systemd-sysextsystemdsystemd-sysext8systemd-sysextsystemd-sysext.serviceActivates System Extension Imagessystemd-sysextOPTIONSCOMMANDsystemd-sysext.serviceDescriptionsystemd-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:Plain directories or btrfs subvolumes containing the OS treeDisk images with a GPT disk label, following the Discoverable Partitions SpecificationDisk images lacking a partition table, with a naked Linux file system (e.g. erofs,
squashfs or ext4)These image formats are the same ones that
systemd-nspawn1
supports via its / switches and those that the
service manager supports via /. 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-sysusers8
definitions. See the Portable Services Documentation
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 uname2
but the used architecture identifiers are the same as for ConditionArchitecture=
described in systemd.unit5.
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.UsesThe 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.
strace1
and
gdb1
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.CommandsThe following commands are understood:When invoked without any command verb, or when is specified
the current merge status is shown, separately for both /usr/ and
/opt/.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.Unmerges all currently installed system extension images from
/usr/ and /opt/, by unmounting the
overlayfs file systems created by
prior.A combination of and : 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 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.A brief list of installed extension images is shown.OptionsOperate 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.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.Exit statusOn success, 0 is returned.See Alsosystemd1,
systemd-nspawn1