1SYSTEMD-SYSEXT(8) systemd-sysext SYSTEMD-SYSEXT(8)
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6 systemd-sysext, systemd-sysext.service - Activates System Extension
7 Images
8
10 systemd-sysext [OPTIONS...]
11 systemd-sysext.service
13
15 systemd-sysext activates/deactivates system extension images. System
16 extension images may – dynamically at runtime — extend the /usr/ and
17 /opt/ directory hierarchies with additional files. This is particularly
18 useful on immutable system images where a /usr/ and/or /opt/ hierarchy
19 residing on a read-only file system shall be extended temporarily at
20 runtime without making any persistent modifications.
21 System extension images should contain files and directories similar in
23 fashion to regular operating system tree. When one or more system
24 extension images are activated, their /usr/ and /opt/ hierarchies are
25 combined via "overlayfs" with the same hierarchies of the host OS, and
26 the host /usr/ and /opt/ overmounted with it ("merging"). When they are
27 deactivated, the mount point is disassembled — again revealing the
28 unmodified original host version of the hierarchy ("unmerging").
29 Merging thus makes the extension's resources suddenly appear below the
30 /usr/ and /opt/ hierarchies as if they were included in the base OS
31 image itself. Unmerging makes them disappear again, leaving in place
32 only the files that were shipped with the base OS image itself.
33 Files and directories contained in the extension images outside of the
35 /usr/ and /opt/ hierarchies are not merged, and hence have no effect
36 when included in a system extension image. In particular, files in the
37 /etc/ and /var/ included in a system extension image will not appear in
38 the respective hierarchies after activation.
39 System extension images are strictly read-only, and the host /usr/ and
41 /opt/ hierarchies become read-only too while they are activated.
42 System extensions are supposed to be purely additive, i.e. they are
44 supposed to include only files that do not exist in the underlying
45 basic OS image. However, the underlying mechanism (overlayfs) also
46 allows removing files, but it is recommended not to make use of this.
47 System extension images may be provided in the following formats:
49 1. Plain directories or btrfs subvolumes containing the OS tree
51 2. Disk images with a GPT disk label, following the Discoverable
53 Partitions Specification[1]
54 3. Disk images lacking a partition table, with a naked Linux file
56 system (e.g. squashfs or ext4)
57 These image formats are the same ones that systemd-nspawn(1) supports
59 via it's --directory=/--image= switches and those that the service
60 manager supports via RootDirectory=/RootImage=. Similar to them they
61 may optionally carry Verity authentication information.
62 System extensions are automatically looked for in the directories
64 /etc/extensions/, /run/extensions/, /var/lib/extensions/,
65 /usr/lib/extensions/ and /usr/local/lib/extensions/. The first two
66 listed directories are not suitable for carrying large binary images,
67 however are still useful for carrying symlinks to them. The primary
68 place for installing system extensions is /var/lib/extensions/. Any
69 directories found in these search directories are considered directory
70 based extension images, any files with the .raw suffix are considered
71 disk image based extension images.
72 During boot OS extension images are activated automatically, if the
74 systemd-sysext.service is enabled. Note that this service runs only
75 after the underlying file systems where system extensions may be
76 located have been mounted. This means they are not suitable for
77 shipping resources that are processed by subsystems running in earliest
78 boot. Specifically, OS extension images are not suitable for shipping
79 system services or systemd-sysusers(8) definitions. See Portable
80 Services[2] for a simple mechanism for shipping system services in disk
81 images, in a similar fashion to OS extensions. Note the different
82 isolation on these two mechanisms: while system extension directly
83 extend the underlying OS image with additional files that appear in a
84 way very similar to as if they were shipped in the OS image itself and
85 thus imply no security isolation, portable services imply service level
86 sandboxing in one way or another. The systemd-sysext.service service is
87 guaranteed to finish start-up before basic.target is reached; i.e. at
88 the time regular services initialize (those which do not use
89 DefaultDependencies=no), the files and directories system extensions
90 provide are available in /usr/ and /opt/ and may be accessed.
91 Note that there is no concept of enabling/disabling installed system
93 extension images: all installed extension images are automatically
94 activated at boot.
