Mounting Partition from Raw Disk Image on Linux

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Mounting Partition from Raw Disk Image on Linux

December 11, 2015 | Article | No Comments

In Linux/Unix or perhaps in every computer system, a term mounting is defined as attaching additional filesystem to the currently accessible filesystem of a computer. As we know, a filesystem is hierarchy of directories (also referred to as a directory tree) that is used to organize files on a computer or storage media.

We might familiar or maybe remember how to mount a partition well. But that’s when the partition is inside a storage media or physical medium. What if the partition we want to access is in form of disk image? Here, we will discuss it.

In this article I use:

  1. Slackware64 14.0
  2. dumped image from 4GB SD-card, using dd

The Theory

Disk Image

A disk image is a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as hard drive, tape drive, floppy disk, optical disc, or USB flash drive. A disk image is usually created by creating a complete sector-by-sector copy of the source medium and thereby perfect replicating the structure and contents of a storage device.

In Linux, we can create disk image using dd utility. Assuming we want to create disk image of a SD-card which is recognized as /dev/sdb as MyDiskImage.img:

dd if=/dev/sdb of=MyDiskImage.img

Partition and Partition Table

As said, disk image is perfect copy of a storage media. Therefore we got very same bit of partition and partition table. Nothing is modified, unless you alter it. Therefore, we can still read the partition table like what we do on physical storage medium.

We can use fdisk to see the partition table of a disk image:

fdisk MyDiskImage.img

Loopback Device

We know that Linux and Unix list all recognized device as device nodes. They are treated like ordinary file and stored on /dev, like /dev/sda for first SCSI storage device we have. However, not many of us know some pseudo-devices on this directory. One of them is loop device /dev/loop*.

This is special node that we will use for mounting an image.

How to Mount

There are some steps we have to do so we can mount a partition inside of disk image.

Know the Offset

We have to know where is the offset of partition. Clearly we need to know where the partition start and where the partition end. Although, we only need the offset of beginning of the partition. To do that, we can use fdisk to peek on partition table. For example:

fdisk MyDiskImage.img

Here’s how my partition looks like.

Disk MyDiskImage.img: 691 MB, 691798016 bytes
255 heads, 63 sectors/track, 84 cylinders, total 1351168 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
Disk identifier: 0x0002c262

                         Device Boot      Start         End      Blocks   Id  System
2013-09-25-wheezy-raspbian.img1            8192      122879       57344    c  W95 FAT32 (LBA)
2013-09-25-wheezy-raspbian.img2          122880     5785599     2831360   83  Linux

What we want to know is the offset, which is in byte-level. However fdisk tell us about sectors. Fortunately we can convert the offset in sector to bytes by multiplying it with the size of sector. In our case, one sector is 512 bytes.

Let say we want to mount partition 2, which is ext4. We calculate the offset for this partition.

122880 * 512 = 62914560

Save this value.

Mount It!

The actual mounting. Mounting process is similar when we mount normal partition. However, this time we use loopback device and use one argument we never use before, “offset”. Basically, offset tell mount utility to skip the data on particular to offset.

Here how we do:

mount -o loop,offset=62914560 MyDiskImage.img /mnt/mount_point

Here we mount partition 2, which is located on 62914560 bytes after the beginning of the file. The partition should be mounted to our mount point (/mnt/mount_point). Unless you have an unknown filesystem, this command shouldn’t be fail.

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A man who is obsessed to low level technology.

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