jammy (8) systemd-sysext.8.gz

Provided by: systemd_249.11-0ubuntu3.12_amd64 bug

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.

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