95 A simple mechanism for version compatibility is enforced: a system
97 extension image must carry a
98 /usr/lib/extension-release.d/extension-release.$name file, which must
99 match its image name, that is compared with the host os-release file:
100 the contained ID= fields have to match, as well as the SYSEXT_LEVEL=
101 field (if defined). If the latter is not defined, the VERSION_ID= field
102 has to match instead. System extensions should not ship a
103 /usr/lib/os-release file (as that would be merged into the host /usr/
104 tree, overriding the host OS version data, which is not desirable). The
105 extension-release file follows the same format and semantics, and
106 carries the same content, as the os-release file of the OS, but it
107 describes the resources carried in the extension image.
108
110 The primary use case for system images are immutable environments where
111 debugging and development tools shall optionally be made available, but
112 not included in the immutable base OS image itself (e.g. strace(1) and
113 gdb(1) shall be an optionally installable addition in order to make
114 debugging/development easier). System extension images should not be
115 misunderstood as a generic software packaging framework, as no
116 dependency scheme is available: system extensions should carry all
117 files they need themselves, except for those already shipped in the
118 underlying host system image. Typically, system extension images are
119 built at the same time as the base OS image — within the same build
120 system.
121 Another use case for the system extension concept is temporarily
123 overriding OS supplied resources with newer ones, for example to
124 install a locally compiled development version of some low-level
125 component over the immutable OS image without doing a full OS rebuild
126 or modifying the nominally immutable image. (e.g. "install" a locally
127 built package with DESTDIR=/var/lib/extensions/mytest make install &&
128 systemd-sysext refresh, making it available in /usr/ as if it was
129 installed in the OS image itself.) This case works regardless if the
130 underlying host /usr/ is managed as immutable disk image or is a
131 traditional package manager controlled (i.e. writable) tree.
132
134 The following commands are understood:
135 status
137 When invoked without any command verb, or when status is specified
138 the current merge status is shown, separately for both /usr/ and
139 /opt/.
140 merge
142 Merges all currently installed system extension images into /usr/
143 and /opt/, by overmounting these hierarchies with an "overlayfs"
144 file system combining the underlying hierarchies with those
145 included in the extension images. This command will fail if the
146 hierarchies are already merged.
147 unmerge
149 Unmerges all currently installed system extension images from /usr/
150 and /opt/, by unmounting the "overlayfs" file systems created by
151 merge prior.
152 refresh
154 A combination of unmerge and merge: if already mounted the existing
155 "overlayfs" instance is unmounted temporarily, and then replaced by
156 a new version. This command is useful after installing/removing
157 system extension images, in order to update the "overlayfs" file
158 system accordingly. If no system extensions are installed when this
159 command is executed, the equivalent of unmerge is executed, without
160 establishing any new "overlayfs" instance. Note that currently
161 there's a brief moment where neither the old nor the new
162 "overlayfs" file system is mounted. This implies that all resources
163 supplied by a system extension will briefly disappear — even if it
164 exists continuously during the refresh operation.
165 list
167 A brief list of installed extension images is shown.
168 -h, --help
170 Print a short help text and exit.
171 --version
173 Print a short version string and exit.
174
176 --root=
177 Operate relative to the specified root directory, i.e. establish
178 the "overlayfs" mount not on the top-level host /usr/ and /opt/
179 hierarchies, but below some specified root directory.
180 --force
182 When merging system extensions into /usr/ and /opt/, ignore version
183 incompatibilities, i.e. force merging regardless of whether the
184 version information included in the extension images matches the
185 host or not.
186 --no-pager
188 Do not pipe output into a pager.
189 --no-legend
191 Do not print the legend, i.e. column headers and the footer with
192 hints.
193 --json=MODE
195 Shows output formatted as JSON. Expects one of "short" (for the
196 shortest possible output without any redundant whitespace or line
197 breaks), "pretty" (for a pretty version of the same, with
198 indentation and line breaks) or "off" (to turn off JSON output, the
199 default).
200
202 On success, 0 is returned.
203
205 systemd(1), systemd-nspawn(1)
206
208 1. Discoverable Partitions Specification
209 https://systemd.io/DISCOVERABLE_PARTITIONS
210 2. Portable Services
212 https://systemd.io/PORTABLE_SERVICES
213systemd 250 SYSTEMD-SYSEXT(8)