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WinBtrfs v1.9

WinBtrfs is a Windows driver for the next-generation Linux filesystem Btrfs. A reimplementation from scratch, it contains no code from the Linux kernel, and should work on any version from Windows XP onwards. It is also included as part of the free operating system ReactOS.

If your Btrfs filesystem is on a MD software RAID device created by Linux, you will also need WinMD to get this to appear under Windows.

See also Quibble, an experimental bootloader allowing Windows to boot from Btrfs, and Ntfs2btrfs, a tool which allows in-place conversion of NTFS filesystems.

First, a disclaimer:

You use this software at your own risk. I take no responsibility for any damage it may do to your filesystem. It ought to be suitable for day-to-day use, but make sure you take backups anyway.

Everything here is released under the GNU Lesser General Public Licence (LGPL); see the file LICENCE for more info. You are encouraged to play about with the source code as you will, and I'd appreciate a note (mark@harmstone.com) if you come up with anything nifty.

See at the end of this document for copyright details of third-party code that's included here.

Features

Todo

Installation

To install the driver, download and extract the latest release, right-click btrfs.inf, and choose Install. The driver is signed, so should work out of the box on modern versions of Windows.

If you using Windows 10 and have Secure Boot enabled, you may have to make a Registry change in order for the driver to be loaded - see below.

WinBtrfs is also available on the following package managers:

choco install winbtrfs

scoop bucket add nonportable
scoop install winbtrfs-np -g

Uninstalling

If you want to uninstall, from a command prompt run:

RUNDLL32.EXE SETUPAPI.DLL,InstallHinfSection DefaultUninstall 132 btrfs.inf

You may need to give the full path to btrfs.inf.

You can also go to Device Manager, find "Btrfs controller" under "Storage volumes", right click and choose "Uninstall". Tick the checkbox to uninstall the driver as well, and let Windows reboot itself.

If you need to uninstall via the registry, open regedit and set the value of HKLM\SYSTEM\CurrentControlSet\services\btrfs\Start to 4, to disable the service. After you reboot, you can then delete the btrfs key and remove C:\Windows\System32\drivers\btrfs.sys.

Compilation

To compile with Visual C++ 2019, open the directory and let CMake do its thing. If you have the Windows DDK installed correctly, it should just work.

To compile with GCC on Linux, you will need a cross-compiler set up, for either i686-w64-mingw32 or x86_64-w64-mingw32. Create a build directory, then use either mingw-x86.cmake or mingw-amd64.cmake as CMake toolchain files to generate your Makefile.

Mappings

The user mappings are stored in the registry key HKLM\SYSTEM\CurrentControlSet\services\btrfs\Mappings. Create a DWORD with the name of your Windows SID (e.g. S-1-5-21-1379886684-2432464051-424789967-1001), and the value of your Linux uid (e.g. 1000). It will take effect next time the driver is loaded.

You can find your current SID by running wmic useraccount get name,sid.

Similarly, the group mappings are stored in under GroupMappings. The default entry maps Windows' Users group to gid 100, which is usually "users" on Linux. You can also specify user SIDs here to force files created by a user to belong to a certain group. The setgid flag also works as on Linux.

Note that processes running under User Access Control tokens create files as the BUILTIN\Administrators SID (S-1-5-32-544), rather as a user account.

LXSS ("Ubuntu on Windows" / "Windows Subsystem for Linux")

The driver will passthrough Linux metadata to recent versions of LXSS, but you will have to let Windows know that you wish to do this. From a Bash prompt on Windows, edit /etc/wsl.conf to look like the following:

[automount]
enabled = true
options = "metadata"
mountFsTab = false

It will then take effect next time you reboot. Yes, you should be able to chroot into an actual Linux installation, if you wish.

Commands

The DLL file shellbtrfs.dll provides the GUI interface, but it can also be used with rundll32.exe to carry out some tasks from the command line, which may be useful if you wish to schedule something to run periodically.

Bear in mind that rundll32 provides no mechanism to return any error codes, so any of these commands may fail silently.

The following commands need various privileges, and so must be run as Administrator to work:

Troubleshooting

On the releases page, there's zip files to download containing the PDBs. Or you can try the symbols server http://symbols.burntcomma.com/ - in windbg, set your symbol path to something like this:

symsrv*symsrv.dll*C:\symbols*http://msdl.microsoft.com/download/symbols;symsrv*symsrv.dll*C:\symbols*http://symbols.burntcomma.com

The driver assumes that all filenames are encoded in UTF-8. This should be the default on most setups nowadays - if you're not using UTF-8, it's probably worth looking into converting your files.

