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fuse: VUOS virtual /dev/fuse

Using FUSE it is possible for users to run file systems implementation in user space. Using the device /dev/fuse a kernel module acts as a bridge to the VFS interface.

fuse virtualizes the Linux kernel support and provides the same protocol on /dev/fuse. Any FUSE filesystem implementation program runs as-is on vuos using this module. There are notable differences:

  • VUOS fuse module does not require kernel code,
  • FUSE mounted file systems are visible for all the processes while vufuse mounted file systems can be seen only by the processes running under the control of the umvu hypervisor.

Note: the name of this module is fuse. In the source code its name is vudevfuse.

vuos provides two modules to support filesystems in userspace: fuse and vufuse.

  • fuse (described in this page) is compatible with the file system implementations developed for Linux FUSE.
  • vufuse (described here) requires the generation of a specific plugin (a shared library), it is faster.

fuse-ext2: ext2/3/4 filesystems

This example uses fuse-ext2, an implementation of ext2/3/4 filesystems for FUSE.

The following shell session performs these steps:

  • create an empty file named img.ext2
  • configure the file as an ext2 file system
  • mount the file system on /mnt
  • a list of /mnt shows that there is only the lost+found directory.
  • create the file /mnt/greetings in the ext2 file system,
  • unmount and remount the partition
  • show the contents of /mnt/greetings
  • unmount the partition
  • show that the file system, is correct

Attention: Mounting a file system is a dangerous operation, valuable data could be lost in case of erroneous operations of software bugs. So fuse-ext2 requires the flag rw+ instead of the default rw to provide read-write access to the file system. Users must be aware of the dangers and voluntarily add that plus sign in the option.

$$ vu_insmod fuse
$$ truncate -s 10M img.ext2
$$ /sbin/mkfs.ext2 img.ext2
mke2fs 1.45.6 (20-Mar-2020)
Discarding device blocks: done
Creating filesystem with 10240 1k blocks and 2560 inodes
Filesystem UUID: 626b8bc8-4bb5-4240-a1e2-ca526c305a67
Superblock backups stored on blocks:
        8193

Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

$$ fuse-ext2 -o rw+ /home/user/img.ext2 /mnt
$$ ls /mnt
lost+found
$$ echo ciao > /mnt/greetings
$$ ls -l /mnt
total 13
-rw-r--r-- 1 user user     5 Aug  7 14:13 greetings
drwx------ 2 root root 12288 Aug  7 14:12 lost+found
$$ cat /mnt/greetings
ciao
$$ vuumount /mnt/
$$ ls /mnt
$$ fuse-ext2 -o rw+ /home/user/img.ext2 /mnt
$$ cat /mnt/greetings
ciao
$$ vuumount /mnt
$$ /sbin/fsck.ext2 -f img.ext2
e2fsck 1.45.6 (20-Mar-2020)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
img.ext2: 12/2560 files (0.0% non-contiguous), 422/10240 blocks
$$

The operation can be graphically represented as follows:

vufuseext exampe

As a second and more realistic example let us mount the file system image BusyBox-1.21.1-amd64-root_fs that we have already introduces in section VDE and Virtual Machines.

This file is available in the tutorial resource page.

The following sequence of commands mounts that file system image on /mnt/.

$$ vu_insmod fuse
$$ fuse-ext2 -o ro /home/user/BusyBox-1.21.1-amd64-root_fs /mnt
$$ ls /mnt
bin  dev  etc  linuxrc  lost+found  proc  root  sbin  sys  usr
$$

fuse2fs: ext2/3/4 filesystems

This example uses fuse2fs, an implementation of ext2/3/4 filesystems for FUSE provided by the e2fsprogs package.

