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:
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 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.
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.
- Discoverable Partition Specification
- Portable Services