Mount a partition of a qcow2 (or raw) disk image
There are situations when you need to access a VM's disk / partition directly, e.g to reset a forgotten root password. For QEMU's qcow2 partitions this can be achieved with the "network block device" (nbd) driver and qemu-nbd from the qemu-utils package. (For "raw" disk images, see below.)
First, load the nbd kernel module:
$ modprobe nbd max_part=12
Now use the command qemu-nbd to make the disk image available as network block device:
$ qemu-nbd --connect=/dev/nbd0 /path/to/image.qcow2
fdisk can be used to list the available partitions inside the disk image:
$ fdisk -l /dev/nbd0
Finally mount the partition (in this case partition #2) to an arbitrary mount point:
$ mount /dev/nbd0p2 /path/to/mountpoint/
chroot into the directory as usual. When you're done, exit the chroot, then unmount the partition and disconnect the network block device:
$ umount /dev/nbd0p2
$ qemu-nbd --disconnect /dev/nbd0
Similarly, a partition inside a raw disk image can be mounted as loop device:
$ losetup /dev/loop0 /path/to/diskimage.img
Make the partition mappings available:
$ kpartx -av /dev/loop0
And mount the partition (here partition #1):
$ mount /dev/mapper/loop0p1 /mnt/image
Alternatively, the partition can be mounted directly: In this case its offset (in bytes) from the beginning of the disk image needs to be passed to the mount command, fdisk will provide the necessary values to calculate it.
Here an example disk image with only one partition:
$ fdisk -l netinst.img
Disk netinst.img: 32 MiB, 33554432 bytes, 65536 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: dos
Disk identifier: 0xb6bdaf05
Device Boot Start End Sectors Size Id Type
netinst.img1 * 2048 65535 63488 31M b W95 FAT32
The "Start" sector of the target partition (here 2048) needs to be multiplied with the disk's sector size, here 512 bytes. In this case the partition's offset is 512*2048=1048576 bytes:
$ mount -o loop,offset=1048576 netinst.img /path/to/mountpoint/