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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd-sysext"
+ xmlns:xi="http://www.w3.org/2001/XInclude">
+
+ <refentryinfo>
+ <title>systemd-sysext</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>systemd-sysext</refentrytitle>
+ <manvolnum>8</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>systemd-sysext</refname>
+ <refname>systemd-sysext.service</refname>
+ <refpurpose>Activates System Extension Images</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <cmdsynopsis>
+ <command>systemd-sysext</command>
+ <arg choice="opt" rep="repeat">OPTIONS</arg>
+ <arg choice="plain">COMMAND</arg>
+ </cmdsynopsis>
+
+ <para><literallayout><filename>systemd-sysext.service</filename></literallayout></para>
+
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para><command>systemd-sysext</command> activates/deactivates system extension images. System extension
+ images may – dynamically at runtime — extend the <filename>/usr/</filename> and
+ <filename>/opt/</filename> directory hierarchies with additional files. This is particularly useful on
+ immutable system images where a <filename>/usr/</filename> and/or <filename>/opt/</filename> hierarchy
+ residing on a read-only file system shall be extended temporarily at runtime without making any
+ persistent modifications.</para>
+
+ <para>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
+ <filename>/usr/</filename> and <filename>/opt/</filename> hierarchies are combined via
+ <literal>overlayfs</literal> with the same hierarchies of the host OS, and the host
+ <filename>/usr/</filename> and <filename>/opt/</filename> 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
+ <filename>/usr/</filename> and <filename>/opt/</filename> 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.</para>
+
+ <para>Files and directories contained in the extension images outside of the <filename>/usr/</filename>
+ and <filename>/opt/</filename> hierarchies are <emphasis>not</emphasis> merged, and hence have no effect
+ when included in a system extension image. In particular, files in the <filename>/etc/</filename> and
+ <filename>/var/</filename> included in a system extension image will <emphasis>not</emphasis> appear in
+ the respective hierarchies after activation.</para>
+
+ <para>System extension images are strictly read-only, and the host <filename>/usr/</filename> and
+ <filename>/opt/</filename> hierarchies become read-only too while they are activated.</para>
+
+ <para>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.</para>
+
+ <para>System extension images may be provided in the following formats:</para>
+
+ <orderedlist>
+ <listitem><para>Plain directories or btrfs subvolumes containing the OS tree</para></listitem>
+ <listitem><para>Disk images with a GPT disk label, following the <ulink
+ url="https://systemd.io/DISCOVERABLE_PARTITIONS">Discoverable Partitions Specification</ulink></para></listitem>
+ <listitem><para>Disk images lacking a partition table, with a naked Linux file system (e.g. squashfs or ext4)</para></listitem>
+ </orderedlist>
+
+ <para>These image formats are the same ones that
+ <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ supports via its <option>--directory=</option>/<option>--image=</option> switches and those that the
+ service manager supports via <option>RootDirectory=</option>/<option>RootImage=</option>. Similar to
+ them they may optionally carry Verity authentication information.</para>
+
+ <para>System extensions are automatically looked for in the directories
+ <filename>/etc/extensions/</filename>, <filename>/run/extensions/</filename>,
+ <filename>/var/lib/extensions/</filename>, <filename>/usr/lib/extensions/</filename> and
+ <filename>/usr/local/lib/extensions/</filename>. 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 <filename>/var/lib/extensions/</filename>. Any directories found in
+ these search directories are considered directory based extension images, any files with the
+ <filename>.raw</filename> suffix are considered disk image based extension images.</para>
+
+ <para>During boot OS extension images are activated automatically, if the
+ <filename>systemd-sysext.service</filename> 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
+ <citerefentry><refentrytitle>systemd-sysusers</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ definitions. See <ulink url="https://systemd.io/PORTABLE_SERVICES">Portable Services</ulink> 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 <filename>systemd-sysext.service</filename> service is guaranteed to finish start-up
+ before <filename>basic.target</filename> is reached; i.e. at the time regular services initialize (those
+ which do not use <varname>DefaultDependencies=no</varname>), the files and directories system extensions
+ provide are available in <filename>/usr/</filename> and <filename>/opt/</filename> and may be
+ accessed.</para>
+
+ <para>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 <filename>.raw</filename>) in <filename>/etc/extensions/</filename> to "mask"
+ an extension with the same name in a system folder with lower precedence.</para>
+
+ <para>A simple mechanism for version compatibility is enforced: a system extension image must carry a
+ <filename>/usr/lib/extension-release.d/extension-release.<replaceable>$name</replaceable></filename>
+ file, which must match its image name, that is compared with the host <filename>os-release</filename>
+ file: the contained <varname>ID=</varname> fields have to match unless <literal>_any</literal> is set
+ for the extension. If the extension <varname>ID=</varname> is not <literal>_any</literal>, the
+ <varname>SYSEXT_LEVEL=</varname> field (if defined) has to match. If the latter is not defined, the
+ <varname>VERSION_ID=</varname> field has to match instead. If the extension defines the
+ <varname>ARCHITECTURE=</varname> field and the value is not <literal>_any</literal> it has to match the kernel's
+ architecture reported by <citerefentry><refentrytitle>uname</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+ but the used architecture identifiers are the same as for <varname>ConditionArchitecture=</varname>
+ described in <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+ System extensions should not ship a <filename>/usr/lib/os-release</filename> file (as that would be merged
+ into the host <filename>/usr/</filename> tree, overriding the host OS version data, which is not desirable).
