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guestfs-release-notes-1.26(1) Virtualization Support guestfs-release-notes-1.26(1)


guestfs-release-notes - libguestfs Release Notes


These release notes only cover the differences from the previous stable/dev branch split (1.24.0). For detailed changelogs, please see the git repository, or the ChangeLog file distributed in the tarball.

New features


virt-customize(1) is a new tool for customizing virtual machine disk images. It lets you install packages, edit configuration files, run scripts, set passwords and so on. virt-builder(1) and virt-sysprep(1) use virt-customize, and command line options across all these tools are now identical.

virt-diff(1) is a new tool for showing the differences between the filesystems of two virtual machines. It is mainly useful when showing what files have been changed between snapshots.

virt-builder(1) has been greatly enhanced. There are many more ways to customize the virtual machine. It can pull templates from multiple repositories. A parallelized internal xzcat implementation speeds up template decompression. Virt-builder uses an optimizing planner to choose the fastest way to build the VM. It is now easier to use virt-builder from other programs. Internationalization support has been added to metadata. More efficient SELinux relabelling of files. Can build guests for multiple architectures. Error messages have been improved. (Pino Toscano)

virt-sparsify(1) has a new --in-place option. This sparsifies an image in place (without copying it) and is also much faster. (Lots of help provided by Paolo Bonzini)

virt-sysprep(1) can delete and scrub files under user control. You can lock user accounts or set random passwords on accounts. Can remove more log files. Can unsubscribe a guest from Red Hat Subscription Manager. New flexible way to enable and disable operations. (Wanlong Gao, Pino Toscano)

virt-win-reg(1) allows you to use URIs to specify remote disk images.

virt-format(1) can now pass the extra space that it recovers back to the host.

guestfish(1) has additional environment variables to give fine control over the "><fs>" prompt. Guestfish reads its (rarely used) configuration file in a different order now so that local settings override global settings. (Pino Toscano)

virt-make-fs(1) was rewritten in C, but is unchanged in terms of functionality and command line usage.

Language bindings

The OCaml bindings have a new "Guestfs.Errno" module, used to check the error number returned by "Guestfs.last_errno".

PHP tests now work. (Pino Toscano)


Inspection can recognize Debian live images.


ARMv7 (32 bit) now supports KVM acceleration.

Aarch64 (ARM 64 bit) is supported, but the appliance part does not work yet.

PPC64 support has been fixed and enhanced.


Denial of service when inspecting disk images with corrupt btrfs volumes
It was possible to crash libguestfs (and programs that use libguestfs as a library) by presenting a disk image containing a corrupt btrfs volume.

This was caused by a NULL pointer dereference causing a denial of service, and is not thought to be exploitable any further.

See commit d70ceb4cbea165c960710576efac5a5716055486 for the fix. This fix is included in libguestfs stable branches ≥ 1.26.0, ≥ 1.24.6 and ≥ 1.22.8, and also in RHEL ≥ 7.0. Earlier versions of libguestfs are not vulnerable.

Better generation of random root passwords and random seeds
When generating random root passwords and random seeds, two bugs were fixed which are possibly security related. Firstly we no longer read excessive bytes from /dev/urandom (most of which were just thrown away). Secondly we changed the code to avoid modulo bias. These issues were not thought to be exploitable. (Both changes suggested by Edwin Török)


GUID parameters are now validated when they are passed to API calls, whereas previously you could have passed any string. (Pino Toscano)

New APIs

"guestfs_add_drive_opts": new "discard" parameter
The new "discard" parameter allows fine-grained control over discard/trim support for a particular disk. This allows the host file to become more sparse (or thin-provisioned) when you delete files or issue the "guestfs_fstrim" API call.
"guestfs_add_domain": new parameters: "cachemode", "discard"
These parameters are passed through when adding the domain's disks.
Discard all blocks on a guestfs device. Combined with the "discard" parameter above, this makes the host file sparse.
Test if discarded blocks read back as zeroes.
For each struct returned through the API, libguestfs now generates "guestfs_compare_*" and "guestfs_copy_*" functions to allow you to compare and copy structs.
Copy attributes (like permissions, xattrs, ownership) from one file to another. (Pino Toscano)
A flexible API for creating empty disk images from scratch. This avoids the need to call out to external programs like qemu-img(1).
Per-backend settings (can also be set via the environment variable "LIBGUESTFS_BACKEND_SETTINGS"). The main use for this is forcing TCG mode in the qemu-based backends, for example:

Get the label or name of a partition (for GPT disk images).

