boot/` or
@ -129,20 +72,22 @@ file system) might be supported by implementations. This is not recommended
because the mounting of `$BOOT` is then dependent on and requires the mounting because the mounting of `$BOOT` is then dependent on and requires the mounting
of the intermediate file system. of the intermediate file system.
**Note:** _`$BOOT` should be considered **shared** among all OS installations **Note:** _`$BOOT` is **shared** among all OS installations of a system.
of a system. Instead of maintaining one `$BOOT` per installed OS (as `/boot/` Instead of maintaining one `$BOOT` per installed OS (as `/boot/` was
was traditionally handled), all installed OS share the same place to drop in traditionally handled), all installed OSes use the same place for boot-time
their boot-time configuration._ configuration._
For systems where the firmware is able to read file systems directly, `$BOOT` For systems where the firmware is able to read file systems directly, `$BOOT`
must be a file system readable by the firmware. For other systems and generic must be a file system readable by the firmware. For other systems and generic
installation and live media, `$BOOT` must be a VFAT (16 or 32) file installation and live media, `$BOOT` must be a VFAT (16 or 32) file system.
system. Applications accessing `$BOOT` should hence not assume that fancier Applications accessing `$BOOT` should hence not assume that fancier file system
file system features such as symlinks, hardlinks, access control or case features such as symlinks, hardlinks, access control or case sensitivity are
sensitivity are supported. supported.
## Boot loader entries
This specification defines two types of boot loader entries. The first type is 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 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 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 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 is useful to cryptographically sign them as one file for the purpose of
@ -161,10 +106,10 @@ Note that the `$BOOT` partition is not supposed to be exclusive territory of
this specification. This specification only defines semantics of the `/loader/` this specification. This specification only defines semantics of the `/loader/`
directory inside the file system (see below), but it doesn't intend to define directory inside the file system (see below), but it doesn't intend to define
ownership of the whole file system exclusively. Boot loaders, firmware, and ownership of the whole file system exclusively. Boot loaders, firmware, and
other software implementing this specification may choose to place other other software implementing this specification may choose to place other files
files and directories in the same file system. For example, boot loaders that and directories in the same file system. For example, boot loaders that
implement this specification might install their own boot code into the `$BOOT` implement this specification might install their own boot code on the same
partition. On systems where `$BOOT` is the ESP this is a particularly common partition; on systems where `$BOOT` is the ESP this is a particularly common
setup. Implementations of this specification must be able to operate correctly setup. Implementations of this specification must be able to operate correctly
if files or directories other than `/loader/` are found in the top level 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 directory. Implementations that add their own files or directories to the file
@ -173,7 +118,7 @@ multiple users of the file system unlikely.
### Type #1 Boot Loader Specification Entries ### Type #1 Boot Loader Specification Entries
We define two directories below `$BOOT`: This specification defines two directories below `$BOOT`:
* `$BOOT/loader/` is the directory containing all files needed for Type #1 * `$BOOT/loader/` is the directory containing all files needed for Type #1
entries entries
@ -187,102 +132,129 @@ the root of the file system. Specifically, if `$BOOT` is the ESP, then
ESP, and not in the `/EFI/` subdirectory._ ESP, and not in the `/EFI/` subdirectory._
Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or
more configuration snippets with the suffix ".conf", one for each boot menu more configuration snippets with the suffix `.conf`, one for each boot menu
item. The file name of the file is used for identification of the boot item but item. The file name of the file is used for identification of the boot item but
shall never be presented to the user in the UI. The file name may be chosen shall never be presented to the user in the UI. The file name may be chosen
freely but should be unique enough to avoid clashes between OS freely but should be unique enough to avoid clashes between OS installations.
installations. More specifically it is suggested to include the machine ID More specifically, it is suggested to include the machine ID (`/etc/machine-id`
(`/etc/machine-id` or the D-Bus machine ID for OSes that lack or the D-Bus machine ID for OSes that lack `/etc/machine-id`), the kernel
`/etc/machine-id`), the kernel version (as returned by `uname -r`) and an OS version (as returned by `uname -r`) and an OS identifier (the `ID=` field of
identifier (The ID field of `/etc/os-release`). Example: `/etc/os-release`).