The following shell session performs these steps:

  • create an empty file named img.ext2
  • configure the file as an ext2 file system
  • mount the file system on /mnt
  • a list of /mnt shows that there is only the lost+found directory.
  • create the file /mnt/greetings in the ext2 file system,
  • unmount and remount the partition
  • show the contents of /mnt/greetings
  • unmount the partition
  • show that the file system, is correct
$$ vu_insmod fuse
$$ truncate -s 10M img.ext2
$$ /sbin/mkfs.ext2 img.ext2
mke2fs 1.45.6 (20-Mar-2020)
Discarding device blocks: done
Creating filesystem with 10240 1k blocks and 2560 inodes
Filesystem UUID: 626b8bc8-4bb5-4240-a1e2-ca526c305a67
Superblock backups stored on blocks:
        8193

Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

$$ fuse2fs -o fakeroot /home/user/img.ext2 /mnt
$$ ls /mnt
lost+found
$$ echo ciao > /mnt/greetings
$$ ls -l /mnt
total 13
-rw-r--r-- 1 user user     5 Aug  7 14:13 greetings
drwx------ 2 root root 12288 Aug  7 14:12 lost+found
$$ cat /mnt/greetings
ciao
$$ vuumount /mnt/
$$ ls /mnt
$$ fuse2fs -o fakeroot /home/user/img.ext2 /mnt
$$ cat /mnt/greetings
ciao
$$ vuumount /mnt
$$ /sbin/fsck.ext2 -f img.ext2
e2fsck 1.45.6 (20-Mar-2020)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
img.ext2: 12/2560 files (0.0% non-contiguous), 422/10240 blocks
$$

The operation can be graphically represented as follows:

vufuseext exampe

As a second and more realistic example let us mount the file system image BusyBox-1.21.1-amd64-root_fs that we have already introduces in section VDE and Virtual Machines.

This file is available in the tutorial resource page.

The following sequence of commands mounts that file system image on /mnt/.

$$ vu_insmod fuse
$$ fuse2fs -o ro /home/user/BusyBox-1.21.1-amd64-root_fs /mnt
Mounting read-only.
$$ ls /mnt
bin  dev  etc  linuxrc  lost+found  proc  root  sbin  sys  usr
$$

fusefatfs: FAT12, FAT16, FAT32 and exFAT.

This module can mount FAT file systems (up to 10).

For example let us download the installation floppy disk of FreeDOS 1.2.

$$ vu_insmod fuse
$$ wget http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.2/official/FD12FLOPPY.zip
...
$$ unzip FD12FLOPPY.zip
Archive:  FD12FLOPPY.zip
  inflating: FLOPPY.img
  inflating: README.md
$$ fusefatfs -o ro /home/user/FLOPPY.img /mnt
$$ ls /mnt
AUTOEXEC.BAT  COMMAND.COM  FDCONFIG.SYS  FDSETUP  KERNEL.SYS  SETUP.BAT

...

$$ vuumount /mnt

archivemount: iso, tar, zip (including compressed archives)

In this exammple the iso image alpine-virt-3.12.0-x86_64.iso is mounted on /mnt:

$$ vu_insmod fuse
$$ archivemount -o ro /home/user/alpine-virt-3.12.0-x86_64.iso /mnt
$$ ls /mnt
apks  boot  efi

...

$$ vuumount /mnt

The following example shows how to mount the compressed archive alpine-minirootfs-3.12.0-x86_64.tar.gz:

$$ vu_insmod fuse
$$ archivemount -o ro /home/user/alpine-minirootfs-3.12.0-x86_64.tar.gz /mnt
$$ ls /mnt
bin  etc   lib    mnt  proc  run   srv  tmp  var
dev  home  media  opt  root  sbin  sys  usr

...

$$ vuumount /mnt

sshfs: mount a remote filesystem using ssh

The following example shows how to mount the filesystem of a remote host using ssh:

$$ vu_insmod fuse
$$ sshfs remote.host.com:/ /mnt
$$ ls /mnt
bin   etc    initrd.img      lib32   lost+found  opt   run   sys  var
boot  extra  initrd.img.old  lib64   media       proc  sbin  tmp  vmlinuz
dev   home   lib             libx32  mnt         root  srv   usr  vmlinuz.old

...

$$ vuumount /mnt

Other file system types

Any FUSE file system implementation should work on VUOS using this module.