+ The <filename>extension-release</filename> file follows the same format and semantics, and carries the same
+ content, as the <filename>os-release</filename> file of the OS, but it describes the resources carried
+ in the extension image.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>Uses</title>
+
+ <para>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.
+ <citerefentry project='man-pages'><refentrytitle>strace</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ and
+ <citerefentry project='man-pages'><refentrytitle>gdb</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ 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.</para>
+
+ <para>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 <command>DESTDIR=/var/lib/extensions/mytest
+ make install &amp;&amp; systemd-sysext refresh</command>, making it available in
+ <filename>/usr/</filename> as if it was installed in the OS image itself.) This case works regardless if
+ the underlying host <filename>/usr/</filename> is managed as immutable disk image or is a traditional
+ package manager controlled (i.e. writable) tree.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>Commands</title>
+
+ <para>The following commands are understood:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><option>status</option></term>
+
+ <listitem><para>When invoked without any command verb, or when <option>status</option> is specified
+ the current merge status is shown, separately for both <filename>/usr/</filename> and
+ <filename>/opt/</filename>.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>merge</option></term>
+ <listitem><para>Merges all currently installed system extension images into
+ <filename>/usr/</filename> and <filename>/opt/</filename>, by overmounting these hierarchies with an
+ <literal>overlayfs</literal> file system combining the underlying hierarchies with those included in
+ the extension images. This command will fail if the hierarchies are already merged.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>unmerge</option></term>
+ <listitem><para>Unmerges all currently installed system extension images from
+ <filename>/usr/</filename> and <filename>/opt/</filename>, by unmounting the
+ <literal>overlayfs</literal> file systems created by <option>merge</option>
+ prior.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>refresh</option></term>
+ <listitem><para>A combination of <option>unmerge</option> and <option>merge</option>: if already
+ mounted the existing <literal>overlayfs</literal> 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 <literal>overlayfs</literal> file system accordingly. If no system extensions
+ are installed when this command is executed, the equivalent of <option>unmerge</option> is
+ executed, without establishing any new <literal>overlayfs</literal> instance. Note that currently
+ there's a brief moment where neither the old nor the new <literal>overlayfs</literal> 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.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>list</option></term>
+
+ <listitem><para>A brief list of installed extension images is shown.</para></listitem>
+ </varlistentry>
+
+ <xi:include href="standard-options.xml" xpointer="help" />
+ <xi:include href="standard-options.xml" xpointer="version" />
+ </variablelist>
+ </refsect1>
+
+ <refsect1>
+ <title>Options</title>
+
+ <variablelist>
+ <varlistentry>
+ <term><option>--root=</option></term>
+
+ <listitem><para>Operate relative to the specified root directory, i.e. establish the
+ <literal>overlayfs</literal> mount not on the top-level host <filename>/usr/</filename> and
+ <filename>/opt/</filename> hierarchies, but below some specified root directory.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><option>--force</option></term>
+
+ <listitem><para>When merging system extensions into <filename>/usr/</filename> and
+ <filename>/opt/</filename>, ignore version incompatibilities, i.e. force merging regardless of
+ whether the version information included in the extension images matches the host or
+ not.</para></listitem>
+ </varlistentry>
+
+ <xi:include href="standard-options.xml" xpointer="no-pager" />
+ <xi:include href="standard-options.xml" xpointer="no-legend" />
+ <xi:include href="standard-options.xml" xpointer="json" />
+ </variablelist>
+ </refsect1>
+
+ <refsect1>
+ <title>Exit status</title>
+
+ <para>On success, 0 is returned.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>