For the very latest versions of Windows 10, Microsoft introduced more onerous requirements for signing, which seemingly aren't available for open-source drivers.

To work around this, go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CI\Policy in Regedit, create a new DWORD value called UpgradedSystem and set to 1, and reboot.

Or you could always just turn off Secure Boot in your BIOS settings.

This is something Microsoft hardcoded into LXSS, presumably to stop people hosing their systems by running mkdir /mnt/c/WiNdOwS.

With the shell extension installed, right-click the drive in Explorer, click Properties, and go to the Btrfs tab. There should be a button which allows you to change the drive letter.

In Regedit, try deleting the relevant entries in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices, then rebooting.

Use the included command line program mkbtrfs.exe. We can't add Btrfs to Windows' own dialog box, unfortunately, as its list of filesystems has been hardcoded. You can also run format /fs:btrfs, if you don't need to set any Btrfs-specific options.

If Windows' Format dialog box refuses to appear, try running format.com with the /fs flag, e.g. format /fs:ntfs D:.

Synology seems to use LVM for its block devices. Until somebody writes an LVM driver for Windows, you're out of luck.

Thecus uses Linux's MD raid for its block devices. You will need to install WinMD as well.

Make sure that you have KB3033929 installed. Or consider installing from an "escrow" ISO which includes all updates.

Paragon's filesystem-reading software is known to disable automount. Disable or uninstall Paragon, then re-enable automount by running diskpart and typing automount enable.

On very old versions of Windows (XP, Server 2003?), Windows ignores Linux partitions entirely. If this is the case for you, try running fdisk on Linux and changing your partition type from 83 to 7.

Changelog

v1.9 (2024-03-15):

v1.8.2 (2023-01-10):

v1.8.1 (2022-08-23):

v1.8 (2022-03-12):

v1.7.9 (2021-10-02):

v1.7.8.1 (2021-06-13):

v1.7.8 (2021-06-09):

v1.7.7 (2021-04-12):

v1.7.6 (2021-01-14):

v1.7.5 (2020-10-31):

v1.7.4 (2020-08-23):

v1.7.3 (2020-05-24):

v1.7.2 (2020-04-10):

v1.7.1 (2020-03-02):

v1.7 (2020-02-26):

v1.6 (2020-02-04):

v1.5 (2019-11-10):

v1.4 (2019-08-31):

v1.3 (2019-06-10):

v1.2.1 (2019-05-06):

v1.2 (2019-05-05):

v1.1 (2018-12-15):

v1.0.2 (2018-05-19):

v1.0.1 (2017-10-15):

v1.0 (2017-09-04):

v0.10 (2017-05-02):

v0.9 (2017-03-05):

v0.8 (2016-12-30):

v0.7 (2016-10-24):

v0.6 (2016-08-21):

v0.5 (2016-07-24):

v0.4 (2016-05-02):

v0.3 (2016-03-25):

v0.2 (2016-03-13):

v0.1 (2016-02-21):

Debug log

WinBtrfs has three levels of debug messages: errors and FIXMEs, warnings, and traces. The release version of the driver only displays the errors and FIXMEs, which it logs via DbgPrint. You can view these messages via the Microsoft program DebugView, available at https://technet.microsoft.com/en-gb/sysinternals/debugview.

If you want to report a problem, it'd be of great help if you could also attach a full debug log. To do this, you will need to use the debug versions of the drivers; copy the files in Debug\x64 or Debug\x86 into x64 or x86. You will also need to set the registry entries in HKLM\SYSTEM\CurrentControlSet\Services\btrfs:

Mount options

The driver will create subkeys in the registry under HKLM\SYSTEM\CurrentControlSet\Services\btrfs for each mounted filesystem, named after its UUID. If you're unsure which UUID refers to which volume, you can check using btrfs fi show on Linux. You can add per-volume mount options to this subkey, which will take effect on reboot. If a value is set in the key above this, it will use this by default.

Contact

I'd appreciate any feedback you might have, positive or negative: mark@harmstone.com.

Copyright

This code contains portions of the following software:

Zlib

Copyright (C) 1995-2017 Jean-loup Gailly and Mark Adler

This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.

LZO

WinBtrfs contains portions of an early version of lzo, which is copyright 1996 Markus Oberhumer. Modern versions are licensed under the GPL, but this was licensed under the LGPL, so I believe it is okay to use.

Zstd

Copyright (c) 2016-present, Facebook, Inc. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

BLAKE2

https://github.com/BLAKE2/BLAKE2 (public domain)

SHA256

https://github.com/amosnier/sha-2 (public domain)