`$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.
Example: `$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.
In order to maximize compatibility with file system implementations and In order to maximize compatibility with file system implementations and
restricted boot loader environments, and to minimize conflicting character use restricted boot loader environments, and to minimize conflicting character use
with other programs, file names shall be chosen from a restricted character with other programs, file names shall be chosen from a restricted character
set: ASCII upper and lower case characters, digits, "+", "-", "_" and 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 Also, the file names should have a length of at least one and at most 255
characters (including file name suffix). characters (including the file name suffix).
These configuration snippets shall be Unix-style text files (i.e. line These configuration snippets shall be UNIX-style text files (i.e. lines
separation with a single newline character), in the UTF-8 encoding. The separated by a single newline character), in the UTF-8 encoding. The
configuration snippets are loosely inspired on Grub1's configuration configuration snippets are loosely inspired by Grub1's configuration syntax.
syntax. Lines beginning with '#' shall be ignored and used for commenting. The Lines beginning with "#" are used for comments and shall be ignored. The first
first word of a line is used as key and shall be separated by one or more word of a line is used as key and is separated by one or more spaces from the
spaces from its value. The following keys are known: value.
* `title` shall contain a human readable title string for this menu item. This #### Type #1 Boot Loader Entry Keys
will be displayed in the boot menu for the item. It is a good idea to
initialize this from the `PRETTY_NAME` of `/etc/os-release`. This name should The following keys are recognized:
be descriptive and does not have to be unique. If a boot loader discovers two
entries with the same title it is a good idea to show more than just the raw * `title` is a human-readable title for this menu item to be displayed in the
title in the UI, for example by appending the `version` field. This field is boot menu. It is a good idea to initialize this from the `PRETTY_NAME=` of
optional. Example: "Fedora 18 (Spherical Cow)". `/etc/os-release`. This name should be descriptive and does not have to be
* `version` shall contain a human readable version string for this menu unique. If a boot loader discovers two entries with the same title it should
item. This is usually the kernel version and is intended for use by OSes to show more than just the raw title in the UI, for example by appending the
install multiple kernel versions at the same time with the same `title` `version` field. This field is optional.
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 Example: `title Fedora 18 (Spherical Cow)`
easily and show it first or preselect it automatically. This field is
optional. Example: `3.7.2-201.fc18.x86_64`. * `version` is a human-readable version for this menu item. This is usually the
* `machine-id` shall contain the machine ID of the OS `/etc/machine-id`. This kernel version and is intended for use by OSes to install multiple kernel
is useful for boot loaders and applications to filter out boot entries, for versions with the same `title` field. This field shall be in a syntax that is
example to show only a single newest kernel per OS, or to group items by OS, useful for Debian-style version sorts, so that the boot loader UI can
or to maybe filter out the currently booted OS in UIs that want to show only determine the newest version easily and show it first or preselect it
other installed operating systems. This ID shall be formatted as 32 lower automatically. This field is optional.
case hexadecimal characters (i.e. without any UUID formatting). This key is
optional. Example: `4098b3f648d74c13b1f04ccfba7798e8`. Example: `version 3.7.2-201.fc18.x86_64`
* `sort-key` shall contain a short string used for sorting entries on
display. This can be defined freely though should typically be initialized * `machine-id` is the machine ID of the OS. This can be used by boot loaders
from `IMAGE_ID=` or `ID=` from `/etc/os-release` of the relevant entry, and applications to filter out boot entries, for example to show only a
possibly suffixed. This field is optional. If set, it is used as primary single newest kernel per OS, to group items by OS, or to filter out the
sorting key for the entries on display (lexicographically increasing). It currently booted OS when showing only other installed operating systems.
does not have to be unique (and usually is not). If non-unique the the This ID shall be formatted as 32 lower case hexadecimal characters
`machine-id` (lexicographically increasing) and `version` (lexicographically (i.e. without any UUID formatting). This key is optional.
decreasing, i.e. newest version first) fields described above are used as
secondary/ternary sorting keys. If this field is not set entries are Example: `machine-id 4098b3f648d74c13b1f04ccfba7798e8`
typically sorted by the `.conf` file name of the entry.
