UAPI.1 Boot Loader Specification | UAPI Group Specifications

UAPI.1 Boot Loader Specification | UAPI Group Specifications
UAPI.1 The Boot Loader Specification
Version
Changes
1.0
Initial Release
This document defines a set of file formats and naming conventions that allow
the boot loader menu entries to be shared between multiple operating systems
and boot loaders installed on one device.
Operating systems cooperatively manage boot loader menu entry directories that
contain drop-in files, making multi-boot scenarios easy to support. Boot menu
entries are defined via two simple formats 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
menu entries 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 boot loader menu entries
Distribution developers, to create appropriate menu entry snippets when
installing or updating kernel packages
UI developers, to implement user interfaces that list and select among the
available boot options
The Partitions
Everything described below is located on one or two partitions. The boot loader
or user-space programs reading the boot loader menu entries should locate them
in the following manner:
On disks with an MBR partition table:
The boot partition — a partition with the type ID of
0xEA
— shall be used
as the single location for boot loader menu entries.
On disks with GPT (GUID Partition Table)
The EFI System Partition (ESP for short) — a partition with a GPT type GUID
of
c12a7328-f81f-11d2-ba4b-00a0c93ec93b
— may be used as one of two locations for
boot loader menu entries.
Optionally, an Extended Boot Loader Partition (XBOOTLDR partition for
short) — a partition with GPT type GUID of
bc13c2ff-59e6-4262-a352-b275fd6f7172
— may be used as the second of two
locations for boot loader menu entries. This partition must be located on
the same disk as the ESP.
Each partition type mentioned above can be present only once on the same disk.
Note:
These partitions are
shared
among all OS installations on the
same disk. Instead of maintaining one boot partition per installed OS (as
/boot/
was traditionally handled), all installed OSes use the same place for
boot loader menu entries.
For systems where the firmware is able to read file systems directly,
the ESP and XBOOTLDR must use a file system readable by the firmware.
For most systems this means VFAT (16 or 32 bit).
The same file system type must be used for both partitions.
Inode types other than directories and regular files
are not allowed as any part of the paths defined by this specification.
If for some reason a file system type allowing those is used,
those must not be created by any tools supporting this specification,
and such paths must be ignored by tools supporting this specification.
Applications should not expect case sensitivity,
and need to be able to deal with case insensitive behaviour of the file system.
Note that the partitions described here are not the exclusive territory of this specification.
This specification only defines semantics of the
/loader/entries/
directory
(along with the companion file
/loader/entries.srel
and the
/EFI/Linux/
directory inside the file system,
but it doesn’t define other contents of the file system.
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 might install their own boot code on the same partition;
this is particularly common in the case of the ESP.
Implementations of this specification must be able to operate correctly if
files or directories other than
/loader/entries/
and
/EFI/Linux/
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.
The
$BOOT
Partition Placeholder
In the text below, the placeholder
$BOOT
will be used to refer to the
partition determined as follows:
On disks with an MBR partition table: → the boot partition, as described above
On disks with a GPT partition table: → the XBOOTLDR partition if it exists
Otherwise, on disks with a GPT partition table: → the ESP
$BOOT
is the
primary
place to put boot menu entry resources into, but
typically not the only one. Most importantly, boot loaders should also pick up
menu entries from the ESP, even if XBOOTLDR exists (for details see below).
Creating These Partitions
An installer for an operating system should use this logic when selecting or
creating partitions:
If a boot partition (in case of MBR) or an XBOOTLDR partition (in case of
GPT) already exists it should be used as
$BOOT
and used as primary
location to place boot loader menu resources in.
Otherwise, if on GPT and an ESP is found and it is large enough (let’s say
at least 1G) it should be used as
$BOOT
and used as primary location to
place boot loader menu resources in.
Otherwise, if on GPT and neither XBOOTLDR nor ESP exist, an ESP should be
created of the appropriate size and be used as
$BOOT
, and used as primary
location to place boot loader menu resources in.
