systemd/man/systemd-dissect.xml

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<refentry id="systemd-dissect" conditional='HAVE_BLKID'
    xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>systemd-dissect</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-dissect</refentrytitle>
    <manvolnum>1</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-dissect</refname>
    <refname>mount.ddi</refname>
    <refpurpose>Dissect Discoverable Disk Images (DDIs)</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="plain"><replaceable>IMAGE</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--mount</option> <arg choice="plain"><replaceable>IMAGE</replaceable></arg> <arg choice="plain"><replaceable>PATH</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--umount</option> <arg choice="plain"><replaceable>PATH</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--attach</option> <arg choice="plain"><replaceable>IMAGE</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--detach</option> <arg choice="plain"><replaceable>PATH</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--list</option> <arg choice="plain"><replaceable>IMAGE</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--mtree</option> <arg choice="plain"><replaceable>IMAGE</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--with</option> <arg choice="plain"><replaceable>IMAGE</replaceable></arg> <arg choice="opt" rep="repeat"><replaceable>COMMAND</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--copy-from</option> <arg choice="plain"><replaceable>IMAGE</replaceable></arg> <arg choice="plain"><replaceable>PATH</replaceable></arg> <arg choice="opt"><replaceable>TARGET</replaceable></arg></command>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>systemd-dissect <arg choice="opt" rep="repeat">OPTIONS</arg> <option>--copy-to</option> <arg choice="plain"><replaceable>IMAGE</replaceable></arg> <arg choice="opt"><replaceable>SOURCE</replaceable></arg> <arg choice="plain"><replaceable>PATH</replaceable></arg></command>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><command>systemd-dissect</command> is a tool for introspecting and interacting with file system OS
    disk images, specifically Discoverable Disk Images (DDIs). It supports four different operations:</para>

    <orderedlist>
      <listitem><para>Show general OS image information, including the image's
      <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> data,
      machine ID, partition information and more.</para></listitem>

      <listitem><para>Mount an OS image to a local directory. In this mode it will dissect the OS image and
      mount the included partitions according to their designation onto a directory and possibly
      sub-directories.</para></listitem>

      <listitem><para>Unmount an OS image from a local directory. In this mode it will recursively unmount
      the mounted partitions and remove the underlying loop device, including all the partition sub-devices.
      </para></listitem>

      <listitem><para>Copy files and directories in and out of an OS image.</para></listitem>
    </orderedlist>

    <para>The tool may operate on three types of OS images:</para>

    <orderedlist>
      <listitem><para>OS disk images containing a GPT partition table envelope, with partitions marked
      according to the <ulink url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions
      Specification</ulink>.</para></listitem>

      <listitem><para>OS disk images containing just a plain file-system without an enveloping partition
      table. (This file system is assumed to be the root file system of the OS.)</para></listitem>

      <listitem><para>OS disk images containing a GPT or MBR partition table, with a single
      partition only. (This partition is assumed to contain the root file system of the OS.)</para></listitem>
    </orderedlist>

    <para>OS images may use any kind of Linux-supported file systems. In addition they may make use of LUKS
    disk encryption, and contain Verity integrity information. Note that qualifying OS images may be booted
    with <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
    <option>--image=</option> switch, and be used as root file system for system service using the
    <varname>RootImage=</varname> unit file setting, see
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>

    <para>Note that the partition table shown when invoked without command switch (as listed below) does not
    necessarily show all partitions included in the image, but just the partitions that are understood and
    considered part of an OS disk image. Specifically, partitions of unknown types are ignored, as well as
    duplicate partitions (i.e. more than one per partition type), as are root and <filename>/usr/</filename>
    partitions of architectures not compatible with the local system. In other words: this tool will display
    what it operates with when mounting the image. To display the complete list of partitions use a tool such
    as <citerefentry
    project='man-pages'><refentrytitle>fdisk</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>

    <para>The <command>systemd-dissect</command> command may be invoked as <command>mount.ddi</command> in
    which case it implements the <citerefentry
    project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry> "external
    helper" interface. This ensures disk images compatible with <command>systemd-dissect</command> can be
    mounted directly by <command>mount</command> and <citerefentry
    project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>. For
    details see below.</para>
  </refsect1>

  <refsect1>
    <title>Commands</title>

    <para>If neither of the command switches listed below are passed the specified disk image is opened and
    general information about the image and the contained partitions and their use is shown.</para>

    <variablelist>
      <varlistentry>
        <term><option>--mount</option></term>
        <term><option>-m</option></term>

