changing contents of initrd image

kushalkoolwal · 08-09-2006, 08:28 PM

Hi, I have an initrd image for my Debian 2.6.13 kernel and I wanted to modify a file in the initrd. I know how to view the contents but I don't know what I have to do in order to change it because it mounts the initrd image as a READ-ONLY filesystem.

Here is what I did:

Code:

root@debian:~# file -z /boot/initrd.img-2.6.13 /boot/initrd.img-2.6.13: Linux Compressed ROM File System data, little endian size 1372160 version #2 sorted_dirs CRC 0x30bc9195, edition 0, 879 blocks, 123 files
root@debian:~#

Looks like my initrd image is cramfs type. so next I tried accessing the contents by following this website's instruction http://www.ducea.com/2006/06/24/insp...n-initrd-file/ :

Code:

# All work is done in a temporary directory
mkdir /tmp/initrdmount
# Mount the image file directly
mount -o loop /boot/initrd.img-2.6.13 /tmp/initrdmount
#Investigate its content, etc.
cd /tmp/initrdmount
#Once done un-mount it:
umount /tmp/initrdmount

Ok I was able to mount it and I could see the contents also:

Code:

root@debian:~# ls /tmp/initrdmount/
bin   dev2   keyscripts  linuxrc.conf  proc    scripts  usr
bin2  devfs  lib         loadmodules   sbin    sys      var
dev   etc    linuxrc     mnt           script  tmp
root@debian:~#

Everything is fine till now(atleast it appears), but now when I tried to create a file in the initrd, I got the following message:

Code:

root@debian:/tmp/initrdmount# touch test
touch: cannot touch `test': Read-only file system
root@debian:/tmp/initrdmount#

Is there any way I can mount it as read-write file system? Also what will be the steps (in summary) once I am able to modify the files inside initrd, to get my new initrd image going.

THanks

bigrigdriver · 08-09-2006, 10:31 PM

I've not found anything in the mount man page that states read-only is the default unless otherwise specified. But, that seems to be the case. So, add -w to the mount command to specify read-write.

# Mount the image file directly
mount -w -o loop /boot/initrd.img-2.6.13 /tmp/initrdmount

kushalkoolwal · 08-10-2006, 04:29 AM

Quote:

Originally Posted by bigrigdriver

I've not found anything in the mount man page that states read-only is the default unless otherwise specified. But, that seems to be the case. So, add -w to the mount command to specify read-write.

# Mount the image file directly
mount -w -o loop /boot/initrd.img-2.6.13 /tmp/initrdmount

Alright I will try that and let you know how did it go.

Thanks for the help anyways.

kushalkoolwal · 08-10-2006, 04:50 PM

Quote:

Originally Posted by bigrigdriver

I've not found anything in the mount man page that states read-only is the default unless otherwise specified. But, that seems to be the case. So, add -w to the mount command to specify read-write.

# Mount the image file directly
mount -w -o loop /boot/initrd.img-2.6.13 /tmp/initrdmount

Nopes it didn't work. Here is what I tried:

Code:

root@debian:/tmp/initrdmount# mount -w -o loop /boot/initrd.img-2.6.13 /tmp/initrdmount/

Code:

root@debian:/tmp/initrdmount# ls bin   dev2   keyscripts  linuxrc.conf  proc    scripts  usr
bin2  devfs  lib         loadmodules   sbin    sys      var
dev   etc    linuxrc     mnt           script  tmp
root@debian:/tmp/initrdmount#

To confirm my worst fear:

Code:

root@debian:/tmp/initrdmount# touch test
touch: cannot touch `test': Read-only file system
root@debian:/tmp/initrdmount#

What could be the problem? Also I don't know if this output helps or not which I already posted in my first post:

Code:

root@debian:~# file -z /boot/initrd.img-2.6.13 /boot/initrd.img-2.6.13:
Linux Compressed ROM File System data,
little endian size 1372160 version #2
sorted_dirs CRC 0x30bc9195, edition 0, 879 blocks, 123 files
root@debian:~#

I just found out some more information about cramfstype. Since my initrd image is cramfstype, it can be only mounted as Rad-Only and I guess that's what the output of file -z is showing to us.

Any more ideas?

bigrigdriver · 08-10-2006, 09:05 PM

The only other thing I can come up with after googling for the last half hour is cramfsprogs. These utilities permit making and extracting a cramfs initrd. I assume that, if you can extract it, you can then edit files then re-make the initrd.