* `linux` refers to the Linux kernel to spawn and shall be a path relative to * `sort-key` is a short string used for sorting entries on display. This should
`$BOOT`. It is recommended that every distribution creates a machine id and typically be initialized from the `IMAGE_ID=` or `ID=` fields of
version specific subdirectory below `$BOOT` and places its kernels and `/etc/os-release`, possibly with an additional suffix. This field is
initial RAM disk images there. Example: optional. If set, it is used as primary sorting key for the entries on
`/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux`. display (lexicographically increasing). It does not have to be unique (and
* `initrd` refers to the initrd to use when executing the kernel. This also usually is not). If non-unique the the `machine-id` (lexicographically
increasing) and `version` (lexicographically decreasing, i.e. newest version
first) fields described above are used as secondary/ternary sorting keys. If
this field is not set entries are typically sorted by the `.conf` file name
of the entry.
Example: `sort-key fedora`
* `linux` is the Linux kernel to spawn and as a path relative to `$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 shall be a path relative to `$BOOT`. This key is optional. This key may
appear more than once in which case all specified images are used, in the appear more than once in which case all specified images are used, in the
order they are listed. Example: order they are listed.
`6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`.
Example: `initrd 6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`
* `efi` refers to an arbitrary EFI program. This also takes a path relative to * `efi` refers to an arbitrary EFI program. This also takes a path relative to
`$BOOT`. If this key is set, and the system is not an EFI system this entry `$BOOT`. If this key is set, and the system is not an EFI system this entry
should be hidden. should be hidden.
* `options` shall contain kernel parameters to pass to the Linux kernel to * `options` shall contain kernel parameters to pass to the Linux kernel to
spawn. This key is optional and may appear more than once in which case all spawn. This key is optional and may appear more than once in which case all
specified parameters are used in the order they are listed. specified parameters are used in the order they are listed.
Example: `options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet`
* `devicetree` refers to the binary device tree to use when executing the * `devicetree` refers to the binary device tree to use when executing the
kernel. This also shall be a path relative to `$BOOT`. This key is kernel. This also shall be a path relative to `$BOOT`. This key is
optional. Example: optional.
`6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`.
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 * `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 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 applied in the same order as they are listed. This key is optional but
depends on the `devicetree` key. Example: depends on the `devicetree` key.
`/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_A.dtbo
/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_B.dtbo` Example: `devicetree-overlay /6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_A.dtbo /6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_B.dtbo`
* `architecture` refers to the architecture this entry is defined for. The
argument should be an architecture identifier, using the architecture * `architecture` refers to the architecture this entry is for. The argument
vocabulary defined by the EFI specification (i.e. `IA32`, `x64`, `IA64`, should be an architecture identifier, using the architecture vocabulary
`ARM`, `AA64`, …). If specified and this does not match (case insensitively) defined by the EFI specification (i.e. `IA32`, `x64`, `IA64`, `ARM`, `AA64`,
the local system architecture this entry should be hidden. …). 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` Each configuration drop-in snippet must include at least a `linux` or an `efi`
key and is otherwise not valid. Here's an example for a complete drop-in file: key. Here is an example for a complete drop-in file:
# /boot/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf # /boot/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf
title Fedora 19 (Rawhide) title Fedora 19 (Rawhide)
sort-key fedora sort-key fedora
machine-id 6a9857a393724b7a981ebb5b8495b9ea machine-id 6a9857a393724b7a981ebb5b8495b9ea
version 3.8.0-2.fc19.x86_64 version 3.8.0-2.fc19.x86_64
options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet
architecture x64 architecture x64
linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd
@ -309,17 +281,19 @@ focus for this specification. More specifically, on non-EFI systems
configuration snippets following this specification cannot be used to spawn configuration snippets following this specification cannot be used to spawn
other operating systems (such as Windows). other operating systems (such as Windows).
