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Merge pull request #23504 from keszybz/bls-reordering

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Refactor the BLS and add a description of version sorts
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Luca Boccassi 2022-05-27 14:36:10 +01:00 committed by GitHub
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35
NEWS
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@ -29,19 +29,19 @@ CHANGES WITH 251:
and backward compatibility broken instead on the assumption that
nobody can be affected given the current state of this interface.
* All kernels supported by systemd mix RDRAND (or similar) into the
entropy pool at early boot. This means that on those systems, even if
/dev/urandom is not yet initialized, it still returns bytes that
are at least as high quality as RDRAND. For that reason, we no longer
have reason to invoke RDRAND from systemd itself, which has
historically been a source of bugs. Furthermore, kernels ≥5.6 provide
the getrandom(GRND_INSECURE) interface for returning random bytes
before the entropy pool is initialized without warning into kmsg,
which is what we attempt to use if available. systemd's direct usage
of RDRAND has been removed. x86 systems ≥Broadwell that are running
an older kernel may experience kmsg warnings that were not seen with
250. For newer kernels, non-x86 systems, or older x86 systems, there
should be no visible changes.
* All kernels supported by systemd mix bytes returned by RDRAND (or
similar) into the entropy pool at early boot. This means that on
those systems, even if /dev/urandom is not yet initialized, it still
returns bytes that are of at least RDRAND quality. For that reason,
we no longer have reason to invoke RDRAND from systemd itself, which
has historically been a source of bugs. Furthermore, kernels ≥5.6
provide the getrandom(GRND_INSECURE) interface for returning random
bytes before the entropy pool is initialized without warning into
kmsg, which is what we attempt to use if available. systemd's direct
usage of RDRAND has been removed. x86 systems ≥Broadwell that are
running an older kernel may experience kmsg warnings that were not
seen with 250. For newer kernels, non-x86 systems, or older x86
systems, there should be no visible changes.
* sd-boot will now measure the kernel command line into TPM PCR 12
rather than PCR 8. This improves usefulness of the measurements on
@ -59,11 +59,10 @@ CHANGES WITH 251:
* busctl capture now writes output in the newer pcapng format instead
of pcap.
* A udev rule that imported hwdb matches for USB devices with
lowercase hexadecimal vendor/product ID digits was added in systemd
250. This has been reverted, since uppercase hexadecimal digits are
supposed to be used, and we already had a rule for that with the
appropriate match.
* A udev rule that imported hwdb matches for USB devices with lowercase
hexadecimal vendor/product ID digits was added in systemd 250. This
has been reverted, since uppercase hexadecimal digits are supposed to
be used, and we already had a rule with the appropriate match.
Users might need to adjust their local hwdb entries.

727
docs/BOOT_LOADER_SPECIFICATION.md
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@ -7,142 +7,93 @@ SPDX-License-Identifier: LGPL-2.1-or-later
# 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._
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.
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:
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
* OS installer developers, to create appropriate partitions and set up the
initial boot loader configuration
* 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 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:
* 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.
## 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:
Everything described below is located on one or two partitions. The boot loader
or user-space programs reading the boot loader configuration should locate them
in the following manner:
* 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`.
* The boot partition — partition with the type ID of 0xEA — shall be used
for boot loader configuration and entries.
* 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
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`.
This placeholder file system shall be determined during _installation time_,
and an fstab entry may be created. It should be mounted to either `/boot/` or
`/efi/`. Additional locations like `/boot/efi/` (with `/boot/` being a separate
file system) might be supported by implementations. This is not recommended
because the mounting of `$BOOT` is then dependent on and requires the mounting
of the intermediate file system.
* The EFI System Partition (ESP for short) — a partition with GPT type GUID
of `c12a7328-f81f-11d2-ba4b-00a0c93ec93b` — should be used for boot loader
configuration and boot entries.
**Note:** _`$BOOT` should be considered **shared** among all OS installations
of a system. Instead of maintaining one `$BOOT` per installed OS (as `/boot/`
was traditionally handled), all installed OS share the same place to drop in
their boot-time configuration._
* Optionally, an Extended Boot Loader Partition (XBOOTLDR partition for
short) — a partition with GPT type GUID of
`bc13c2ff-59e6-4262-a352-b275fd6f7172` — may be used as an additional
location for boot loader entries. This partition must be located on the
same disk as the ESP.