Otherwise, a boot partition (in case of MBR) or XBOOTLDR partition (in case
of GPT) should be created of an appropriate size, and be used as
$BOOT
and used as primary location to place boot loader menu resources in.
These partitions shall be determined during
installation time
, and
/etc/fstab
entries may be created.
Mount Points
It is recommended to mount
$BOOT
(either XBOOTLDR or the ESP) to
/boot/
If both XBOOTLDR and the ESP are present, the ESP should be mounted to
/efi/
Effectively, this means that
/boot/
is the location where new boot entries shall be written to.
For the boot loader itself, the situation is more complicated,
because its files may be located under
/boot/
or
/efi/
depending on whether XBOOTLDR exists.
(Mounting the ESP to
/boot/efi/
, as was traditionally done, is not
recommended. Such a nested setup complicates an implementation via direct
autofs
mounts — as implemented by
systemd
for example —, as establishing
the inner
autofs
will trigger the outer one. Mounting the two partitions via
autofs
is recommended because the simple VFAT file system has weak data
integrity properties and should remain unmounted whenever possible.)
From Linux, the file systems must be mounted with
MS_NOEXEC
MS_NODEV
MS_NOSUID
MS_NOSYMFOLLOW
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 combine the kernel binary
with the configuration, initrd, and other components of the boot loader entry.
This is also useful because the file can be cryptographically signed
for the purposes of SecureBoot (“Type #2”, Unified Kernel Images).
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.
Type #1 Boot Loader Specification Entries
/loader/entries/
in
$BOOT
is the primary directory containing Type #1
drop-in snippets defining boot entries, one
.conf
file for each boot menu
item. Each OS may provide one or more such entries.
If the ESP is separate from
$BOOT
it may also contain a
/loader/entries/
directory, where the boot loader should look for boot entry snippets, as an
additional source. The boot loader should enumerate both directories and
present a merged list to the user. Note that this is done for compatibility
only: while boot loaders should look in both places, OSes should only add their
files to
$BOOT
Note:
In all cases the
/loader/entries/
directory should be located
directly in the root of the ESP or XBOOTLDR filesystem. Specifically, the
/loader/entries/
directory should
not
be located under the
/EFI/
subdirectory on the ESP.
The file name of the boot entry snippets 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
entry-token
(see
kernel-install
or machine ID (see
/etc/machine-id
),
and the kernel version (as returned by
uname -r
, including the OS
identifier), so that the whole filename is
$BOOT/loader/entries/-.conf
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 boot loader menu snippets shall be UNIX-style text files
(i.e. lines separated by a single newline character),
in the UTF-8 encoding.
The boot loader menu 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.
The rest of the line contains the value, a literal string.
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
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 is used for sorting entries,
so that the boot loader can order entries by age or select the newest one
automatically. This field is optional.
See
Sorting
below.
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
os-release
possibly with an additional suffix. This field is optional.
Example:
sort-key fedora
linux
specifies the Linux kernel image to execute.
The value is a path relative to the root of the file system
containing the boot entry snippet itself.
It is recommended that every distribution creates
a subdirectory specific to the entry-token or machine-id,
and underneath that, subdirectories specific to the kernel version,
and places places the kernel and initrd images there
(see below).
Example:
linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
initrd
specifies the initrd to use when executing the kernel (
cpio
image).
The value is a path relative to the root of the file system
containing the boot entry snippet itself.
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
initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/modules
efi
refers to an arbitrary EFI program. If this key is set, and the system
is not an EFI system, this entry should be hidden.
uki
is a combination of the
linux
key, the
efi
key and Type #2 semantics: if this key is used the
specified path shall refer to a
unified kernel image (UKI)
. This informs the
boot loader to use UKI semantics (as exposed by Type #2) even though it is referenced via a Type #1
entry. This is useful to parameterize the UKI differently, to place it in a directory other than
/EFI/Linux
, or to share a single UKI image among multiple menu entries.
