x SuSE Linux 13.1-RELEASE x
x SuSE Linux 13.1-RELEASEx
SYSTEMD-SYSEXT(8) systemd-sysext SYSTEMD-SYSEXT(8)
NAME
systemd-sysext, systemd-sysext.service, systemd-confext, systemd-
confext.service - Activates System Extension Images
SYNOPSIS
systemd-sysext [OPTIONS...] COMMAND
systemd-sysext.service
systemd-confext [OPTIONS...] COMMAND
systemd-confext.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. erofs, 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 searched for in the directories /etc/extensions/,
/run/extensions/ and /var/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. When invoked in the initrd, the
additional directory /.extra/sysext/ is included in the directories
that are searched for extension images. Note however, that by default a
tighter image policy applies to images found there, though, see below.
This directory is populated by systemd-stub(7) with extension images
found in the system's EFI System Partition.
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 the Portable
Services Documentation[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.
The systemd-confext concept follows the same principle as the systemd-
sysext(1) functionality but instead of working on /usr and /opt,
confext will extend only /etc. Files and directories contained in the
confext images outside of the /etc/ hierarchy are not merged, and hence
have no effect when included in the image. Formats for these images are
of the same as sysext images. The merged hierarchy will be mounted with
"nosuid" and (if not disabled via --noexec=false) "noexec".
Confexts are looked for in the directories /run/confexts/,
/var/lib/confexts/, /usr/lib/confexts/ and /usr/local/lib/confexts/.
The first listed directory is not suitable for carrying large binary
images, however is still useful for carrying symlinks to them. The
primary place for installing configuration extensions is
/var/lib/confexts/. Any directories found in these search directories
are considered directory based confext images; any files with the .raw
suffix are considered disk image based confext images.
Again, just like sysext images, the confext images will contain a
/etc/extension-release.d/extension-release.$name file, which must match
the image name (with the usual escape hatch of xattr), and again with
content being one or more of ID=, VERSION_ID=, and CONFEXT_LEVEL.
Confext images will then be checked and matched against the base OS
layer.
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.
For the confext case, the OSConfig project aims to perform runtime
reconfiguration of OS services. Sometimes, there is a need to swap
certain configuration parameter values or restart only a specific
service without deployment of new code or a complete OS deployment. In
other words, we want to be able to tie the most frequently configured
options to runtime updateable flags that can be changed without a
system reboot. This will help reduce servicing times when there is a
need for changing the OS configuration.
COMMANDS
The following commands are understood by both the sysext and confext
concepts:
status
When invoked without any command verb, or when status is specified
the current merge status is shown, separately (for both /usr/ and
/opt/ of sysext and for /etc/ of confext).
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. For confext, the merge happens into
the /etc/ directory instead.
unmerge
Unmerges all currently installed system extension images from /usr/
and /opt/ for sysext and /etc/, for confext, 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 for sysext or /etc/ for confext, but below some
specified root directory.
--force
When merging system extensions into /usr/ and /opt/ for sysext and
/etc/ for confext, ignore version incompatibilities, i.e. force
merging regardless of whether the version information included in
the images matches the host or not.
--image-policy=policy
Takes an image policy string as argument, as per systemd.image-
policy(7). The policy is enforced when operating on system
extension disk images. If not specified defaults to
"root=verity+signed+encrypted+unprotected+absent:usr=verity+signed+encrypted+unprotected+absent"
for system extensions, i.e. only the root and /usr/ file systems in
the image are used. For configuration extensions defaults to
"root=verity+signed+encrypted+unprotected+absent". When run in the
initrd and operating on a system extension image stored in the
/.extra/sysext/ directory a slightly stricter policy is used by
default: "root=signed+absent:usr=signed+absent", see above for
details.
--noexec=BOOL
When merging configuration extensions into /etc/ the "MS_NOEXEC"
mount flag is used by default. This option can be used to disable
it.
--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), systemd-stub(7)
NOTES
1. Discoverable Partitions Specification
https://uapi-group.org/specifications/specs/discoverable_partitions_specification
2. Portable Services Documentation
https://systemd.io/PORTABLE_SERVICES
systemd 254 SYSTEMD-SYSEXT(8)
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