For systems where the firmware is able to read file systems directly, `$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.
In the text below, `$BOOT` will be used to refer to (the root of) the first of
the two partitions (the boot partition on MBR disks and the ESP on GPT disks),
and `$XBOOTLDR` will be used to refer to (the root of) the optional second
partition.
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, matching the characteristics described above. On disks
with GPT, only the ESP partition without the 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.
Those file systems shall be determined during _installation time_, and an fstab
entry may be created. If only one partition is used, it should be mounted on
`/boot/`. If both XBOOTLDR partition and the ESP are used, they should be
mounted on `/boot` and `/efi`, or on `/boot` and `/boot/efi`.
**Note:** _Those file systems are **shared** among all OS installations on the
system. Instead of maintaining one boot partition 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, the ESP
must — and the XBOOTLDR partition should — be a file system readable by the
firmware. For most systems this means VFAT (16 or 32 bit). Applications
accessing both partitions 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
@ -157,132 +108,151 @@ from the user. Only entries matching the feature set of boot loader and system
shall be considered and displayed. This allows image builders to put together
images that transparently support multiple different architectures.
Note that the `$BOOT` partition is not supposed to be exclusive territory of
Note that the boot partitions are not supposed to be the exclusive territory of
this specification. This specification only defines semantics of the `/loader/`
directory inside the file system (see below), but it doesn't intend to define
ownership of the whole file system exclusively. Boot loaders, firmware, and
other software implementing this specification may choose to place other
files and directories in the same file system. For example, boot loaders that
implement this specification might install their own boot code into the `$BOOT`
partition. On systems where `$BOOT` is the ESP this is a particularly common
setup. Implementations of this specification must be able to operate correctly
if files or directories other than `/loader/` are found in the top level
directory. Implementations that add their own files or directories to the file
systems should use well-named directories, to make name collisions between
multiple users of the file system unlikely.
ownership of the whole 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 that implement this
specification 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/`
are found in the top level directory. Implementations that add their own files
or directories to the file systems should use well-named directories, to make
name collisions between multiple users of the file system unlikely.
### Type #1 Boot Loader Specification Entries
We define two directories below `$BOOT`:
* `$BOOT/loader/` is the directory containing all files needed for Type #1
entries
* `$BOOT/loader/entries/` is the directory containing the drop-in
snippets. This directory contains one `.conf` file for each boot menu item.
`$ESP/loader/` is the main directory containing the configuration for the boot
loader.
**Note:** _In all cases the `/loader/` directory should be located directly in
the root of the file system. Specifically, if `$BOOT` is the ESP, then
`/loader/` directory should be located directly in the root directory of the
ESP, and not in the `/EFI/` subdirectory._
the root of the file system. Specifically, the `/loader/` directory should
**not** be located under the `/EFI/` subdirectory on the ESP._
Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or
more configuration snippets with the suffix ".conf", one for each boot menu
item. The file name of the file is used for identification of the boot item but
shall never be presented to the user in the UI. The file name may be chosen
freely but should be unique enough to avoid clashes between OS
installations. More specifically it is suggested to include the machine ID
(`/etc/machine-id` or the D-Bus machine ID for OSes that lack
`/etc/machine-id`), the kernel version (as returned by `uname -r`) and an OS
identifier (The ID field of `/etc/os-release`). Example:
`$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.
`$BOOT/loader/entries/` and `$XBOOTLDR/loader/entries/` are the directories
containing the drop-in snippets defining boot entries, one `.conf` file for
each boot menu item. Each OS may provide one or more such entries. The boot
loader should enumerate both directories and provide a merged list.
