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NAME | SYNOPSIS | DESCRIPTION | USES | MUTABILITY | COMMANDS | OPTIONS | EXIT STATUS | SEE ALSO | NOTES | COLOPHON |
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SYSTEMD-SYSEXT(8) systemd-sysext SYSTEMD-SYSEXT(8)
systemd-sysext, systemd-sysext.service, systemd-confext, systemd-
confext.service - Activates System Extension Images
systemd-sysext [OPTIONS...] COMMAND
systemd-sysext.service
systemd-confext [OPTIONS...] COMMAND
systemd-confext.service
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 by default. On
mutable host file systems, /usr/ and /opt/ hierarchies become
read-only while extensions are merged, unless mutability is
enabled. Mutability may be enabled via the --mutable= option; see
"Mutability" below for more information.
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[2]
page 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). EXTENSION_RELOAD_MANAGER= can be set to 1 if
the extension requires a service manager reload after application
of the extension. Note that for the reasons mentioned earlier:
Portable Services[2] remain the recommended way to ship system
services. 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".
Just like sysexts, confexts are strictly read-only by default.
Merging confexts on mutable host file systems will result in
/etc/ becoming read-only. As with sysexts, mutability can be
enabled via the --mutable= option. Refer to "Mutability" below
for more information.
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 the
user.extension-release.strict xattr(7)), 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.
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.
With systemd-confext one can 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. It
also provides a reliable tool for managing configuration because
all old configuration files disappear when the systemd-confext
image is removed.
By default, merging system extensions on mutable host file
systems will render /usr/ and /opt/ hierarchies read-only.
Merging configuration extensions will have the same effect on
/etc/. Mutable mode allows writes to these locations when
extensions are merged.
The following modes are supported:
1. disabled: Force immutable mode even if write routing
directories exist below /var/lib/extensions.mutable/. This is
the default.
2. auto: Automatic mode. Mutability is disabled by default and
only enabled if a corresponding write routing directory
exists below /var/lib/extensions.mutable/.
3. enabled: Force mutable mode and automatically create write
routing directories below /var/lib/extensions.mutable/ when
required.
4. import: Force immutable mode like disabled above, but merge
the contents of directories below
/var/lib/extensions.mutable/ into the host file system.
5. ephemeral: Force mutable mode like enabled above, but instead
of using write routing directory below
/var/lib/extensions.mutable/, systemd-sysext will use empty
ephemeral directories. This means that the modifications made
in the merged hierarchies will be gone when the hierarchies
are unmerged.
6. ephemeral-import: Force mutable mode like ephemeral above,
but instead of ignoring the contents of write routing
directories under /var/lib/extensions.mutable/, merge them
into the host file system, like import does.
See "Options" below on specifying modes using the --mutable=
command line option.
With exception of the ephemeral mode, the mutable mode routes
writes to subdirectories in /var/lib/extensions.mutable/.
Writes to /usr/ are directed to
/var/lib/extensions.mutable/usr/
writes to /opt/ are directed to
/var/lib/extensions.mutable/opt/, and
writes to /etc/ land in /var/lib/extensions.mutable/etc/.
If usr/, opt/, or etc/ in /var/lib/extensions.mutable/ are
symlinks, then writes are directed to the symlinks' targets.
Consequently, to retain mutability of a host file system, create
symlinks
/var/lib/extensions.mutable/etc/ → /etc/
/var/lib/extensions.mutable/usr/ → /usr/
/var/lib/extensions.mutable/opt/ → /opt/
to route writes back to the original base directory hierarchy.
Alternatively, a temporary file system may be mounted to
/var/lib/extensions.mutable/, or symlinks in
/var/lib/extensions.mutable/ may point to sub-directories on a
temporary file system (e.g. below /tmp/) to only allow ephemeral
changes. Note that this is not the same as ephemeral mode,
because the temporary file system will still exist after
unmerging.
Added in version 256.
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).
Added in version 248.
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.
Added in version 248.
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.
Added in version 248.
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.
Added in version 248.
list
A brief list of installed extension images is shown.
Added in version 248.
-h, --help
Print a short help text and exit.
--version
Print a short version string and exit.
--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.
Added in version 248.
--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.
Added in version 248.
--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.
Added in version 254.
--mutable=BOOL|auto|import
Set mutable mode.
no
force immutable mode even with write routing directories
present. This is the default.
Added in version 256.
auto
enable mutable mode individually for /usr/, /opt/, and
/etc/ if write routing sub-directories or symlinks are
present in /var/lib/extensions.mutable/; disable
otherwise. See "Mutability" above for more information on
write routing.
Added in version 256.
yes
force mutable mode. Write routing directories will be
created in /var/lib/extensions.mutable/ if not present.
Added in version 256.
import
immutable mode, but with contents of write routing
directories in /var/lib/extensions.mutable/ also merged
into the host file system.
Added in version 256.
ephemeral
force mutable mode, but with contents of write routing
directories in /var/lib/extensions.mutable/ being
ignored, and modifications of the host file system being
discarded after unmerge.
Added in version 256.
ephemeral-import
force mutable mode, with contents of write routing
directories in /var/lib/extensions.mutable/ being merged
into the host file system, but with the modifications
made to the host file system being discarded after
unmerge.
Added in version 256.
Added in version 256.
--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.
Added in version 254.
--no-reload
When used with merge, unmerge or refresh, do not reload
daemon after executing the changes even if an extension that
is applied requires a reload via the
EXTENSION_RELOAD_MANAGER= set to 1.
Added in version 255.
--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).
On success, 0 is returned.
systemd(1), systemd-nspawn(1), systemd-stub(7), importctl(1)
1. Discoverable Partitions Specification
https://uapi-group.org/specifications/specs/discoverable_partitions_specification
2. Portable Services
https://systemd.io/PORTABLE_SERVICES
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 257~devel SYSTEMD-SYSEXT(8)
Pages that refer to this page: importctl(1), portablectl(1), systemd-cryptenroll(1), org.freedesktop.portable1(5), os-release(5), systemd.directives(7), systemd.image-policy(7), systemd.index(7), systemd-stub(7), systemd-repart(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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