### Standard-conformance Marker File
Unfortunately, there are implementations of boot loading infrastructure that Unfortunately, there are implementations of boot loading infrastructure that
are also using the /loader/entries/ directory, but place files in them that are are also using the `/loader/entries/` directory, but installing files that do
not valid by this specification. In order to minimize confusion a boot loader not follow this specification. In order to minimize confusion, a boot loader
implementation may place a file /loader/entries.srel next to the implementation may place the file `/loader/entries.srel` next to the
/loader/entries/ directory containing the ASCII string "type1" (suffixed `/loader/entries/` directory containing the ASCII string `type1` (followed by a
with a UNIX newline). Tools that need to determine whether an existing UNIX newline). Tools that need to determine whether an existing directory
directory implements the semantics described here may check for this file and implements the semantics described here may check for this file and contents:
contents: if it exists and contains the mentioned string, it shall assume a if it exists and contains the mentioned string, it shall assume a
standards compliant implementation is in place. If it exists but contains a standards-compliant implementation is in place. If it exists but contains a
different string it shall assume non-standard semantics are implemented. If the different string it shall assume other semantics are implemented. If the file
file does not exist no assumptions should be made. does not exist, no assumptions should be made.
### Type #2 EFI Unified Kernel Images ### Type #2 EFI Unified Kernel Images
@ -327,31 +301,31 @@ 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 loader, a kernel image, an initramfs image, and the kernel command line. See
the description of the `--uefi` option in the description of the `--uefi` option in
[dracut(8)](http://man7.org/linux/man-pages/man8/dracut.8.html). Such unified [dracut(8)](http://man7.org/linux/man-pages/man8/dracut.8.html). Such unified
images will be searched for under `$BOOT/EFI/Linux/` and must have the 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 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 systems with EFI firmware. Ignore this section if you work on systems not
supporting EFI. supporting EFI.
Type #2 file names should be chosen from the same restricted character set as Type #2 file names should be chosen from the same restricted character set as
Type #1 described above (but use a different file name suffix of `.efi` instead Type #1 described above (but with the file name suffix of `.efi` instead of
of `.conf`). `.conf`).
Images of this type have the advantage that all metadata and payload that makes Images of this type have the advantage that all metadata and payload that makes
up the boot entry is monopolized in a single PE file that can be signed up the boot entry is contained in a single PE file that can be signed
cryptographically as one for the purpose of EFI SecureBoot. cryptographically as one for the purpose of EFI SecureBoot.
A valid unified kernel image must contain two PE sections: A valid unified kernel image must contain two PE sections:
* `.cmdline` section with the kernel command line * `.cmdline` section with the kernel command line,
* `.osrel` section with an embedded copy of the * `.osrel` section with an embedded copy of the
[os-release](https://www.freedesktop.org/software/systemd/man/os-release.html) [os-release](https://www.freedesktop.org/software/systemd/man/os-release.html)
file describing the image file describing the image.
The `PRETTY_NAME=` and `VERSION_ID=` fields in the embedded os-release file are The `PRETTY_NAME=` and `VERSION_ID=` fields in the embedded `os-release` file
used the same as `title` and `version` in the "boot loader specification" are used the same as `title` and `version` in the Type #1 entries. The
entries. The `.cmdline` section is used instead of the `options` field. `linux` `.cmdline` section is used instead of the `options` field. `linux` and `initrd`
and `initrd` fields are not necessary, and there is no counterpart for the fields are not necessary, and there is no counterpart for the `machine-id`
`machine-id` field. field.
On EFI, any such images shall be added to the list of valid boot entries. On EFI, any such images shall be added to the list of valid boot entries.
@ -374,43 +348,116 @@ path separator. This needs to be converted to an EFI-style "\\" separator in
EFI boot loaders. EFI boot loaders.