The file name is used for identification of the boot item but shall never be
presented to the user in the UI. The file name may be chosen freely but should
be unique enough to avoid clashes between OS installations. More specifically,
it is suggested to include the `entry-token` (see
[kernel-install](https://www.freedesktop.org/software/systemd/man/kernel-install.html))
or machine ID (see
[/etc/machine-id](https://www.freedesktop.org/software/systemd/man/machine-id.html)),
and the kernel version (as returned by `uname -r`, including the OS
identifier), so that the whole filename is
`$BOOT/loader/entries/<entry-token-or-machine-id>-<version>.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 file name suffix).
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. 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:
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
[os-release](https://www.freedesktop.org/software/systemd/man/os-release.html).
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](#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](https://www.freedesktop.org/software/systemd/man/os-release.html),
possibly with an additional suffix. This field is optional.
Example: `sort-key fedora`
* `linux` is the Linux kernel to spawn and as a path relative to file system
root. It is recommended that every distribution creates a machine id and
version specific subdirectory 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 key is
optional. This key may appear more than once in which case all specified
images are used, in the order they are listed.
Example: `initrd 6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`
* `efi` refers to an arbitrary EFI program. If this key is set, and the system
is not an EFI system, this entry should be hidden.
* `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 `$BOOT` and places its kernels and
initial RAM disk images there. Example:
`/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux`.
* `initrd` refers to the initrd to use when executing the kernel. This also
shall be a path relative to `$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`.
* `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`.
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:
`/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
@ -298,28 +268,40 @@ i.e. it is a good idea that both images shipped as UEFI PE images and those
which are not don't make unnecessary assumption on the underlying firmware,
i.e. don't hard depend on legacy BIOS calls or UEFI boot services.
Note that these configuration snippets may only reference kernels (and EFI
programs) that reside on the same file system as the configuration snippets,
i.e. everything referenced must be contained in the same file system. This is
by design, as referencing other partitions or devices would require a
non-trivial language for denoting device paths. If kernels/initrds are to be
read from other partitions/disks the boot loader can do this in its own native
configuration, using its own specific device path language, and this is out of
focus for this specification. More specifically, on non-EFI systems
configuration snippets following this specification cannot be used to spawn
other operating systems (such as Windows).
When Type #1 configuration 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.
### Recommended Directory Layout for Additional Files
It is recommened to place the kernel and other other files comprising a single
boot loader entry in a separate directory:
`/<entry-token-or-machine-id>/<version>/`. This naming scheme uses the same
elements as the boot loader configuration 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 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
@ -327,31 +309,31 @@ 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
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 `$BOOT/EFI/Linux/` and must have the
extension `.efi`. Support for images of this type is of course specific to
systems with EFI firmware. Ignore this section if you work on systems not
supporting EFI.
images are installed in the`$BOOT/EFI/Linux/` and `$XBOOTLDR/EFI/Linux/`
directories 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.
@ -374,43 +356,225 @@ path separator. This needs to be converted to an EFI-style "\\" separator in
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 `$BOOT` and `$XBOOTLDR`, then simply reads all the
files `$BOOT/loader/entries/*.conf` and `$XBOOTLDR/loader/entries/*.conf`, and
populates its boot menu. On EFI, it then extends this with any unified kernel
images found in `$BOOT/EFI/Linux/*.efi` and `$XBOOTLDR/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.
installer_ installs the kernel and initrd images to `$XBOOTLDR` (if used) or
`$BOOT`. It is recommended to place these files in a vendor and OS and
installation specific directory. It then generates a configuration snippet,
placing it in `$BOOT/loader/entries/xyz.conf`, with "xyz" as concatenation of
machine id and version information (see above). The files created by a kernel
package are tied to the kernel package and should be removed along with it.
For "EFI Unified Kernel Images" (Type #2), the vendor or kernel package
installer creates the combined image and drops it into `$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.
## Sorting
## Out of Focus
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:
if `sort-key` is set on both entries, use in order of priority,
the `sort-key` (A-Z, increasing [alphanumerical order](#alphanumerical-order)),
`machine-id` (A-Z, increasing alphanumerical order),
and `version` keys (decreasing [version order](#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._
### Alphanumerical order
Free-form strings and machine IDs should be compared using a method equivalent
to [strcmp(3)](https://man7.org/linux/man-pages/man3/strcmp.3.html) on their
UTF-8 represenations. 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 following method should be used to compare version strings. The algorithm
is based on rpm's `rpmvercmp()`, but not identical.
