boot-loader-spec: undo redefinition of $BOOT

In 53c26db4dac0f5b79ca2a57364ee7df78a14bbfd the meaning of $BOOT was
redefined. I think that's quite problematic, since the concept is
implemented in code and interface of bootctl. Thus, I think we should
stick to the original definition, which is: "where to *place* boot menu
entries" (as opposed to "where to *read* boot menu entries from").

The aforementioned change was done to address two things afaiu:

1. it focussed on a $BOOT as the single place to put boot entries in,
   instead of mentioning that both ESP and $BOOT are expected to be
   the source

2. it mentioned the /loader/ dir (as location for boot loader resources)
   itself as part of the spec, which however only really makes sense in
   the ESP. /loader/entries/ otoh makes sense in either the ESP or
   $BOOT.

With this rework I try to address these two issues differently:

1. I intend to make clear the $BOOT is the "primary" place to put stuff
   in, and is what should be mounted to /boot/.

2. The ESP (if different from $BOOT) is listed as "secondary" source to
   read from, and is what should be mounted to /efi/. NB we now make the
   distinction between "where to put" (which is single partition) and
   "where to read from".

3. This drops any reference of the /loader/ dir witout the /entries/
   suffix. Only the full /loader/entries/ dir (and its companion file
   /loader/entries.srel) are now mentioned. Thus isolated /loader/
   directory hence becomes irrelevant in the spec, and the fact that
   sd-boot maintains some files there (and only in the ESP) is kept out
   of the spec, because it is irrelevant to other boot loaders.

4. It puts back the suggestion to mount $BOOT to /boot/ and the ESP to
   /efi/ (and suggests adding a symlink or bind mount if both are the
   same partition). Why? Because the dirs are semantically unrelated:
   it's OK and common to have and ESP but no $BOOT, hence putting ESP
   inside of a useless, non-existing "ghost" dir /boot/ makes little
   sense. More importantly though, because these partitions are
   typically backed by VFAT we want to maintain them as an autofs, with
   a short idle delay, so that the file systems are unmounted (and thus
   fully clean) at almost all times. This doesn't work if they are
   nested within each other, as the establishment of the inner autofs
   would pin the outer one, making the excercise useless. Now I don't
   think the spec should mention autofs (since that is an implementation
   detail), but it should arrange things so that this specific, very
   efficient, safe and robust implementation can be implemented.

The net result should be easy from an OS perspective:

1. *Put* boot loader entries in /boot/, always.

2. *Read* boot loader entries from both /boot/ and /efi/ -- if these are distinct.

3. The only things we define in the spec are /loader/entries/*.conf and
   /EFI/Linux/*.efi in these two partitions (well, and the companion
   file /loader/entries.srel

4. /efi/ and /boot/ because not nested can be autofs.

5. bootctl code and interface (in particular --esp-path= and
   --boot-path=) match the spec again. `bootctl -x` and `bootctl -p`
   will now print the path to $BOOT and ESP again, matching the concepts
   in the spec again.

From the sd-boot perspective things are equally easy:

1. Read boot enrties from ESP and XBOOTLDR.

2. Maintain boot loader config/other resources in ESP only.

And that's it.

Fixes: #24247

1 files changed, 185 insertions, 134 deletions

diff --git a/docs/BOOT_LOADER_SPECIFICATION.md b/docs/BOOT_LOADER_SPECIFICATION.md
index 70e0c7ba1d..9188033768 100644
--- a/docs/BOOT_LOADER_SPECIFICATION.md
+++ b/docs/BOOT_LOADER_SPECIFICATION.md
@@ -8,12 +8,12 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 # The Boot Loader Specification
 
 This document defines a set of file formats and naming conventions that allow
-the boot loader configuration to be shared between multiple operating systems
+the boot loader menu entries to be shared between multiple operating systems
 and boot loaders installed on one device.
 
-Operating systems cooperatively manage a boot loader configuration directory
-that contains drop-in files, making multi-boot scenarios easy to support. Boot
-menu items are defined via a simple format that can be understood by different
+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.
@@ -23,72 +23,105 @@ includes a boot loader.
 The target audience for this specification is:
 
 * Boot loader developers, to write a boot loader that directly reads its
-  configuration from these files
+  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 configuration
-* Distribution developers, to create appropriate configuration snippets when
+  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 Boot Partition
+## The Partitions
 
 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
+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 — partition with the type ID of 0xEA — shall be used
-    for boot loader configuration and entries.
+  * 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 GPT type GUID
-    of `c12a7328-f81f-11d2-ba4b-00a0c93ec93b` — should be used for boot loader
-    configuration and boot entries.
+  * 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 an additional
-    location for boot loader entries. This partition must be located on the
-    same disk as the ESP.
+    `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.
 
-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.
+There may be at most one partition of each of the types listed above on the
+same disk.
 
-An installer for the operating system should use this logic when selecting or
+**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
+must — and the MBR boot and GPT 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.
+
+### The `$BOOT` Partition Placeholder
+
+In the text below, the placeholder `$BOOT` will be used to refer to the
+partition determined as follows:
+
+ 1. On disks with an MBR partition table: → the boot partition, as described above
+
+ 2. On disks with a GPT partition table: → the XBOOTLDR partition if it exists
+
+ 3. 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 `$BOOT` is not found, a new suitably sized partition (let's say 500MB)
-    should be created, matching the characteristics described above. On disks
-    with GPT, only the ESP partition without the XBOOTLDR partition should be
-    created.
+  * If a boot partition (in case of MBR) or an XBOOTLDR partition (in case of
+    GPT) already exists it should be used as `$BOOT` and used as primary
+    location to place boot loader menu resources in.
 