If this key is set, and the system is not an EFI system, this entry should be hidden.
Example:
uki /fooos/bar.efi
uki-url
is similar to
uki
, but takes an RFC 3986 URI instead of a file path. It may be used to
reference remotely stored UKIs, which shall be downloaded using network boot support of the boot loader or
firmware on activation.
Example:
uki-url http://example.com/fooos.efi
Optionally, in place of an URL a simple filename prefixed with
may be specified. In this case, if the
boot loader itself was booted via the network and knows its originating URL it shall replace the last
path component of that URL (i.e. the filename part) with the specified filename.
Example:
uki-url :fooos.efi
This example would mean: if the boot loader was acquired from
such a line would be treated equivalent to
uki-url http://example.com/somedir/fooos.efi
If this key is set, and the system is not an EFI system, or the boot loader/firmware does not support
network booting, this entry
shall
be hidden. Similarly, if the
syntax is used, but the boot loader
was not invoked via network booting (or doesn’t know its originating URL) entries of this type
shall
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 combined 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 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
profile
refers to the profile number to use in multi-profile EFI programs.
See the section
multi-profile UKIs
in
the UKI Specification
for more information.
This should only be used when
uki
or
uki-url
is set to a multi-profile UKI.
Behaviour is implementation-defined for any other case and implementors may choose to
ignore the field or issue a warning.
Example:
profile 1
extra
refers to additional resources to pass to the invoked kernel. This takes a path relative to the
root of the file system containing the snippet itself, referring to a regular file. The file name must be
suffixed with a recognized suffix which indicates the type of additional resource. Currently, recognized
suffixes are
.confext.raw
(for systemd-style configuration extension DDIs),
.sysext.raw
(for
systemd-style system extenion configuration extension DDIs) and
.cred
(for systemd-style encrypted system
credential files). In future additional suffixes will be defined and implementations must gracefully handle
unrecognized suffixes. This key may appear multiple times.
Example:
extra /6a9857a393724b7a981ebb5b8495b9ea/somedata.cred
Each boot loader menu entry 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
extra /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/somedata.cred
extra /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/somethingelse.cred
On EFI systems all Linux kernel images
should
be EFI PE images, or in case of the
uki
or
uki-url
key
must
be EFI PE images conforming to the
UKI Specification
. 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.
When Type #1 boot loader menu entry snippets refer to other files (for
linux
initrd
efi
devicetree
, and
devicetree-overlay
), those files must be
located on the same partition, and the paths must be absolute paths relative to
the root of that file system. The naming of those files can be chosen by the
installer. A recommended scheme is described in the next section. Paths should
be normalized, i.e. not include
..
or a sequence of more than one
. Paths may be prefixed with a
, but this is optional and has the same
effect as paths without it: all paths are always relative to the root directory
of the partition they are referenced from.
Even though the backing file system is typically case-insensitive (i.e. VFAT)
it is strongly recommended to reference files in the casing actually used for
the directories/files, so that placing these files on other file systems is
still safe and robust.
Recommended Directory Layout for Additional Files
It is recommended to place the kernel and other other files comprising a single
boot loader entry in a separate directory:
///
. This naming scheme uses the same
elements as the boot loader menu entry snippet, providing the same level of
uniqueness.
Example:
$BOOT/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
$BOOT/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd
Other naming schemes are possible. In particular, traditionally a flat naming
scheme with files in the root directory was used. This is not recommended
because it is hard to avoid conflicts in a multi-boot installation.
Standard-conformance Marker File
Unfortunately, there are implementations of boot loading infrastructure that
are also using the
/loader/entries/
directory, but install 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, and possibly other, optional resources. See the
UKI Specification
for details. The primary place for
such unified images is the
/EFI/Linux/
directory in
$BOOT
. Operating
systems should place unified EFI kernels only in the
$BOOT
partition. Boot
loaders should also look in the
/EFI/Linux/
of the ESP — if it is different
from
$BOOT
— and present a merged list of menu entries from both partitions.
