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SYSTEMD-SYSEXT(8) systemd-sysext SYSTEMD-SYSEXT(8)

NAME
systemd-sysext, systemd-sysext.service - Activates System Extension Images

SYNOPSIS
systemd-sysext [OPTIONS...]

systemd-sysext.service

DESCRIPTION
systemd-sysext activates/deactivates system extension images. System extension images may –
dynamically at runtime — extend the /usr/ and /opt/ directory hierarchies with additional
files. This is particularly useful on immutable system images where a /usr/ and/or /opt/
hierarchy residing on a read-only file system shall be extended temporarily at runtime
without making any persistent modifications.

System extension images should contain files and directories similar in fashion to regular
operating system tree. When one or more system extension images are activated, their /usr/
and /opt/ hierarchies are combined via "overlayfs" with the same hierarchies of the host OS,
and the host /usr/ and /opt/ overmounted with it ("merging"). When they are deactivated, the
mount point is disassembled — again revealing the unmodified original host version of the
hierarchy ("unmerging"). Merging thus makes the extension's resources suddenly appear below
the /usr/ and /opt/ hierarchies as if they were included in the base OS image itself.
Unmerging makes them disappear again, leaving in place only the files that were shipped with
the base OS image itself.

Files and directories contained in the extension images outside of the /usr/ and /opt/
hierarchies are not merged, and hence have no effect when included in a system extension
image. In particular, files in the /etc/ and /var/ included in a system extension image will
not appear in the respective hierarchies after activation.

System extension images are strictly read-only, and the host /usr/ and /opt/ hierarchies
become read-only too while they are activated.

System extensions are supposed to be purely additive, i.e. they are supposed to include only
files that do not exist in the underlying basic OS image. However, the underlying mechanism
(overlayfs) also allows removing files, but it is recommended not to make use of this.

System extension images may be provided in the following formats:

1. Plain directories or btrfs subvolumes containing the OS tree

2. Disk images with a GPT disk label, following the Discoverable Partitions Specification[1]

3. Disk images lacking a partition table, with a naked Linux file system (e.g. squashfs or
ext4)

These image formats are the same ones that systemd-nspawn(1) supports via it's
--directory=/--image= switches and those that the service manager supports via
RootDirectory=/RootImage=. Similar to them they may optionally carry Verity authentication
information.

System extensions are automatically looked for in the directories /etc/extensions/,
/run/extensions/, /var/lib/extensions/, /usr/lib/extensions/ and /usr/local/lib/extensions/.
The first two listed directories are not suitable for carrying large binary images, however
are still useful for carrying symlinks to them. The primary place for installing system
extensions is /var/lib/extensions/. Any directories found in these search directories are
considered directory based extension images, any files with the .raw suffix are considered
disk image based extension images.

During boot OS extension images are activated automatically, if the systemd-sysext.service is
enabled. Note that this service runs only after the underlying file systems where system
extensions are searched are mounted. This means they are not suitable for shipping resources
that are processed by subsystems running in earliest boot. Specifically, OS extension images
are not suitable for shipping system services or systemd-sysusers(8) definitions. See
Portable Services[2] for a simple mechanism for shipping system services in disk images, in a
similar fashion to OS extensions. Note the different isolation on these two mechanisms: while
system extension directly extend the underlying OS image with additional files that appear in
a way very similar to as if they were shipped in the OS image itself and thus imply no
security isolation, portable services imply service level sandboxing in one way or another.
The systemd-sysext.service service is guaranteed to finish start-up before basic.target is
reached; i.e. at the time regular services initialize (those which do not use
DefaultDependencies=no), the files and directories system extensions provide are available in
/usr/ and /opt/ and may be accessed.

Note that there is no concept of enabling/disabling installed system extension images: all
installed extension images are automatically activated at boot.

A simple mechanism for version compatibility is enforced: a system extension image must carry
a /usr/lib/extension-release.d/extension-release.$name file, which must match its image name,
that is compared with the host os-release file: the contained ID= fields have to match, as
well as the SYSEXT_LEVEL= field (if defined). If the latter is not defined, the VERSION_ID=
field has to match instead. System extensions should not ship a /usr/lib/os-release file (as
that would be merged into the host /usr/ tree, overriding the host OS version data, which is
not desirable). The extension-release file follows the same format and semantics, and carries
the same content, as the os-release file of the OS, but it describes the resources carried in
the extension image.

USES
The primary use case for system images are immutable environments where debugging and
development tools shall optionally be made available, but not included in the immutable base
OS image itself (e.g. strace(1) and gdb(1) shall be an optionally installable addition in
order to make debugging/development easier). System extension images should not be
misunderstood as a generic software packaging framework, as no dependency scheme is
available: system extensions should carry all files they need themselves, except for those
already shipped in the underlying host system image. Typically, system extension images are
built at the same time as the base OS image — within the same build system.

Another use case for the system extension concept is temporarily overriding OS supplied
resources with newer ones, for example to install a locally compiled development version of
some low-level component over the immutable OS image without doing a full OS rebuild or
modifying the nominally immutable image. (e.g. "install" a locally built package with
DESTDIR=/var/lib/extensions/mytest make install && systemd-sysext refresh, making it
available in /usr/ as if it was installed in the OS image itself.) This case works regardless
if the underlying host /usr/ is managed as immutable disk image or is a traditional package
manager controlled (i.e. writable) tree.

COMMANDS
The following commands are understood:

status
When invoked without any command verb, or when status is specified the current merge
status is shown, separately for both /usr/ and /opt/.

merge
Merges all currently installed system extension images into /usr/ and /opt/, by
overmounting these hierarchies with an "overlayfs" file system combining the underlying
hierarchies with those included in the extension images. This command will fail if the
hierarchies are already merged.

unmerge
Unmerges all currently installed system extension images from /usr/ and /opt/, by
unmounting the "overlayfs" file systems created by merge prior.

refresh
A combination of unmerge and merge: if already mounted the existing "overlayfs" instance
is unmounted temporarily, and then replaced by a new version. This command is useful
after installing/removing system extension images, in order to update the "overlayfs"
file system accordingly. If no system extensions are installed when this command is
executed, the equivalent of unmerge is executed, without establishing any new "overlayfs"
instance. Note that currently there's a brief moment where neither the old nor the new
"overlayfs" file system is mounted. This implies that all resources supplied by a system
extension will briefly disappear — even if it exists continuously during the refresh
operation.

list
A brief list of installed extension images is shown.

-h, --help
Print a short help text and exit.

--version
Print a short version string and exit.

OPTIONS
--root=
Operate relative to the specified root directory, i.e. establish the "overlayfs" mount
not on the top-level host /usr/ and /opt/ hierarchies, but below some specified root
directory.

--force
When merging system extensions into /usr/ and /opt/, ignore version incompatibilities,
i.e. force merging regardless of whether the version information included in the
extension images matches the host or not.

--no-pager
Do not pipe output into a pager.

--no-legend
Do not print the legend, i.e. column headers and the footer with hints.

--json=MODE
Shows output formatted as JSON. Expects one of "short" (for the shortest possible output
without any redundant whitespace or line breaks), "pretty" (for a pretty version of the
same, with indentation and line breaks) or "off" (to turn off JSON output, the default).

EXIT STATUS
On success, 0 is returned.