NAME¶
systemd-dissect, mount.ddi - Dissect Discoverable Disk Images
(DDIs)
SYNOPSIS¶
systemd-dissect [OPTIONS...]
IMAGE
systemd-dissect [OPTIONS...]
--mount IMAGE
PATH
systemd-dissect [OPTIONS...]
--umount PATH
systemd-dissect [OPTIONS...]
--attach IMAGE
systemd-dissect [OPTIONS...]
--detach PATH
systemd-dissect [OPTIONS...]
--list IMAGE
systemd-dissect [OPTIONS...]
--mtree IMAGE
systemd-dissect [OPTIONS...]
--with IMAGE
[COMMAND...]
systemd-dissect [OPTIONS...]
--copy-from IMAGE
PATH [TARGET]
systemd-dissect [OPTIONS...]
--copy-to IMAGE
[SOURCE] PATH
systemd-dissect [OPTIONS...]
--discover
systemd-dissect [OPTIONS...]
--validate IMAGE
DESCRIPTION¶
systemd-dissect is a tool for introspecting and interacting
with file system OS disk images, specifically Discoverable Disk Images
(DDIs). It supports four different operations:
1.Show general OS image information, including the
image's
os-release(5) data, machine ID, partition information and
more.
2.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.
3.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.
4.Copy files and directories in and out of an OS
image.
The tool may operate on three types of OS images:
1.OS disk images containing a GPT partition table
envelope, with partitions marked according to the Discoverable Partitions
Specification[1].
2.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.)
3.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.)
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
systemd-nspawn(1)'s --image= switch, and be used as root file
system for system service using the RootImage= unit file setting, see
systemd.exec(5).
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 /usr/ 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 fdisk(8).
The systemd-dissect command may be invoked as
mount.ddi in which case it implements the mount(8)
"external helper" interface. This ensures disk images compatible
with systemd-dissect can be mounted directly by mount and
fstab(5). For details see below.
COMMANDS¶
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.
--mount, -m
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
Discoverable Partitions Specification[1] 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.
To unmount an OS image mounted like this use the --umount
operation.
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.
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.)
All mounted file systems are checked with the appropriate
fsck(8) implementation in automatic fixing mode, unless explicitly
turned off (--fsck=no) or read-only operation is requested
(--read-only).
Note that this functionality is also available in mount(8)
via a command such as mount -t ddi myimage.raw targetdir/, as well as
in fstab(5). For details, see below.
-M
This is a shortcut for --mount --mkdir.
--umount, -u
Unmount an OS image from the specified directory. This
command expects one argument: a directory where an OS image was mounted.
All mounted partitions will be recursively unmounted, and the
underlying loop device will be removed, along with all its partition
sub-devices.
-U
This is a shortcut for --umount --rmdir.
--attach
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 losetup
--find --show, 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.
--detach
Detach the specified disk image from a loopback block
device. This undoes the effect of --attach 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.
--list, -l
Prints the paths of all the files and directories in the
specified OS image or directory to standard output.
--mtree
Generates a BSD
mtree(8) 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,
chattr(1) file flags,
btrfs(5) 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.
--with
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 --read-only to switch to read-only
operation. The invoked process will have the $SYSTEMD_DISSECT_ROOT
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.
--copy-from, -x
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 ("-"), 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.
--copy-to, -a
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 ("-"), 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.
As with --mount file system checks are implicitly run
before the copy operation begins.
--discover
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 /usr/lib/machines/, /usr/lib/portables/,
/usr/lib/confexts/, /var/lib/machines/, /var/lib/portables/,
/var/lib/extensions/ and so on.
--validate
Validates the partition arrangement of a disk image
(DDI), and ensures it matches the image policy specified via
--image-policy=, 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.
-h, --help
Print a short help text and exit.
--version
Print a short version string and exit.
OPTIONS¶
The following options are understood:
--read-only, -r
Operate in read-only mode. By default --mount will
establish writable mount points. If this option is specified they are
established in read-only mode instead.
--fsck=no
Turn off automatic file system checking. By default when
an image is accessed for writing (by --mount or --copy-to) the
file systems contained in the OS image are automatically checked using the
appropriate fsck(8) command, in automatic fixing mode. This behavior
may be switched off using --fsck=no.
--growfs=no
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
--mount or
--copy-to) 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
Discoverable Partitions
Specification[1]. This behavior may be switched off using
--growfs=no. File systems are grown automatically on access if all of
the following conditions are met:
1.The file system is mounted writable
2.The file system currently is smaller than the
partition it is contained in (and thus can be grown)
3.The image contains a GPT partition table
4.The file system is stored on a partition defined by
the Discoverable Partitions Specification
5.Bit 59 of the GPT partition flags for this partition
is set, as per specification
6.The --growfs=no option is not passed.
--mkdir
If combined with --mount 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.
--rmdir
If combined with --umount the specified directory
where the OS image is mounted is removed after unmounting the OS image.
--discard=
Takes one of "disabled", "loop",
"all", "crypto". If "disabled" the image is
accessed with empty block discarding turned off. If "loop"
discarding is enabled if operating on a regular file. If "crypt"
discarding is enabled even on encrypted file systems. If "all"
discarding is unconditionally enabled.
--in-memory
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.
--root-hash=, --root-hash-sig=,
--verity-data=
Configure various aspects of Verity data integrity for
the OS image. Option --root-hash= specifies a hex-encoded top-level
Verity hash to use for setting up the Verity integrity protection. Option
--root-hash-sig= 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 --verity-data= 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 Discoverable Partitions
Specification[1].
--loop-ref=
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 ".lo_file_name" field for the block device. Note this is
distinct from the /sys/class/block/loopX/loop/backing_file 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.
--mtree-hash=no
If combined with --mtree, turns off inclusion of
file hashes in the mtree output. This makes the --mtree faster when
operating on large images.
--image-policy=policy
Takes an image policy string as argument, as per
systemd.image-policy(7). The policy is enforced when operating on the
disk image specified via
--image=, see above. If not specified defaults
to the "*" policy, i.e. all recognized file systems in the image are
used.
--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, a non-zero failure code otherwise. If
the --with command is used the exit status of the invoked command is
propagated.
INVOCATION AS /SBIN/MOUNT.DDI¶
The systemd-dissect executable may be symlinked to
/sbin/mount.ddi. If invoked through that it implements mount(8)'s
"external helper" interface for the (pseudo) file system type
"ddi". This means conformant disk images may be mounted directly
via
# mount -t ddi myimage.raw targetdir/
in a fashion mostly equivalent to:
# systemd-dissect --mount myimage.raw targetdir/
Note that since a single DDI may contain multiple file systems it
should later be unmounted with umount -R targetdir/, for recursive
operation.
This functionality is particularly useful to mount DDIs
automatically at boot via simple /etc/fstab entries. For example:
/path/to/myimage.raw /images/myimage/ ddi defaults 0 0
When invoked this way the mount options "ro",
"rw", "discard", "nodiscard" map to the
corresponding options listed above (i.e. --read-only,
--discard=all, --discard=disabled). Mount options are
not generically passed on to the file systems inside the images.
EXAMPLES¶
Example 1. Generate a tarball from an OS disk
image
# systemd-dissect --with foo.raw tar cz . >foo.tar.gz
NOTES¶
- 1.
- Discoverable Partitions Specification