        <listitem><para>Mount the specified OS image to the specified directory. This will dissect the image,
        determine the OS root file system — as well as possibly other partitions — and mount them to the
        specified directory. If the OS image contains multiple partitions marked with the <ulink
        url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink>
        multiple nested mounts are established. This command expects two arguments: a path to an image file
        and a path to a directory where to mount the image.</para>

        <para>To unmount an OS image mounted like this use the <option>--umount</option> operation.</para>

        <para>When the OS image contains LUKS encrypted or Verity integrity protected file systems
        appropriate volumes are automatically set up and marked for automatic disassembly when the image is
        unmounted.</para>

        <para>The OS image may either be specified as path to an OS image stored in a regular file or may
        refer to block device node (in the latter case the block device must be the "whole" device, i.e. not
        a partition device). (The other supported commands described here support this, too.)</para>

        <para>All mounted file systems are checked with the appropriate <citerefentry
        project='man-pages'><refentrytitle>fsck</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        implementation in automatic fixing mode, unless explicitly turned off (<option>--fsck=no</option>) or
        read-only operation is requested (<option>--read-only</option>).</para>

        <para>Note that this functionality is also available in <citerefentry
        project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry> via a
        command such as <command>mount -t ddi myimage.raw targetdir/</command>, as well as in <citerefentry
        project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>. For
        details, see below.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-M</option></term>

        <listitem><para>This is a shortcut for <option>--mount --mkdir</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--umount</option></term>
        <term><option>-u</option></term>

        <listitem><para>Unmount an OS image from the specified directory. This command expects one argument:
        a directory where an OS image was mounted.</para>

        <para>All mounted partitions will be recursively unmounted, and the underlying loop device will be
        removed, along with all its partition sub-devices.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>-U</option></term>

        <listitem><para>This is a shortcut for <option>--umount --rmdir</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--attach</option></term>

        <listitem><para>Attach the specified disk image to an automatically allocated loopback block device,
        and print the path to the loopback block device to standard output. This is similar to an invocation
        of <command>losetup --find --show</command>, but will validate the image as DDI before attaching, and
        derive the correct sector size to use automatically. Moreover, it ensures the per-partition block
        devices are created before returning. Takes a path to a disk image file.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--detach</option></term>

        <listitem><para>Detach the specified disk image from a loopback block device. This undoes the effect
        of <option>--attach</option> above. This expects either a path to a loopback block device as an
        argument, or the path to the backing image file. In the latter case it will automatically determine
        the right device to detach.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--list</option></term>
        <term><option>-l</option></term>

        <listitem><para>Prints the paths of all the files and directories in the specified OS image or
        directory to standard output.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--mtree</option></term>
        <term><option>-l</option></term>

        <listitem><para>Generates a BSD
        <citerefentry project='die-net'><refentrytitle>mtree</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        compatible file manifest of the specified disk image or directory. This is useful for comparing image
        contents in detail, including inode information and other metadata. While the generated manifest will
        contain detailed inode information, it currently excludes extended attributes, file system
        capabilities, MAC labels,
        <citerefentry project='man-pages'><refentrytitle>chattr</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        file flags,
        <citerefentry project='url'><refentrytitle url='https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)'>btrfs</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        subvolume information, and various other file metadata. File content information is shown via a
        SHA256 digest. Additional fields might be added in future. Note that inode information such as link
        counts, inode numbers and timestamps is excluded from the output on purpose, as it typically
        complicates reproducibility.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--with</option></term>

        <listitem><para>Runs the specified command with the specified OS image mounted. This will mount the
        image to a temporary directory, switch the current working directory to it, and invoke the specified
        command line as child process. Once the process ends it will unmount the image again, and remove the
        temporary directory. If no command is specified a shell is invoked. The image is mounted writable,
        use <option>--read-only</option> to switch to read-only operation. The invoked process will have the
        <varname>$SYSTEMD_DISSECT_ROOT</varname> environment variable set, containing the absolute path name
        of the temporary mount point, i.e. the same directory that is set as the current working
        directory.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--copy-from</option></term>
        <term><option>-x</option></term>

        <listitem><para>Copies a file or directory from the specified OS image or directory into the
        specified location on the host file system. Expects three arguments: a path to an image file or
        directory, a source path (relative to the image's root directory) and a destination path (relative to
        the current working directory, or an absolute path, both outside of the image). If the destination
        path is omitted or specified as dash (<literal>-</literal>), the specified file is written to
        standard output. If the source path in the image file system refers to a regular file it is copied to
        the destination path. In this case access mode, extended attributes and timestamps are copied as
        well, but file ownership is not. If the source path in the image refers to a directory, it is copied
        to the destination path, recursively with all containing files and directories. In this case the file
        ownership is copied too.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--copy-to</option></term>
        <term><option>-a</option></term>

