# The Boot Loader Specification
-_TL;DR: Currently there's no common boot scheme across architectures and
-platforms for open-source operating systems. There's also little cooperation
-between multiple distributions in dual-boot (or triple, … multi-boot)
-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
-can cooperatively manage a boot loader configuration directory, that accepts
-drop-in files for boot menu items that are defined in a format that is shared
-between various 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. The target audience for this specification is:
+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 at runtime from these drop-in snippets
+ configuration from these files
* Firmware developers, to add generic boot loading support directly to the
firmware itself
-* Distribution and Core OS developers, in order to create these snippets at
- OS/kernel package installation time
-* UI developers, for implementing a user interface that discovers the available
- boot options
-* OS Installer developers, to prepare their installation media and for setting
- up the initial drop-in directory
-
-## Why is there a need for this specification?
-
-Of course, without this specification things already work mostly fine. But here's why we think this specification is needed:
-
-* To make the boot more robust, as no explicit rewriting of configuration files
- 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.
+* 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
-## Why not simply rely on the EFI boot menu logic?
+## The boot partition
-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:
+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:
-* 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
+* On disks with an MBR partition table:
-Everything described below is located on a placeholder file system `$BOOT`. The
-installer program should pick `$BOOT` according to the following rules:
+ * A partition with the type ID of 0xEA shall be used as `$BOOT`.
-* On disks with an MBR partition table:
- * If the OS is installed on a disk with an MBR partition table, and a
- partition with the type id of 0xEA already exists it should be used as
- `$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 `$BOOT`.
* On disks with GPT (GUID Partition Table)
- * If the OS is installed on a disk with GPT, and an Extended Boot Loader
- Partition (or XBOOTLDR partition for short), i.e. a partition with GPT type
- GUID of `bc13c2ff-59e6-4262-a352-b275fd6f7172`, already exists, it should
+
+ * If an Extended Boot Loader Partition (XBOOTLDR partition for short) — a
+ partition with GPT type GUID of `bc13c2ff-59e6-4262-a352-b275fd6f7172` —
+ is found, it should be used as `$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 `$BOOT`.
- * Otherwise, if the OS is installed on a disk with GPT, and an EFI System
- Partition (or ESP for short), i.e. a partition with GPT type UID of
- `c12a7328-f81f-11d2-ba4b-00a0c93ec93b` already exists and is large enough
- (let's say 250MB) and otherwise qualifies, it should be used as `$BOOT`.
- * Otherwise, if the OS is installed on a disk with GPT, and if the ESP
- already exists but is too small, a new suitably sized (let's say 500MB)
- XBOOTLDR partition shall be created and used as `$BOOT`.
- * Otherwise, if the OS is installed on a disk with GPT, and no ESP exists
- yet, a new suitably sized (let's say 500MB) ESP should be created and used
- as `$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 `$BOOT`.
This placeholder file system shall be determined during _installation time_,
and an fstab entry may be created. It should be mounted to either `/boot/` or
because the mounting of `$BOOT` is then dependent on and requires the mounting
of the intermediate file system.
-**Note:** _`$BOOT` should be considered **shared** among all OS installations
-of a system. Instead of maintaining one `$BOOT` per installed OS (as `/boot/`
-was traditionally handled), all installed OS share the same place to drop in
-their boot-time configuration._
+**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 `$BOOT` should hence not assume that fancier
-file system features such as symlinks, hardlinks, access control or case
-sensitivity are supported.
+installation and live media, `$BOOT` must be a VFAT (16 or 32) file system.
+Applications accessing `$BOOT` should hence not assume that fancier file system
+features such as symlinks, hardlinks, access control or case sensitivity are
+supported.
+
+## Boot loader entries
This specification defines two types of boot loader entries. The first type is
-text based, very simple and suitable for a variety of firmware, architecture
+text based, very simple, and suitable for a variety of firmware, architecture
and image types ("Type #1"). The second type is specific to EFI, but allows
single-file images that embed all metadata in the kernel binary itself, which
is useful to cryptographically sign them as one file for the purpose of
this specification. This specification only defines semantics of the `/loader/`
directory inside the file system (see below), but it doesn't intend to define
ownership of the whole file system exclusively. Boot loaders, firmware, and
-other software implementing this specification may choose to place other
-files and directories in the same file system. For example, boot loaders that
-implement this specification might install their own boot code into the `$BOOT`
-partition. On systems where `$BOOT` is the ESP this is a particularly common
+other software implementing this specification may choose to place other files
+and directories in the same file system. For example, boot loaders that
+implement this specification might install their own boot code on the same
+partition; on systems where `$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
### Type #1 Boot Loader Specification Entries
-We define two directories below `$BOOT`:
+This specification defines two directories below `$BOOT`:
* `$BOOT/loader/` is the directory containing all files needed for Type #1
entries
ESP, and not in the `/EFI/` subdirectory._
Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or
-more configuration snippets with the suffix ".conf", one for each boot menu
+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:
-`$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.
+freely but should be unique enough to avoid clashes between OS installations.