## Logic ## Locating boot entries
A _boot loader_ needs a file system driver to discover and read `$BOOT`, then A _boot loader_ locates `$BOOT`, then simply reads all files
simply reads all files `$BOOT/loader/entries/*.conf`, and populates its boot `$BOOT/loader/entries/*.conf`, and populates its boot menu with this. On EFI,
menu with this. On EFI, it then extends this with any unified kernel images it then extends this with any unified kernel images found in
found in `$BOOT/EFI/Linux/*.efi`. It may also add additional entries, for `$BOOT/EFI/Linux/*.efi`. It may also add additional entries, for example a
example a "Reboot into firmware" option. Optionally it may sort the menu based "Reboot into firmware" option. Optionally it may sort the menu based on the
on the `sort-key`, `machine-id` and `version` fields, and possibly others. It `sort-key`, `machine-id` and `version` fields, and possibly others. It uses the
uses the file name to identify specific items, for example in case it supports file name to identify specific items, for example in case it supports storing
storing away default entry information somewhere. A boot loader should away default entry information somewhere. A boot loader should generally not
generally not modify these files. modify these files.
For "Boot Loader Specification Entries" (Type #1), the _kernel package For "Boot Loader Specification Entries" (Type #1), the _kernel package
installer_ installs the kernel and initrd images to `$BOOT` (it is recommended installer_ installs the kernel and initrd images to `$BOOT` (it is recommended
to place these files in a vendor and OS and installation specific directory) to place these files in a vendor and OS and installation specific directory)
and then generates a configuration snippet for it, placing this in and then generates a configuration snippet for it, placing this in
`$BOOT/loader/entries/xyz.conf`, with xyz as concatenation of machine id and `$BOOT/loader/entries/xyz.conf`, with "xyz" as concatenation of machine id and
version information (see above). The files created by a kernel package are version information (see above). The files created by a kernel package are tied
private property of the kernel package and should be removed along with it. to the kernel package and should be removed along with it.
For "EFI Unified Kernel Images" (Type #2), the vendor or kernel package For "EFI Unified Kernel Images" (Type #2), the vendor or kernel package
installer creates the combined image and drops it into `$BOOT/EFI/Linux/`. This installer should create the combined image and drop it into
file is also private property of the kernel package and should be removed along `$BOOT/EFI/Linux/`. This file is also tied to the kernel package and should be
with it. removed along with it.
A _UI application_ intended to show available boot options shall operate A _UI application_ intended to show available boot options shall operate
similar to a boot loader, but might apply additional filters, for example by similarly to a boot loader, but might apply additional filters, for example by
filtering out the booted OS via the machine ID, or by suppressing all but the filtering the booted OS via the machine ID, or by suppressing all but the
newest kernel versions. newest kernel versions.
An _OS installer_ picks the right place for `$BOOT` as defined above (possibly An _OS installer_ picks the right place for `$BOOT` as defined above (possibly
creating a partition and file system for it) and pre-creates the creating a partition and file system for it) and creates the `/loader/entries/`
`/loader/entries/` directory in it. It then installs an appropriate boot loader directory in it. It then installs an appropriate boot loader that can read
that can read these snippets. Finally, it installs one or more kernel packages. these snippets. Finally, it installs one or more kernel packages.
## Out of Focus ## 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: There are a couple of items that are out of focus for this specification:
@ -419,6 +466,7 @@ There are a couple of items that are out of focus for this specification:
communicate to the boot loader the default boot loader entry temporarily or 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 persistently. Defining a common scheme for this is certainly a good idea, but
out of focus for this specification. out of focus for this specification.
* This specification is just about "Free" Operating systems. Hooking in other * This specification is just about "Free" Operating systems. Hooking in other
operating systems (like Windows and macOS) into the boot menu is a different operating systems (like Windows and macOS) into the boot menu is a different
story and should probably happen outside of this specification. For example, story and should probably happen outside of this specification. For example,
@ -426,6 +474,7 @@ There are a couple of items that are out of focus for this specification:
runtime without explicit configuration (like `systemd-boot` does it), or via runtime without explicit configuration (like `systemd-boot` does it), or via
native configuration (for example via explicit Grub2 configuration generated native configuration (for example via explicit Grub2 configuration generated
once at installation). once at installation).
* This specification leaves undefined what to do about systems which are * This specification leaves undefined what to do about systems which are
upgraded from an OS that does not implement this specification. As the upgraded from an OS that does not implement this specification. As the
previous boot loader logic was largely handled by in distribution-specific previous boot loader logic was largely handled by in distribution-specific