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vudev VUOS virtual devices

vudev is the module to create virtual devices.

The standard way to interact with devices in UNIX is through special files i.e. entries in the file systems that appear as files. The purpose of special files is just to give a pathname that can be used by programs to identify and access the device. Special files can appear in the file system using vudev submodules.

vudevramdisk

vudevramdisk allocates a RAM area and makes it available as a virtual disk.

The meaning of the shell session below is:

  • add the vudev and vufuse VUOS modules.
  • use vumount to create the device
  • show that it effectively appears as a block device
  • the hex-dump shows a 10MB empty area
$$ vu_insmod vudev vufuse
$$ vumount -t vudevramdisk -o size=10M none /dev/ramdisk
$$ ls -l /dev/ramdisk
brw------- 0 user user 0, 0 Aug  8 09:06 /dev/ramdisk
$$ od -x /dev/ramdisk
0000000 0000 0000 0000 0000 0000 0000 0000 0000
*
200000000
$$

Now it is possible to create a file system structure on the virtual file system and then mount it.

$$ /sbin/mkfs.ext2 /dev/ramdisk
mke2fs 1.45.6 (20-Mar-2020)
warning: Unable to get device geometry for /dev/ramdisk
Creating filesystem with 32768 1k blocks and 8192 inodes
Filesystem UUID: a3233045-566a-4f6d-aed7-e2806fd31904
Superblock backups stored on blocks:
        8193, 24577

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

$$ vumount -t vufuseext2 -o rw+ /dev/ramdisk /mnt
$$ ls /mnt
lost+found
$$

Graphically the structure can be depicted as follows:

vuos ramdisk + fuse

vudevpartx

This submodule is able to manage both the partitioning scheme MBR and GPT. So it is possible to mount a disk image and have access to its partitions.

Let us create an empty 200MB file and create a GPT partitiontable on it by fdisk.

$$ vu_insmod vudev vufuse
$$ truncate -s 200M diskimg
$$ /sbin/fdisk diskimg

Welcome to fdisk (util-linux 2.36).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x4870e71c.

Command (m for help): g
Created a new GPT disklabel (GUID: 1D18D7AF-C42D-A349-B53C-328F229BCCD8).

Command (m for help): n
Partition number (1-128, default 1):
First sector (2048-409566, default 2048):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-409566, default 409566): +100M

Created a new partition 1 of type 'Linux filesystem' and of size 100 MiB.

Command (m for help): n
Partition number (2-128, default 2): 2
First sector (206848-409566, default 206848):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (206848-409566, default 409566):

Created a new partition 2 of type 'Linux filesystem' and of size 99 MiB.

Command (m for help): p
Disk diskimg: 200 MiB, 209715200 bytes, 409600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 1D18D7AF-C42D-A349-B53C-328F229BCCD8

Device      Start    End Sectors  Size Type
diskimg1     2048 206847  204800  100M Linux filesystem
diskimg2   206848 409566  202719   99M Linux filesystem

Command (m for help): w
The partition table has been altered.
Syncing disks.

$$

vudevpartx mounts /home/user/diskimg as /dev/hda, i.e. each partition of the disk image can be individually mounted as /dev/hda1, /dev/hda2, etc.

$$ vumount -t vudevpartx /home/user/diskimg /dev/hda
$$ /sbin/mkfs.ext4 /dev/hda2
mke2fs 1.45.6 (20-Mar-2020)
warning: Unable to get device geometry for /dev/hda2
Creating filesystem with 101356 1k blocks and 25376 inodes
Filesystem UUID: 86ab39fd-be6f-4f54-8e2c-565a25c7cb9f
Superblock backups stored on blocks:
        8193, 24577, 40961, 57345, 73729

Allocating group tables: done
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done

$$ mount -t vufuseext2 -o rw+ /dev/hda2 /mnt
$$ ls /mnt
lost+found
$$

vuos partx + fuse

A Practical example: modify config.txt in a Raspbian/RaspPiOS image

Let us suppose that the disk image is /tmp/2020-05-27-raspios-buster-lite-armhf.img.

This is the recipe:

$$ vu_insmod vudev vufuse
$$ mount -t vudevpartx /tmp/2020-05-27-raspios-buster-lite-armhf.img /dev/hdp
$$ mount -t vufusefatfs -o rw+ /dev/hdp1 /mnt
$$ vi /mnt/config.txt 
... update the contents of config.txt
$$ vuumount /mnt
$$ vuumount /dev/hdp

other virtual devices

Note: wip we are currently developing the support for more virtual devices.