SYSTEMD-SYSEXT(8) systemd-sysext SYSTEMD-SYSEXT(8)
NAME
systemd-sysext, systemd-sysext.service - Activates System Extension
Images
SYNOPSIS
systemd-sysext [OPTIONS...] COMMAND
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 overlaying or 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 its --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 may be
located have been 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. However, you can place an empty directory named like
the extension (no .raw) in /etc/extensions/ to "mask" an extension with
the same name in a system folder with lower precedence.
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 unless "_any" is set for the
extension. If the extension ID= is not "_any", the SYSEXT_LEVEL= field
(if defined) has to match. If the latter is not defined, the
VERSION_ID= field has to match instead. If the extension defines the
ARCHITECTURE= field and the value is not "_any" it has to match the
kernel's architecture reported by uname(2) but the used architecture
identifiers are the same as for ConditionArchitecture= described in
systemd.unit(5). 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.
SEE ALSO
systemd(1), systemd-nspawn(1)
NOTES
1. Discoverable Partitions Specification
https://systemd.io/DISCOVERABLE_PARTITIONS
2. Portable Services
https://systemd.io/PORTABLE_SERVICES
systemd 252 SYSTEMD-SYSEXT(8)
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