Introduction
is the GRand Unified Bootloader. For those unfamiliar, a bootloader is
a critical piece of software used when a computer turns on. Its job is
to load an operating system. The bootloader resides on a disk of some
sort (floppy, hard) and is called by the BIOS, which is the real low-level program that runs on startup. GRUB is installed at a specific location on these devices.
Image files are byte-for-byte representations of block devices, such
as a hard disk or floppy disk. They can be used for a variety of
things, such as backup, offline investigation, or virtual machines. I
was using the latter (specifically, and ). These programs act like a computer within a computer, and take an image file to be the disk for the virtual machine.
How do these come together? When building an image file from scratch, I wanted to install GRUB on it. There are a lot of about how to install GRUB on a floppy image or on a real HDD, but not on a HDD
image. Perhaps this info is already out there, but since I didn’t find
it, I figured I’d tell you all what klueska and I came up with.
Details
The following was done on a regular Linux box. Use root/sudo when
you need to. Normally, I’ll explain what I’ll do, and then show the
commands I used to do it.
Creating the Image
This will create an 8MB image. Use whatever size you want. These
values will result in one cylinder (based on the old
sector/cylinder/head sizing). Fdisk complained on smaller images, so YMMV. I have a folder called mnt/ in my current directory, which is where I store both the images and the image mount point.
dd if=/dev/zero of=mnt/hdd.img bs=512 count=16065
This creates a loopback device that connects to the image file. This
helps some utilities that are expecting to work with a device instead
of a file. For instance, fdisk was a little happier with this.
losetup /dev/loop1 mnt/hdd.img
Fdisk the image, just like it’s a regular block device. The easiest thing is to just create one Linux primary partition.
fdisk /dev/loop1
Okay, enough with the slow stuff; it’s time to put your “daddy
pants” on. Here’s the problem. You need to make a file system on that
partition, but you do not have a device that points to just that
partition, like you would for a normal hard drive. You only have the
loopback device, which points to the whole image. It’s like you only
have /dev/hda, and not /dev/hda1. What to do? Create another loopback
device, and have the device start from a certain offset within the
image.
This determines the offset, in sectors, for the beginning of the
partition. Then multiply the sector offset by 512, since losetup
offsets by bytes.
fdisk -ul /dev/loop1
This will have us point loop2 to the partition on the disk (if there is only one partition), as explained in more detail . Then we simply make a filesystem on it.
losetup -o 32256 /dev/loop2 /dev/loop1
mkfs /dev/loop2
If you have multiple partitions, you will need to tell losetup how far to go into the target with the --sizelimit
flag. By default, it will map all the way to the end of the target,
which will clobber any following partitions. To figure out the size,
look at the Blocks column output of fdisk -ul, and
multiply that by 1024. Thanks to Anonymous for pointing this out in the
comments below. Here’s an example (for about 400 cylinders):
losetup -o 32256 --sizelimit 3290079232 /dev/loop2 /dev/loop1
mkfs /dev/loop2
This will mount it so you can examine it and so you’re ready to
install grub. Make sure you have mkdir’d mnt/hdd, yada yada yada.
mount /dev/loop2 mnt/hdd/
Installing GRUB
This creates the file structure on the image for GRUB and copies local copies of critical GRUB
files to the appropriate folder. It also copies whatever kernel you
want to load into the root of the image’s file system. Adjust files and
paths to your liking.
mkdir -p mnt/hdd/boot/grub
cp -r /boot/grub/stage1 /boot/grub/stage2 /boot/grub/menu.lst mnt/hdd/boot/grub
cp -r the_kernel mnt/hdd/
Don’t forget to edit the menu.lst file (a grub.conf) to suit your
kernel. Mine looked something like this. Look elsewhere for more
guidance.
default 0
timeout 10
title=LonnyOS
root (hd0,0)
kernel /the_kernel
Now install GRUB for real.
grub --device-map=/dev/null
This will open up a GRUB environment. Enter the following:
device (hd0) mnt/hdd.img
root (hd0,0)
setup (hd0)
Note that we set the device to the actual image file, and not the loopback device. GRUB can work with either. Normally, utilities work better with the device, but due to a in GRUB,
it was flipping out (Error 22) when given the loopback device. It
worked fine when installing on the hdd.img. For more details, look
elsewhere (my references and google are decent starting points).
Using the Image
You can easily mount the image and use it, even while using the image directly for a virtual machine. I ran losetup -a
to see which loopback devices I had, then the -d flag to delete
whichever I don’t need. Now I’ll loopback directly into the image, and
mount it. And then make sure my regular user account has all the access
necessary. Also, be careful of any necessary sizelimits to losetup, as mentioned above.
losetup -o 32256 /dev/loop0 mnt/hdd.img
mount /dev/loop0 mnt/hdd
chown -R brho:brho mnt/hdd
As I muck around with the_kernel, I can just cp it into mnt/hdd/, and quickly run KVM or Bochs to test the new kernel.
kvm mnt/hdd.img
bochs -q 'ata0-master: type=disk, mode=flat, path="./mnt/hdd.img", cylinders=1, heads=255, spt=63'
One thing to note: after you copy the_kernel to mnt/hdd, the hdd.img
is not actually updated instantly. This is because filesystems do not
immediately flush their changes to disk, and the write() syscall will
return early. If you want to immediately run your VM with your new
image, simply sync your disks to make sure all writes are flushed.
sync
Conclusion
Hopefully this has helped you. If you hose your system, you’re on
your own / standard disclaimers apply. But if not, perhaps you are the
happy owner of a fresh GRUB hdd image. More importantly, you should know how to make one and (roughly) how the process worked.
References
This article was edited after publication by the author on 04 Feb 2009.
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