Warning: Do not loop mount untrusted filesystems¶

Untrusted filesystems and untrusted disk images should not be loop mounted because they could contain exploits that attack your host kernel. Use the tools from libguestfs(3) instead since it safely isolates untrusted filesystems from the host.

Loop mount a filesystem from a compressed file¶

If you have a filesystem or disk image in xz-compressed format then you can use nbdkit-xz-filter(1) and nbdkit-file-plugin(1) to loop mount it as follows:

 nbdkit --filter=xz file disk.xz
 nbd-client localhost /dev/nbd0
 mount /dev/nbd0p1 /mnt

Loop mount a filesystem from a web server¶

You can use nbdkit-curl-plugin(1) to loop mount a filesystem from a disk image on a web server:

 nbdkit [--filter=xz] curl https://example.com/disk.img
 nbd-client localhost /dev/nbd0
 mount /dev/nbd0p1 /mnt

Use --filter=xz if the remote image is XZ-compressed.

Create a giant btrfs filesystem¶

nbdkit is useful for testing the limits of Linux filesystems. Using nbdkit-memory-plugin(1) you can create virtual disks stored in RAM with a virtual size up to 2⁶³-1 bytes, and then create filesystems on these:

 nbdkit memory $(( 2**63 - 1 ))
 nbd-client localhost /dev/nbd0

Partition the device using GPT, creating a single partition with all default settings:

 gdisk /dev/nbd0

Make a btrfs filesystem on the disk and mount it:

 mkfs.btrfs -K /dev/nbd0p1
 mount /dev/nbd0p1 /mnt

Inject errors into Linux devices¶

Using nbdkit-error-filter(1) you can see how Linux devices react to errors:

 nbdkit --filter=error \
        memory 64M \
        error-rate=100% error-file=/tmp/inject
 nbd-client localhost /dev/nbd0
 mkfs -t ext4 /dev/nbd0
 mount /dev/nbd0 /mnt

Inject errors by touching /tmp/inject, and stop injecting errors by removing this file.

Write Linux block devices in shell script¶

Using nbdkit-sh-plugin(3) you can write custom Linux block devices in shell script for testing. For example the following shell script creates a disk which contains a bad sector:

 #!/bin/bash -
 case "$1" in
     thread_model) echo parallel ;;
     get_size) echo 64M ;;
     pread)
         if [ $4 -le 100000 ] && [ $(( $4+$3 )) -gt 100000 ]; then
             echo EIO Bad block >&2
             exit 1
         else
             dd if=/dev/zero count=$3 iflag=count_bytes
         fi ;;
     *) exit 2 ;;
 esac

Create a loop from this shell script using:

 nbdkit sh ./bad-sector.sh
 nbd-client localhost /dev/nbd0

You can then try running tests such as:

 badblocks /dev/nbd0