Build changes

The following extra packages are required to build libguestfs 1.26:
supermin ≥ 5
Supermin version 5 is required to build this version of libguestfs.
flex, bison
Virt-builder now uses a real parser to parse its metadata file, so these tools are required.
This is now a required build dependency, where previously it was (in theory) optional.


PO message extraction rewritten to be more robust. (Pino Toscano)

"podwrapper" gives an error if the --insert or --verbatim argument pattern is not found.

Libguestfs now passes the qemu -enable-fips option to enable FIPS, if qemu supports it.

"./configure --without-qemu" can be used if you don't want to specify a default hypervisor.

Copy-on-write [COW] overlays, used for example for read-only drives, are now created through an internal backend API (".create_cow_overlay").

Libvirt backend uses some funky C macros to generate XML. These are simpler and safer.

The ChangeLog file format has changed. It is now just the same as "git log", instead of using a custom format.

Appliance start-up has changed:

  • The libguestfs appliance now initializes LVM the same way as it is done on physical machines.
  • The libguestfs appliance does not write an empty string to /proc/sys/kernel/hotplug when starting up.

    Note that you must configure your kernel to have "CONFIG_UEVENT_HELPER_PATH=""" otherwise you will get strange LVM errors (this applies as much to any Linux machine, not just libguestfs). (Peter Rajnoha)

Libguestfs can now be built on arches that have ocamlc(1) but not ocamlopt(1). (Hilko Bengen, Olaf Hering)

You cannot use "./configure --disable-daemon --enable-appliance". It made no sense anyway. Now it is expressly forbidden by the configure script.

The packagelist file uses "m4" for macro expansion instead of "cpp".

Bugs fixed
java bindings inspect_list_applications2 throws java.lang.ArrayIndexOutOfBoundsException:
[RFE] enable subscription manager clean or unregister operation to sysprep
virt-resize does not preserve GPT partition names
mount-local should give a clearer error if root is not mounted
virt-sparsify overwrites block devices if used as output files
libguestfs: error: invalid backend: appliance
guestfs_pvs prints "unknown device" if a physical volume is missing
Recommended default clock/timer settings
ruby-libguestfs throws "expecting 0 or 1 arguments" on
Cannot inspect cirros 0.3.1 disk image fully
LIBVIRT_DEFAULT_URI=qemu:///system breaks libguestfs
virt-builder network (eg. --install) doesn't work if resolv.conf sets nameserver
When SSSD is installed, libvirt configuration requires authentication, but not clear to user
virt-make-fs fails making fat/vfat whole disk: Device partition expected, not making filesystem on entire device '/dev/sda' (use -I to override)
virt-sysprep to delete more logfiles
RFE: libguestfs inspection does not recognize Free4NAS live CD
RFE: virt-sysprep/virt-builder should have an option to lock a user account
libguestfs fails examining libvirt guest with ceph drives: rbd: image name must begin with a '/'
virt-builder fails if $HOME/.cache doesn't exist
libguestfs: do not use versioned jar file
All libguestfs LVM operations fail on Debian/Ubuntu
Need update helpout of part-set-gpt-type
virt-sysprep does not correctly set the hostname on Debian/Ubuntu
guestfish prints literal "\n" in error messages
guestmount: "touch" command fails: touch: setting times of `timestamp': Invalid argument
[RFE] function to get partition name
list-devices returns devices of different types out of order


guestfs-examples(1), guestfs-faq(1), guestfs-performance(1), guestfs-recipes(1), guestfs-testing(1), guestfs(3), guestfish(1),


Richard W.M. Jones


Copyright (C) 2009-2020 Red Hat Inc.


This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.


To get a list of bugs against libguestfs, use this link:

To report a new bug against libguestfs, use this link:

When reporting a bug, please supply:

  • The version of libguestfs.
  • Where you got libguestfs (eg. which Linux distro, compiled from source, etc)
  • Describe the bug accurately and give a way to reproduce it.
  • Run libguestfs-test-tool(1) and paste the complete, unedited output into the bug report.
2021-03-31 libguestfs-1.44.1