        <listitem><para>Copies a file or directory from the specified location in the host file system into
        the specified OS image or directory. Expects three arguments: a path to an image file or directory, a
        source path (relative to the current working directory, or an absolute path, both outside of the
        image) and a destination path (relative to the image's root directory). If the source path is omitted
        or specified as dash (<literal>-</literal>), the data to write is read from standard input. If the
        source path in the host file system refers to a regular file, it is copied to the destination path.
        In this case access mode, extended attributes and timestamps are copied as well, but file ownership
        is not. If the source path in the host file system refers to a directory it is copied to the
        destination path, recursively with all containing files and directories. In this case the file
        ownership is copied too.</para>

        <para>As with <option>--mount</option> file system checks are implicitly run before the copy
        operation begins.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--discover</option></term>

        <listitem><para>Show a list of DDIs in well-known directories. This will show machine, portable
        service and system/configuration extension disk images in the usual directories
        <filename>/usr/lib/machines/</filename>, <filename>/usr/lib/portables/</filename>,
        <filename>/usr/lib/confexts/</filename>, <filename>/var/lib/machines/</filename>,
        <filename>/var/lib/portables/</filename>, <filename>/var/lib/extensions/</filename> and so
        on.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--validate</option></term>

        <listitem><para>Validates the partition arrangement of a disk image (DDI), and ensures it matches the
        image policy specified via <option>--image-policy=</option>, if one is specified. This parses the
        partition table and probes the file systems in the image, but does not attempt to mount them (nor to
        set up disk encryption/authentication via LUKS/Verity). It does this taking the configured image
        dissection policy into account. Since this operation does not mount file systems, this command –
        unlike all other commands implemented by this tool – requires no privileges other than the ability to
        access the specified file. Prints "OK" and returns zero if the image appears to be in order and
        matches the specified image dissection policy. Otherwise prints an error message and returns
        non-zero.</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>

    <para>The following options are understood:</para>

    <variablelist>
      <varlistentry>
        <term><option>--read-only</option></term>
        <term><option>-r</option></term>

        <listitem><para>Operate in read-only mode. By default <option>--mount</option> will establish
        writable mount points. If this option is specified they are established in read-only mode
        instead.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--fsck=no</option></term>

        <listitem><para>Turn off automatic file system checking. By default when an image is accessed for
        writing (by <option>--mount</option> or <option>--copy-to</option>) the file systems contained in the
        OS image are automatically checked using the appropriate <citerefentry
        project='man-pages'><refentrytitle>fsck</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        command, in automatic fixing mode. This behavior may be switched off using
        <option>--fsck=no</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--growfs=no</option></term>

        <listitem><para>Turn off automatic growing of accessed file systems to their partition size, if
        marked for that in the GPT partition table. By default when an image is accessed for writing (by
        <option>--mount</option> or <option>--copy-to</option>) the file systems contained in the OS image
        are automatically grown to their partition sizes, if bit 59 in the GPT partition flags is set for
        partition types that are defined by the <ulink
        url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink>. This
        behavior may be switched off using <option>--growfs=no</option>. File systems are grown automatically
        on access if all of the following conditions are met:</para>
        <orderedlist>
          <listitem><para>The file system is mounted writable</para></listitem>
          <listitem><para>The file system currently is smaller than the partition it is contained in (and thus can be grown)</para></listitem>
          <listitem><para>The image contains a GPT partition table</para></listitem>
          <listitem><para>The file system is stored on a partition defined by the Discoverable Partitions Specification</para></listitem>
          <listitem><para>Bit 59 of the GPT partition flags for this partition is set, as per specification</para></listitem>
          <listitem><para>The <option>--growfs=no</option> option is not passed.</para></listitem>
        </orderedlist>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--mkdir</option></term>

        <listitem><para>If combined with <option>--mount</option> the directory to mount the OS image to is
        created if it is missing. Note that the directory is not automatically removed when the disk image is
        unmounted again.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--rmdir</option></term>

        <listitem><para>If combined with <option>--umount</option> the specified directory where the OS image
        is mounted is removed after unmounting the OS image.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--discard=</option></term>

