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 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(8) 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.
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.
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.
MUTABILITY
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.
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).
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.
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.
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).
EXIT STATUS
On success, 0 is returned.
SEE ALSO
systemd(1), systemd-nspawn(1), systemd-stub(7), importctl(1)
NOTES
1. Discoverable Partitions Specification
https://uapi-group.org/specifications/specs/discoverable_partitions_specification
2. Portable Services
https://systemd.io/PORTABLE_SERVICES
systemd 257.9 SYSTEMD-SYSEXT(8)
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