ASCII letters (`a-z`, `A-Z`) and digits (`0-9`) form alphanumerical components of the version.
Minus (`-`) separates the version and release parts.
Dot (`.`) separates parts of version or release.
Tilde (`~`) is a prefix that always compares lower.
Caret (`^`) is a prefix that always compares higher.
Both strings are compared from the beginning until the end, or until the
strings are found to compare as different. In a loop:
1. Any characters which are outside of the set of listed above (`a-z`, `A-Z`, `0-9`, `-`, `.`, `~`, `^`)
are skipped in both strings. In particular, this means that non-ASCII characters
that are Unicode digits or letters are skipped too.
2. If one of the strings has ended: if the other string hasn't, the string that
has remaining characters compares higher. Otherwise, the strings compare
equal.
3. If the remaining part of one of strings starts with `~`:
if other remaining part does not start with `~`,
the string with `~` compares lower. Otherwise, both tilde characters are skipped.
4. The check from point 2. is repeated here.
5. If the remaining part of one of strings starts with `-`:
if the other remaining part does not start with `-`,
the string with `-` compares lower. Otherwise, both minus characters are skipped.
6. If the remaining part of one of strings starts with `^`:
if the other remaining part does not start with `^`,
the string with `^` compares higher. Otherwise, both caret characters are skipped.
6. If the remaining part of one of strings starts with `.`:
if the other remaining part does not start with `.`,
the string with `.` compares lower. Otherwise, both dot characters are skipped.
7. If either of the remaining parts starts with a digit, numerical prefixes are
compared numerically. Any leading zeroes are skipped.
The numerical prefixes (until the first non-digit character) are evaluated as numbers.
If one of the prefixes is empty, it evaluates as 0.
If the numbers are different, the string with the bigger number compares higher.
Otherwise, the comparison continues at the following characters at point 1.
8. Leading alphabetical prefixes are compared alphabetically.
The substrings are compared letter-by-letter.
If both letters are the same, the comparison continues with the next letter.
Capital letters compare lower than lower-case letters (`A < a`).
When the end of one substring has been reached (a non-letter character or the end
of the whole string), if the other substring has remaining letters, it compares higher.
Otherwise, the comparison continues at the following characters at point 1.
Examples (with '' meaning the empty string):
* `11 == 11`
* `systemd-123 == systemd-123`
* `bar-123 < foo-123`
* `123a > 123`
* `123.a > 123`
* `123.a < 123.b`
* `123a > 123.a`
* `11α == 11β`
* `A < a`
* '' < `0`
* `0.` > `0`
* `0.0` > `0`
* `0` < `~`
* '' < `~`
Note: [systemd-analyze](https://www.freedesktop.org/software/systemd/man/systemd-analyze.html)
implements this version comparison algorithm as
```
systemd-analyze compare-versions <version-a> <version-b>
```
## 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.
### Why is the version comparsion 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.
### Out of Focus
There are a couple of items that are out of focus for this specification:
@ -419,6 +583,7 @@ 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
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,
@ -426,6 +591,7 @@ 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
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
@ -434,6 +600,13 @@ There are a couple of items that are out of focus for this specification:
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 configuration
snippets following this specification cannot be used to spawn other operating
systems (such as Windows).
## Links

25
src/fundamental/string-util-fundamental.c
View File

@ -124,8 +124,8 @@ sd_int strverscmp_improved(const sd_char *a, const sd_char *b) {
* (newer) 124-1
*/
if (isempty(a) || isempty(b))
return CMP(strcmp_ptr(a, b), 0);
a = strempty(a);
b = strempty(b);
for (;;) {
const sd_char *aa, *bb;
@ -150,7 +150,7 @@ sd_int strverscmp_improved(const sd_char *a, const sd_char *b) {
}
/* If at least one string reaches the end, then longer is newer.
* Note that except for '~' prefixed segments, a string has more segments is newer.
* Note that except for '~' prefixed segments, a string which has more segments is newer.