Regardless if placed in the primary or secondary location: the files 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 unified kernel image in the
/EFI/Linux/
directory contains PE sections
described in the
UKI Specification
Of these sections, the
.osrel
section is used to generate boot entries.
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. Absent
these fields, other metadata may be used to display the images as boot entries.
For a full list of mandatory and allowed fields, see the
UKI Specification
Additional Notes
Note that these boot entry snippets and unified kernels do not need to be the
only menu entry sources 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
boot loader menu entry 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 boot loader menu entry 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
boot loader
locates the XBOOTLDR partition and the ESP, then simply reads
all the files
/loader/entries/*.conf
in them, and populates its boot menu
(and handle gracefully if one of the two partitions is missing). On EFI, it
then extends this with any unified kernel images found in
/EFI/Linux/*.efi
in
the two partitions. 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. It
then generates a boot loader menu entry snippet, placing it in
$BOOT/loader/entries/xyz.conf
, with “xyz” as concatenation of
entry-token/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.
UI application
intended to show available boot options shall operate
similarly to a boot loader (and thus search both
$BOOT
and the ESP if
distinct), 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 and the
/loader/entries.srel
file in it (the latter only if the
directory didn’t exist yet). It then installs an appropriate boot loader that
can read these snippets. Finally, it installs one or more kernel packages.
Boot counting
The main idea is that when boot entries are initially installed, they are
marked as “indeterminate” and assigned a number of boot attempts. Each time the
boot loader tries to boot an entry, it decreases this count by one. If the
operating system considers the boot as successful, it removes the counter
altogether and the entry becomes “good”. Otherwise, once the assigned number of
boots is exhausted, the entry is marked as “bad”.
Which boots are “successful” is determined by the operating system. systemd
provides a generic mechanism that can be extended with arbitrary checks and
actions, see
Automatic Boot Assessment
but the boot counting mechanism described in this specification can
also be used with other implementations.
The boot counting data is stored in the name of the boot loader entry. A boot
loader entry file name may contain a plus (
) followed by a number. This may
optionally be followed by a minus (
) followed by a second number. The dot
) and file name suffix (
conf
or
efi
) must immediately follow. Boot
counting is enabled for entries which match this pattern.
The first number is the “tries left” counter signifying how many attempts to boot
this entry shall still be made. The second number is the “tries done” counter,
showing how many failed attempts to boot it have already been made. Each time
a boot loader entry marked this way is booted, the first counter is decremented,
and the second one incremented. (If the second counter is missing,
then it is assumed to be equivalent to zero.) If the “tries left” counter is
above zero the entry is still considered “indeterminate”. A boot entry with the
“tries left” counter at zero is considered “bad”.
When decrementing the “tries left” counter, implementations should preserve
the total number of digits by padding with leading zeroes if needed. For example,
+10
becomes
+09
instead of
+9
. This ensures that the filename length
and structure remain constant during counter updates.
The “tries done” counter should be pre-initialized with fixed number of digits,
initially all zero. The number of digits should be sufficient for the expected range
of increments (e.g.
-00
or
-0000
). During boot counting, this counter is
incremented without changing its width. If the counter would overflow its allocated
width, it shall be capped at the maximum representable value (e.g.
-99
for two digits)
and processing shall continue as normal.
If the boot attempt completed successfully the entry’s counters are removed
from the name (entry state becomes “good”), thus turning off boot counting for
this entry.
Sorting
The boot loader menu should generally show entries in some order meaningful to
the user. The
title
key is free-form and not suitable to be used as the
primary sorting key. Instead, the boot loader should use the following rules:
Entries which are subject to boot counting and are marked as “bad”, should
be sorted later than all other entries. Entries which are marked as
“indeterminate” or “good” (or were not subject to boot counting at all),
are thus sorted earlier.
If
sort-key
is set on both entries, use in order of priority,
the
sort-key
(A-Z, increasing
alphanumerical order
),
machine-id
(A-Z, increasing alphanumerical order),
and
version
keys (decreasing
version order
).
