Hacking RAM disks

To boot Linux, you only need two things:

• The Linux kernel itself, and
• A root filesystem with an init program of some sort.

As any book about embedded Linux will tell you, the init process will typically configure devices, launch other programs and generally set up what you think of as the Linux environment.

Many O/S distributions, including Android, use a small RAM disk as the initial root filesystem and the init program will enumerate the devices and typically mount other filesystems.

In previous kernel versions, the RAM disks were typically ext2 filesystems, but the current convention is to use the initramfs file format, which is essentially a cpio archive.

To speed the boot, the cpio archive is typically gzipped and when used with the U-Boot boot loader, wrapped with a U-Boot header containing a CRC by the mkimage tool.

In the course of working with an O/S that uses a RAM disk, you will often want to extract the content at least to examine it. You may also want to modify some of the contents and re-create it, but you can't do this directly.

In this post, I'll walk through the process of doing just that.

I'll use the file uramdisk.img from the SD card tree of the images provided in the Freescale Android release R13.4-Beta release.

If you run file on the image, you'll see it reported as a PPC/U-Boot image file:

~/$ file uramdisk.img
uramdisk.img: u-boot/PPCBoot image

That's because mkimage adds 64 bytes of header. We can use dd to strip it like so:

~$ dd bs=1 skip=64 if=uramdisk.img of=uramdisk-no-header.img
167778+0 records in
167778+0 records out
167778 bytes (168 kB) copied, 0.91557 s, 183 kB/s
~$ ls -l uramdisk*
-rw-r--r-- 1 ericn ericn   167842 2012-09-07 12:21 uramdisk.img
-rw-r--r-- 1 ericn gitosis 167778 2012-09-07 12:25 uramdisk-no-header.img
~$ file uramdisk-no-header.img
uramdisk-no-header.img: gzip compressed data, from Unix

As you can see, after stripping the 64-byte header, file identifies the image as a gzipped data.

If we unzip it, we'll see that it's a cpio archive and we can list its content:

~$ zcat uramdisk-no-header.img > uramdisk-uncompressed.img
~$ file uramdisk-uncompressed.img
uramdisk-uncompressed.img: ASCII cpio archive (SVR4 with no CRC)
~$ cpio -t 
You can extract them like so:
~$ mkdir myramdisk
bash: ../uramdisk-uncompressed.img: No such file or directory
~$ cd myramdisk/
~/myramdisk$ sudo cpio -i --no-absolute-filenames 
Note the use of sudo above. You'll generally want to do that in order to preserve root ownership.


Also note that I tailed the file default.prop, which is a file you might want to hack in an Android image. In particular, ro.secure and ro.debuggable are two properties that you might want to set differently when you're working with an Android image.


If you edit that file, you can re-create the RAM disk using these steps:
~/myramdisk$ shopt -s dotglob
~/myramdisk$ sudo find . | sudo cpio -H newc -o | gzip > ../uramdisk.cpio.gz
551 blocks
~/myramdisk$ mkimage -A arm -O linux -T ramdisk -n "Initial Ram Disk"
             -d ../uramdisk.cpio.gz ../uramdisk.img.new
Image Name:   Initial Ram Disk
Created:      Fri Sep  7 12:47:00 2012
Image Type:   ARM Linux RAMDisk Image (gzip compressed)
Data Size:    169051 Bytes = 165.09 kB = 0.16 MB
Load Address: 0x00000000
Entry Point:  0x00000000
 The second command-line above is a little hairy, with a four-stage pipeline that lists all files (including those beginning with dot because of the preceding shopt) and hands them off to cpio. Note that the -H newc is needed for historical reasons. Since cpio sends its output to stdout, we pipe that to gzip and the gzipped output to a new file named uramdisk.cpio.gz. 

 The last command is the mkimage U-Boot tool, which adds the 64-byte U-Boot header back onto the output.

 I hoped this quick note helps you understand the basics of a RAM disk. We'll likely make use of this in future posts, and want it around for reference.