* So, this check must be after the '~' check. */
if (*a == '\0' || *b == '\0')
return CMP(*a, *b);
@ -187,12 +187,6 @@ sd_int strverscmp_improved(const sd_char *a, const sd_char *b) {
}
if (is_digit(*a) || is_digit(*b)) {
/* Skip leading '0', to make 00123 equivalent to 123. */
while (*a == '0')
a++;
while (*b == '0')
b++;
/* Find the leading numeric segments. One may be an empty string. So,
* numeric segments are always newer than alpha segments. */
for (aa = a; is_digit(*aa); aa++)
@ -200,6 +194,17 @@ sd_int strverscmp_improved(const sd_char *a, const sd_char *b) {
for (bb = b; is_digit(*bb); bb++)
;
/* Check if one of the strings was empty, but the other not. */
r = CMP(a != aa, b != bb);
if (r != 0)
return r;
/* Skip leading '0', to make 00123 equivalent to 123. */
while (*a == '0')
a++;
while (*b == '0')
b++;
/* To compare numeric segments without parsing their values, first compare the
* lengths of the segments. Eg. 12345 vs 123, longer is newer. */
r = CMP(aa - a, bb - b);
@ -228,7 +233,7 @@ sd_int strverscmp_improved(const sd_char *a, const sd_char *b) {
return r;
}
/* The current segments are equivalent. Let's compare the next one. */
/* The current segments are equivalent. Let's move to the next one. */
a = aa;
b = bb;
}

2
src/login/logind-session-device.c
View File

@ -120,7 +120,7 @@ static int session_device_open(SessionDevice *sd, bool active) {
assert(sd->type != DEVICE_TYPE_UNKNOWN);
assert(sd->node);
/* open device and try to get an udev_device from it */
/* open device and try to get a udev_device from it */
fd = open(sd->node, O_RDWR|O_CLOEXEC|O_NOCTTY|O_NONBLOCK);
if (fd < 0)
return -errno;

25
src/test/test-string-util.c
View File

@ -852,8 +852,8 @@ static void test_strverscmp_improved_newer(const char *older, const char *newer)
TEST(strverscmp_improved) {
static const char * const versions[] = {
"",
"~1",
"",
"ab",
"abb",
"abc",
@ -917,6 +917,29 @@ TEST(strverscmp_improved) {
/* invalid characters */
assert_se(strverscmp_improved("123_aa2-67.89", "123aa+2-67.89") == 0);
/* some corner cases */
assert_se(strverscmp_improved("123.", "123") > 0); /* One more version segment */
assert_se(strverscmp_improved("12_3", "123") < 0); /* 12 < 123 */
assert_se(strverscmp_improved("12_3", "12") > 0); /* 3 > '' */
assert_se(strverscmp_improved("12_3", "12.3") > 0); /* 3 > '' */
assert_se(strverscmp_improved("123.0", "123") > 0); /* 0 > '' */
assert_se(strverscmp_improved("123_0", "123") > 0); /* 0 > '' */
assert_se(strverscmp_improved("123..0", "123.0") < 0); /* '' < 0 */
/* empty strings or strings with ignored characters only */
assert_se(strverscmp_improved("", NULL) == 0);
assert_se(strverscmp_improved(NULL, "") == 0);
assert_se(strverscmp_improved("0_", "0") == 0);
assert_se(strverscmp_improved("_0_", "0") == 0);
assert_se(strverscmp_improved("_0", "0") == 0);
assert_se(strverscmp_improved("0", "0___") == 0);
assert_se(strverscmp_improved("", "_") == 0);
assert_se(strverscmp_improved("_", "") == 0);
assert_se(strverscmp_improved("_", "_") == 0);
assert_se(strverscmp_improved("", "~") > 0);
assert_se(strverscmp_improved("~", "") < 0);
assert_se(strverscmp_improved("~", "~") == 0);
/* non-ASCII digits */
(void) setlocale(LC_NUMERIC, "ar_YE.utf8");
assert_se(strverscmp_improved("1٠١٢٣٤٥٦٧٨٩", "1") == 0);