Merge pull request #10415 from poettering/boot-loader-spec-tweaks
docs: tweaks to the boot loader spec
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Lennart Poettering
GitHub
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a57e48a133
4
TODO
4
TODO
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@ -21,6 +21,10 @@ Janitorial Clean-ups:
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Features:
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* bootctl: implement Type #2 boot loader entry discovery
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* bootctl,sd-boot: actually honour the "architecture" key
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* consider splitting out all temporary file creation APIs (we have so many in
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fileio.h and elsewhere!) into a new util file of its own.
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@ -43,20 +43,36 @@ Everything described below is located on a placeholder file system `$BOOT`. The
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* Otherwise, if the OS is installed on a disk with GPT disk label, and if the ESP partition already exists but is too small, a new suitably sized (let's say 500MB) partition with GPT type GUID of bc13c2ff-59e6-4262-a352-b275fd6f7172 shall be created and it should be used as `$BOOT`.
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* Otherwise, if the OS is installed on a disk with GPT disk label, and no ESP partition exists yet, a new suitably sized (let's say 500MB) ESP should be created and should be used as `$BOOT`.
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This placeholder file system shall be determined during _installation time_, and an fstab entry may be created. It should be mounted to either /boot or /efi. Additional locations like /boot/efi, with /boot being a separate file system, might be supported by implementations. This is not recommended because the mounting of `$BOOT` is then dependent on and requires the mounting of the intermediate file system.
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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 `$BOOT` is then dependent on and requires the mounting of the intermediate file system.
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**Note:** _`$BOOT` should be considered **shared** among all OS installations of a system. Instead of maintaining one `$BOOT` per installed OS (as `/boot` was traditionally handled), all installed OS share the same place to drop in their boot-time configuration._
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**Note:** _`$BOOT` should be considered **shared** among all OS installations of a system. Instead of maintaining one `$BOOT` per installed OS (as `/boot/` was traditionally handled), all installed OS share the same place to drop in their boot-time configuration._
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For systems where the firmware is able to read file systems directly, `$BOOT` must be a file system readable by the firmware. For other systems, `$BOOT` must be a VFAT (16 or 32) file system. Applications accessing `$BOOT` should hence not assume that fancier file system features such as symlinks, hardlinks, access control or case sensitivity are supported.
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### Boot loader specification entries
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This specification defines two types of boot loader entries. The first type is
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text based, very simple and suitable for a variety of firmware, architecture
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and image types ("Type #1"). The second type is specific to EFI, but allows
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single-file images that embedd all metadata in the kernel binary itself, which
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is useful to cryptographically sign them as one file for the purpose of
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SecureBoot ("Type #2").
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Not all boot loader entries will apply to all systems. For example, Type #1
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entries that use the `efi` key and all Type #2 entries only apply to EFI
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systems. Entries using the `architecture` key might specify an architecture that
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doesn't match the local one. Boot loaders should ignore all entries that don't
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match the local platform and what the boot loader can support, and hide them
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from the user. Only entries matching the feature set of boot loader and system
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shall be considered and displayed. This allows image builders to put together
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images that transparently support multiple different architectures.
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### Type #1 Boot Loader Specification Entries
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We define two directories below `$BOOT`:
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* `$BOOT/loader/` is the directory containing all files defined by this specification
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* `$BOOT/loader/` is the directory containing all files needed for Type #1 entries
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* `$BOOT/loader/entries/` is the directory containing the drop-in snippets. This directory contains one `.conf` file for each boot menu item.
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**Note:** _In all cases the /loader directory should be located directly in the root of the file system. Specifically, if `$BOOT` is the ESP, then /loader directory should be located directly in the root directory of the ESP, and not in the EFI/ subdirectory._
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**Note:** _In all cases the `/loader/` directory should be located directly in the root of the file system. Specifically, if `$BOOT` is the ESP, then `/loader/` directory should be located directly in the root directory of the ESP, and not in the `/EFI/` subdirectory._
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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`.
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@ -65,32 +81,44 @@ These configuration snippets shall be Unix-style text files (i.e. line separatio
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* `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)".
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* `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`.
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* `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`.
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* `linux` refers to the kernel to spawn and shall be a path relative to the `$BOOT` directory. It is recommended that every distribution creates a machine id and version specific subdirectory below `$BOOT` and places its kernels and initial RAM disk images there. Example: `/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux`.
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* `linux` refers to the Linux kernel to spawn and shall be a path relative to the `$BOOT` directory. It is recommended that every distribution creates a machine id and version specific subdirectory below `$BOOT` and places its kernels and initial RAM disk images there. Example: `/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux`.
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* `initrd` refers to the initrd to use when executing the kernel. This also shall be a path relative to the `$BOOT` directory. This key is optional. This key may appear more than once in which case all specified images are used, in the order they are listed. Example: `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`.
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* `efi` to spawn arbitrary EFI programs. This also takes a path relative to `$BOOT`. This key is only available on EFI systems.
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* `efi` refers to an arbitrary EFI program. This also takes a path relative to `$BOOT`. If this key is set, and the system is not an EFI system this entry should be hidden.
