Extracting and Repacking a Red Pitaya's initramdisk

It took me some time to figure out on how to quickly extract, modify and then repack a Red Pitaya’s initramdisk without installing a full build environment. In this post I provide some easy to use Linux shell scripts to do just that.

First you will need some tools, i.e.

• cpio
• mkbootimg

The latter is part of the u-boot-tools package and may be installed through a package manager such as aptitude by issuing

sudo apt-get install u-boot-tools

on the command line. The former should already be available on most systems and can be installed through the package cpio if it is not.

The Red Pitaya’s initramdisk may be easily grabbed by copying the uramdisk.image.gz file from the official SD-card image or from a running Red Pitaya’s /opt folder.

How it works

For extraction, the first step is to remove the 64 byte u-boot header from the packed ramdisk which is simply done by using dd. Then it is uncompressed using gunzip and finally extracted by cpio. The sequence of commands is

( dd bs=64 skip=1 count=0; dd bs=4096 ) < $infile > $tempfile1
cat $tempfile1 | gunzip > $tempfile2
cd $outfolder; sudo cpio -id --no-absolute-filenames < $tempfile2

Note that the sudo invocation of cpio is necessary here as most files and folders inside the initramdisk are owned by root. The arguments to cpio mean

• -i: extract
• -d: create leading directories
• --no-absolute-filenames: Don’t create files on absolute paths as it may overwrite your system files otherwise!

For repackaging, the steps are essentially reversed. The u-boot header is added by a call to mkbootimg instead

cd $infolder; sudo find . | sudo cpio -H newc -o > $tempfile1
cat $tempfile1 | gzip > $tempfile2
mkimage -A arm -T ramdisk -C gzip -d $tempfile2 $outfile

The arguments to cpio mean

• -o: create archive
• -H newc: Choose the newc format for the archive

The following scripts make it easy to automate extraction and repackaging.

Extraction

To extract the initramdisk to a folder you may use the following script I provide here. Simply save it to a file unmkramdiskimg.sh and make it executable. You may also download the script by issuing the commands

wget https://sonntam.github.io/assets/unmkramdiskimg.sh
chmod u+x unmkramdiskimg.sh

Packaging

For packaging you may use the counterpart script mkramdiskimg.sh from here. The same applies here as for the extraction script. Type

wget https://sonntam.github.io/assets/mkramdiskimg.sh
chmod u+x mkramdiskimg.sh

to get an executable copy in your current folder.

Usage

To extract, simply pass the filename of the packed initramdisk and a target folder to extract to the extraction script:

./unmkramdiskimg.sh uramdisk.image.gz initramdisk

Note: If the target folder already exists and isn’t empty, you have to specify the “-f” modifier to force extraction.

To repackage, pass the name of the folder containing the rootfs and a target filename to save the initramdisk to:

./mkramdiskimg.sh initramdisk uramdisk.image.gz

Then put the resulting image file back on your Red Pitaya’s SD card. If you want to do this on a running Red Pitaya then you first have to mount the SD card as writable

mount -o remount,rw /opt
<copy uramdisk image>

and finally don’t forget to either remount the SD card as read-only again (otherwise the SD card’s file system may get corrupted on power-loss) or reboot the system:

mount -o remount,ro /opt
reboot

Now your modified image is ready to be used!