If
sort-key
is set on one entry, it sorts earlier.
At the end, if necessary, when
sort-key
is not set or those fields are not
set or are all equal, the boot loader should sort using the file name of the
entry (decreasing version sort), with the suffix removed.
Note:
This description assumes that the boot loader shows entries in a
traditional menu, with newest and “best” entries at the top, thus entries with
a higher version number are sorter
earlier
. The boot loader is free to
use a different direction (or none at all) during display.
Note:
The boot loader should allow booting “bad” entries, e.g. in case no
other entries are left or they are unusable for other reasons. It may
deemphasize or hide such entries by default.
Note:
“Bad” boot entries have a suffix of “+0-
”, where
is the
number of failed boot attempts. Removal of the suffix is not necessary for
comparisons described by the last point above. In the unlikely scenario that we
have multiple such boot entries that differ only by the boot counting data, we
would sort them by
Alphanumerical Order
Free-form strings and machine IDs should be compared using a method equivalent
to
strcmp(3)
on their
UTF-8 representations. If just one of the strings is unspecified or empty, it
compares lower. If both strings are unspecified or empty, they compare equal.
Version Order
The version format and the comparison algorithm are defined in the
Version Format Specification
. A specific
separator character between the version and other tokens in a string is not
mandated by this specification, as it often depends on existing kernel packages
which differ between distributions and packaging formats.
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 boot loader menus 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 menu entries 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 menu entries 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 menu entries 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 specification 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 firmware 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.
Why is the version comparison logic so complicated?
The
sort-key
allows us to group entries by “operating system”, e.g. all
versions of Fedora together, no matter if they identify themselves as “Fedora
Workstation” or “Fedora Rawhide (prerelease)”. The
sort-key
was introduced
only recently, so we need to provide a meaningful order for entries both with
and without it. Since it is a new concept, it is assumed that entries with
sort-key
are newer.
In a traditional menu with entries displayed vertically, we want names to be
sorter alpabetically (CentOS, Debian, Fedora, OpenSUSE, …), it would be strange
to have them in reverse order. But when multiple kernels are available for the
same installation, we want to display the latest kernel with highest priority,
i.e. earlier in the list.
Why do you use file renames to store the counter? Why not a regular file?
Mainly two reasons: it’s relatively likely that renames can be implemented
atomically even in simpler file systems, as renaming generally avoids
allocating or releasing data blocks. Writing to file contents has a much bigger
chance to be result in incomplete or corrupt data. Moreover renaming has the
benefit that the boot count metadata is directly attached to the boot loader
entry file, and thus the lifecycle of the metadata and the entry itself are
bound together. This means no additional clean-up needs to take place to drop
the boot loader counting information for an entry when it is removed.
This is also why the size of the counters should remain fixed while boot counting
is active: renames that change the filename length may no longer be atomic on some
file systems. On FAT32, for example, long filenames are stored using multiple
consecutive directory records, and growing or shrinking a filename can require
allocating or releasing these entries. To keep such updates effectively atomic,
the specification requires fixed-width counters and pre-allocation of the “tries done”
field so that counter updates do not change the filename length.
Why not use EFI variables for storing the boot counter?
The memory chips used to back the persistent EFI variables are generally not of
the highest quality, hence shouldn’t be written to more than necessary. This
means we can’t really use it for changes made regularly during boot, but should
use it only for seldom-made configuration changes.
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.
Referencing kernels or initrds on other partitions other than the partition
containing the Type #1 boot loader entry. This is by design, as specifying
other partitions or devices would require a non-trivial language for denoting
device paths. In particular this means that on non-EFI systems boot loader
menu entry snippets following this specification cannot be used to spawn
other operating systems (such as Windows).
Links
GUID Partition Table
Boot Loader Interface
Discoverable Partitions Specification
systemd-boot(7)
bootctl(1)
systemd-gpt-auto-generator(8)