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* `options` shall contain kernel parameters to pass to the Linux kernel to spawn. This key is optional and may appear more than once in which case all specified parameters are used in the order they are listed.
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* `devicetree` refers to the binary device tree to use when executing the
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kernel. This also shall be a path relative to the `$BOOT` directory. This
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key is optional. Example: `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`.
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* `architecture` refers to the UEFI architecture this entry is defined for. If specified and this does not match the local UEFI system architecture the entry is hidden.
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* `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.
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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:
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# /boot/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf
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title Fedora 19 (Rawhide)
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version 3.8.0-2.fc19.x86_64
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machine-id 6a9857a393724b7a981ebb5b8495b9ea
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options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2
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linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
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initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd
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title Fedora 19 (Rawhide)
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version 3.8.0-2.fc19.x86_64
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machine-id 6a9857a393724b7a981ebb5b8495b9ea
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options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2
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architecture x64
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linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
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initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd
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On EFI systems all kernel images shall be EFI images. In order to be compatible 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.
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On EFI systems all Linux kernel images should be EFI images. In order to increase compatibility with EFI systems it is highly recommended only to install EFI kernel images, even on non-EFI systems, if that's applicable and supported on the specific architecture.
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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).
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### Unified kernel images
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### Type #2 EFI Unified Kernel Images
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A unified kernel image is a single UEFI executable combining an UEFI 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 images will be searched for under `$BOOT/EFI/Linux` and must have the extension `.efi`.
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A unified kernel image is a single EFI PE executable combining an EFI stub
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loader, a kernel image, an initramfs image, and the kernel command line. See
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the description of the `--uefi` option in
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[dracut(8)](http://man7.org/linux/man-pages/man8/dracut.8.html). Such unified
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images will be searched for under `$BOOT/EFI/Linux/` and must have the
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extension `.efi`. Support for images of this type is of course specific to
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systems with EFI firmware. Ignore this section if you work on systems not
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supporting EFI.
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Images of this type have the advantage that all metadata and payload that makes
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up the boot entry is monopolized in a single PE file that can be signed
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cryptographically as one for the purpose of EFI SecureBoot.
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A valid unified kernel image must contain two PE sections:
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@ -99,7 +127,7 @@ A valid unified kernel image must contain two PE sections:
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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.
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Any such images shall be added to the list of valid boot entries.
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On EFI, any such images shall be added to the list of valid boot entries.
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### Additional notes
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@ -112,11 +140,28 @@ Note that all paths used in the configuration snippets use a Unix-style "/" as p
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## Logic
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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. It then extends this with any unified kernel images found in `$BOOT/EFI/Linux`. It may also add additional entries, for example "Reboot into firmware". Optionally it may sort the menu based on the `machine-id` and `version` fields, and possibly others. It uses the file name to identify specific items, for example in case it supports storing away default entry information somewhere. A boot loader should generally not modify these files.
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A _boot loader_ needs a file system driver to discover and read `$BOOT`, then
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simply reads all files `$BOOT/loader/entries/*.conf`, and populates its boot
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menu with this. On EFI, it then extends this with any unified kernel images
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found in `$BOOT/EFI/Linux/*.efi`. It may also add additional entries, for
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example a "Reboot into firmware" option. Optionally it may sort the menu based
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on the `machine-id` and `version` fields, and possibly others. It uses the file
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name to identify specific items, for example in case it supports storing away
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default entry information somewhere. A boot loader should generally not modify
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these files.
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For "boot loader specification" entries, 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.
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For "Boot Loader Specification Entries" (Type #1), the _kernel package
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installer_ installs the kernel and initrd images to `$BOOT` (it is recommended
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to place these files in a vendor and OS and installation specific directory)
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and then generates a configuration snippet for it, placing this in
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`$BOOT/loader/entries/xyz.conf`, with xyz as concatenation of machine id and
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version information (see above). The files created by a kernel package are
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private property of the kernel package and should be removed along with it.
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For "unified kernel images", the _kernel install_ creates the combined image and drops it into `$BOOT/EFI/Linux`. This file is also private property of the kernel package and should be removed along with it.
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For "EFI Unified Kernel Images" (Type #2), the vendor or kernel package
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installer creates the combined image and drops it into `$BOOT/EFI/Linux/`. This
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file is also private property of the kernel package and should be removed along
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with it.
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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 newest kernel versions.
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@ -128,7 +173,7 @@ An _OS installer_ picks the right place for `$BOOT` as defined above (possibly c
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There are a couple of items that are out of focus for this specification:
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* 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.
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* 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 <strike>Gummiboot</strike> systemd-boot does it), or via native configuration (for example via explicit Grub2 configuration generated once at installation).
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* 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).
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* 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.
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@ -136,5 +181,3 @@ There are a couple of items that are out of focus for this specification:
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[systemd-boot(7)](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)<br>
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[bootctl(1)](https://www.freedesktop.org/software/systemd/man/bootctl.html)
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[Obsolete patch adding Boot Loader Specification support to GNU grub 2](http://pkgs.fedoraproject.org/cgit/grub2.git/tree/0460-blscfg-add-blscfg-module-to-parse-Boot-Loader-Specif.patch?h=f20)
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