+More specifically, it is suggested to include the machine ID (`/etc/machine-id`
+or the D-Bus machine ID for OSes that lack `/etc/machine-id`), the kernel
+version (as returned by `uname -r`) and an OS identifier (the `ID=` field of
+`/etc/os-release`).
+
+Example: `$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 file name suffix).
-
-These configuration snippets shall be Unix-style text files (i.e. line
-separation with a single newline character), in the UTF-8 encoding. The
-configuration snippets are loosely inspired on Grub1's configuration
-syntax. Lines beginning with '#' shall be ignored and used for commenting. The
-first word of a line is used as key and shall be separated by one or more
-spaces from its value. The following keys are known:
-
-* `title` shall contain a human readable title string for this menu item. This
- 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
- 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 in the UI, for example by appending the `version` field. This field is
- optional. Example: "Fedora 18 (Spherical Cow)".
-* `version` shall contain a human readable version string for this menu
- item. This is usually the kernel version and is intended for use by OSes to
- install multiple kernel versions at the same time 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: `3.7.2-201.fc18.x86_64`.
-* `machine-id` shall contain the machine ID of the OS `/etc/machine-id`. This
- is useful for boot loaders and applications to filter out boot entries, for
- example to show only a single newest kernel per OS, or to group items by OS,
- or to maybe filter out the currently booted OS in UIs that want to show 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: `4098b3f648d74c13b1f04ccfba7798e8`.
-* `sort-key` shall contain a short string used for sorting entries on
- display. This can be defined freely though should typically be initialized
- from `IMAGE_ID=` or `ID=` from `/etc/os-release` of the relevant entry,
- possibly suffixed. 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.
-* `linux` refers to the Linux kernel to spawn and shall be 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:
- `/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux`.
-* `initrd` refers to the initrd to use when executing the kernel. This also
+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 `$BOOT`. This key is optional. This key may
appear more than once in which case all specified images are used, in the
- order they are listed. Example:
- `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`.
+ order they are listed.
+
+ Example: `initrd 6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`
+
* `efi` refers to an arbitrary EFI program. This also takes a path relative to
`$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:
- `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`.
+ 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:
- `/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
- vocabulary defined by the EFI specification (i.e. `IA32`, `x64`, `IA64`,
- `ARM`, `AA64`, …). If specified and this does not match (case insensitively)
- the local system architecture this entry should be hidden.
+ 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 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
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
+ 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
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 place files in them that are
-not valid by this specification. In order to minimize confusion a boot loader
-implementation may place a file /loader/entries.srel next to the
-/loader/entries/ directory containing the ASCII string "type1" (suffixed
-with 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 non-standard semantics are implemented. If the
-file does not exist no assumptions should be made.
+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
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 will be searched for under `$BOOT/EFI/Linux/` and must have the
+images are installed in the`$BOOT/EFI/Linux/` directory and must have the
extension `.efi`. Support for images of this type is of course specific to
systems with EFI firmware. Ignore this section if you work on systems not
supporting EFI.
Type #2 file names should be chosen from the same restricted character set as
-Type #1 described above (but use a different file name suffix of `.efi` instead
-of `.conf`).
+Type #1 described above (but with the file name suffix of `.efi` instead of
+`.conf`).
Images of this type have the advantage that all metadata and payload that makes
-up the boot entry is monopolized in a single PE file that can be signed
+up the boot entry is contained in a single PE file that can be signed
cryptographically as one for the purpose of EFI SecureBoot.
A valid unified kernel image must contain two PE sections:
-* `.cmdline` section with the kernel command line
+* `.cmdline` section with the kernel command line,
* `.osrel` section with an embedded copy of the
[os-release](https://www.freedesktop.org/software/systemd/man/os-release.html)
- file describing the image
+ file describing the image.
-The `PRETTY_NAME=` and `VERSION_ID=` fields in the embedded os-release file are
-used the same as `title` and `version` in the "boot loader specification"
-entries. The `.cmdline` section is used instead of the `options` field. `linux`
-and `initrd` fields are not necessary, and there is no counterpart for the
-`machine-id` field.
+The `PRETTY_NAME=` and `VERSION_ID=` fields in the embedded `os-release` file
+are used the same as `title` and `version` in the Type #1 entries. The
+`.cmdline` section is used instead of the `options` field. `linux` and `initrd`
+fields are not necessary, and there is no counterpart for the `machine-id`
+field.
On EFI, any such images shall be added to the list of valid boot entries.
EFI boot loaders.
-## Logic
+## Locating boot entries
-A _boot loader_ needs a file system driver to discover and read `$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.
+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
-`$BOOT/loader/entries/xyz.conf`, with xyz as concatenation of machine id and
-version information (see above). The files created by a kernel package are
-private property of the kernel package and should be removed along with it.
+`$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 creates the combined image and drops it into `$BOOT/EFI/Linux/`. This
-file is also private property of the kernel package and should be removed along
-with it.
+installer should create the combined image and drop it into
+`$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
-similar 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
+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 pre-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.
+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:
-## Out of Focus
+* 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:
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,
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
projects / thirdparty / systemd.git / commitdiff