        <listitem><para>Takes one of <literal>disabled</literal>, <literal>loop</literal>,
        <literal>all</literal>, <literal>crypto</literal>. If <literal>disabled</literal> the image is
        accessed with empty block discarding turned off. If <literal>loop</literal> discarding is enabled if
        operating on a regular file. If <literal>crypt</literal> discarding is enabled even on encrypted file
        systems. If <literal>all</literal> discarding is unconditionally enabled.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--in-memory</option></term>

        <listitem><para>If specified an in-memory copy of the specified disk image is used. This may be used
        to operate with write-access on a (possibly read-only) image, without actually modifying the original
        file. This may also be used in order to operate on a disk image without keeping the originating file
        system busy, in order to allow it to be unmounted.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--root-hash=</option></term>
        <term><option>--root-hash-sig=</option></term>
        <term><option>--verity-data=</option></term>

        <listitem><para>Configure various aspects of Verity data integrity for the OS image. Option
        <option>--root-hash=</option> specifies a hex-encoded top-level Verity hash to use for setting up the
        Verity integrity protection. Option <option>--root-hash-sig=</option> specifies the path to a file
        containing a PKCS#7 signature for the hash. This signature is passed to the kernel during activation,
        which will match it against signature keys available in the kernel keyring.  Option
        <option>--verity-data=</option> specifies a path to a file with the Verity data to use for the OS
        image, in case it is stored in a detached file. It is recommended to embed the Verity data directly
        in the image, using the Verity mechanisms in the <ulink
        url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--loop-ref=</option></term>

        <listitem><para>Configures the "reference" string the kernel shall report as backing file for the
        loopback block device. While this is supposed to be a path or filename referencing the backing file,
        this is not enforced and the kernel accepts arbitrary free-form strings, chosen by the user. Accepts
        arbitrary strings up to a length of 63 characters. This sets the kernel's
        <literal>.lo_file_name</literal> field for the block device. Note this is distinct from the
        <filename>/sys/class/block/loopX/loop/backing_file</filename> attribute file that always reports a
        path referring to the actual backing file. The latter is subject to mount namespace translation, the
        former is not.</para>

        <para>This setting is particularly useful in combination with the <option>--attach</option> command,
        as it allows later referencing the allocated loop device via
        <filename>/dev/disk/by-loop-ref/…</filename> symlinks. Example: first, set up the loopback device
        via <command>systemd-dissect attach --loop-ref=quux foo.raw</command>, and then reference it in a
        command via the specified filename: <command>cfdisk /dev/disk/by-loop-ref/quux</command>.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--mtree-hash=no</option></term>

        <listitem><para>If combined with <option>--mtree</option>, turns off inclusion of file hashes in the
        mtree output. This makes the <option>--mtree</option> faster when operating on large images.
        </para></listitem>
      </varlistentry>

      <xi:include href="standard-options.xml" xpointer="image-policy-open" />
      <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, a non-zero failure code otherwise. If the <option>--with</option>
    command is used the exit status of the invoked command is propagated.</para>
  </refsect1>

  <refsect1>
    <title>Invocation as <command>/sbin/mount.ddi</command></title>

    <para>The <command>systemd-dissect</command> executable may be symlinked to
    <filename>/sbin/mount.ddi</filename>. If invoked through that it implements <citerefentry
    project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
    "external helper" interface for the (pseudo) file system type <literal>ddi</literal>. This means
    conformant disk images may be mounted directly via</para>

    <programlisting># mount -t ddi myimage.raw targetdir/</programlisting>

    <para>in a fashion mostly equivalent to:</para>

    <programlisting># systemd-dissect --mount myimage.raw targetdir/</programlisting>

    <para>Note that since a single DDI may contain multiple file systems it should later be unmounted with
    <command>umount -R targetdir/</command>, for recursive operation.</para>

    <para>This functionality is particularly useful to mount DDIs automatically at boot via simple
    <filename>/etc/fstab</filename> entries. For example:</para>

    <programlisting>/path/to/myimage.raw /images/myimage/ ddi defaults 0 0</programlisting>

    <para>When invoked this way the mount options <literal>ro</literal>, <literal>rw</literal>,
    <literal>discard</literal>, <literal>nodiscard</literal> map to the corresponding options listed above
    (i.e. <option>--read-only</option>, <option>--discard=all</option>,
    <option>--discard=disabled</option>). Mount options are <emphasis>not</emphasis> generically passed on to
    the file systems inside the images.</para>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Generate a tarball from an OS disk image</title>

      <programlisting># systemd-dissect --with foo.raw tar cz . >foo.tar.gz</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <ulink url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink>,
      <citerefentry project='man-pages'><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>umount</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>fdisk</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>