-  * 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.
+  * 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.
 
-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`.
+  * 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.
 
-**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._
+  * 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.
 
-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.
+These partitions shall be determined during _installation time_, and
+`/etc/fstab` entries may be created.
+
+### Mount Points
+
+It is recommended to mount `$BOOT` to `/boot/`, and the ESP to `/efi/`. If
+`$BOOT` and the ESP are the same, then either a bind mount or a symlink should
+be established making the partition available under both paths.
+
+(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.)
 
 ## Boot Loader Entries
 
@@ -108,37 +141,43 @@ 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 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. 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.
+Note that the three partitions described above are not supposed to be 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 intend to define contents of the rest 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 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/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.
 
 ### Type #1 Boot Loader Specification Entries
 
-`$ESP/loader/` is the main directory containing the configuration for the boot
-loader.
-
-**Note:** _In all cases the `/loader/` directory should be located directly in
-the root of the file system. Specifically, the `/loader/` directory should
-**not** be located under the `/EFI/` subdirectory on the ESP._
-
-`$BOOT/loader/entries/` and `$XBOOTLDR/loader/entries/` are the directories
-containing the drop-in snippets defining boot entries, one `.conf` file for
-each boot menu item. Each OS may provide one or more such entries. The boot
-loader should enumerate both directories and provide a merged list.
-
-The file name is used for identification of the boot item but shall never be
-presented to the user in the UI. The file name may be chosen freely but should
-be unique enough to avoid clashes between OS installations. More specifically,
-it is suggested to include the `entry-token` (see
+`/loader/entries/` in `$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 file system. 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](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)),
@@ -155,9 +194,9 @@ set: ASCII upper and lower case characters, digits, "+", "-", "_" and ".".
 Also, the file names should have a length of at least one and at most 255
 characters (including the file name suffix).
 
-These configuration snippets shall be UNIX-style text files (i.e. lines
+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
-configuration snippets are loosely inspired by Grub1's configuration syntax.
+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.
@@ -202,16 +241,17 @@ The following keys are recognized:
 
   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.
+* `linux` is the Linux kernel image to execute and takes a path relative to the
+  root of the file system containing the boot entry snippet itself. It is
+  recommended that every distribution creates an entry-token/machine-id and
+  version specific subdirectory and places its kernels and initrd images there
+  (see below).
 
   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.
+* `initrd` is the initrd `cpio` image to use when executing the kernel. 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`
 
@@ -220,7 +260,7 @@ The following keys are recognized:
 
 * `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.
+  specified parameters are combined in the order they are listed.
 
   Example: `options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet`
 
@@ -244,7 +284,7 @@ The following keys are recognized:
 
   Example: `architecture aa64`
 
-Each configuration drop-in snippet must include at least a `linux` or an `efi`
+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
@@ -268,11 +308,15 @@ 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 configuration snippets refer to other files (for `linux`,
+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.
+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
@@ -284,7 +328,7 @@ still safe and robust.
 It is recommended 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
+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`
@@ -297,8 +341,8 @@ 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 installing files that do
-not follow this specification. In order to minimize confusion, a boot loader
+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
@@ -311,13 +355,16 @@ 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, an initramfs image, and the kernel command line. See
-the description of the `--uefi` option in
-[dracut(8)](https://man7.org/linux/man-pages/man8/dracut.8.html). Such unified
-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.
+loader, a kernel image, an initrd image, and the kernel command line. See
+[systemd-stub(7)](https://www.freedesktop.org/software/systemd/man/systemd-stub.html)
+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
@@ -327,7 +374,7 @@ 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 valid unified kernel image must contain two PE sections:
+A valid unified kernel image in the `/EFI/Linux/` directory must contain two PE sections:
 
 * `.cmdline` section with the kernel command line,
 * `.osrel` section with an embedded copy of the
@@ -344,42 +391,44 @@ On EFI, any such images shall be added to the list of valid boot entries.
 
 ### Additional Notes
 
-Note that these configurations snippets do not need to be the only
-configuration source 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.
+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
-configuration 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).
+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 configuration snippets use a Unix-style "/" as
-path separator. This needs to be converted to an EFI-style "\\" separator in
-EFI boot loaders.
+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
 
-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.
+A _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 `$XBOOTLDR` (if used) or
-`$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 `$BOOT/loader/entries/xyz.conf`, with "xyz" as concatenation of
-machine id and version information (see above). The files created by a kernel
-package are tied to the kernel package and should be removed along with it.
+installer_ installs the kernel and initrd images to `$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
@@ -387,14 +436,16 @@ installer should create the combined image and drop it into
 removed along with it.
 
 A _UI application_ intended to show available boot options shall operate
-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.
+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 in it. It then installs an appropriate boot loader that can read
-these snippets. Finally, it installs one or more kernel packages.
+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
 
@@ -567,18 +618,18 @@ This specification brings the following advantages:
   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
+  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 configuration which is essential
+* 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 configuration too, UI can be written that
+  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 configuration of the various boot loaders, which
+* 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.
 
@@ -678,9 +729,9 @@ There are a couple of items that are out of focus for this specification:
 * 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).
+  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