How to extract/unpack/uncompress the contents of the initramfs image on RHEL7 and RHEL8? How do I list initramfs image file content? How do I modify the contents of an initrd or initramfs? How do I view an initrd or initramfs? How to fix cpio premature end of archive error. How to list initramfs content in RHEL 7 and RHEL 8 using lsinitrd.

Fix: cpio premature end of archive, List (lsinitrd) & Extract initramfs in RHEL Linux

 

You may observe error “cpio premature end of archive” while trying to extract initramfs. I got this error while trying to extract initramfs on my Red Hat Enterprise Linux 7 host but the same problem can also occur in RHEL 8 as in both the initramfs is available in ASCII format unlike old initramfs from RHEL 5 and 6 where the initramfs was in a different format.

With RHEL 7 the initramfs file now stores both CPU microcode and the initial boot image in the one “combined” image file. The CPU microcode is stored with CPIO compression, then the boot image is stored with its own separate compression.

In this article I will share different methods to extract initramfs image file in different variants of Red Hat Enterprise Linux and to fix cpio premature end of archive error. These commands can also be used in other Linux distributions if the file format of the initramfs is same as RHEL.

 

Identify compression format of the image

Before we extract initramfs it is important to identify the compression format of the image so that we can accordingly extract initramfs image.
Use the file command on the initramfs/initrd to identify the compression format:

# file /boot/initramfs-($uname -r).img

The $(uname -r) will use the file for the current kernel version. If you wish to identify compression format of some other initramfs image then you can also give the full file name

# file /boot/initramfs-3.10.0-957.el7.x86_64.img

 

Possible initramfs compression format

Below are some of the possible initramfs image file format available over the various Red Hat generations.

# file /boot/initramfs-($uname -r).img
/boot/initramfs-3.10.0-327.el7.x86_64.img: gzip compressed data, from Unix, last modified: Fri Oct 7 03:36:28 2016, max compression

There may also be an xz/LZMA-format image which displays as:

# file /boot/initramfs-($uname -r).img
/boot/initramfs-2.6.32-754.el6.x86_64.img: LZMA compressed data

Or ASCII format image which is displayed as:

# file /boot/initramfs-$(uname -r).img
/boot/initramfs-4.18.0-80.el8.x86_64.img: ASCII cpio archive (SVR4 with no CRC)

 

Uncompress/Extract initramfs with Gzip format

Create a temporary directory where you wish to extract initramfs

# mkdir /tmp/initrd

# cd /tmp/initrd

make sure gzip is installed on your setup or you can install it using yum

# yum install gzip
NOTE:
On RHEL system you must have an active subscription to RHN or you can configure a local offline repository using which “yum” package manager can install the provided rpm and it’s dependencies.

Next execute the below command to extract initramfs

# zcat /boot/initramfs-3.10.0-327.el7.x86_64.img | cpio -i --no-absolute-filenames
88386 blocks

 

Uncompress/Extract initramfs with xz/LZMA format

Again you can create a temporary directory where you wish to extraxt initramfs content

# mkdir /tmp/initrd

# cd /tmp/initrd

To uncompress initramfs execute the below command

# xz -dc < /boot/initrd-$(uname -r).img | cpio -idmv

 

Fix “cpio premature end of archive” error

In RHEL6 and older, the contents of the initramfs or initrd image could be extracted using cpio as shown below:

# zcat initramfs-2.6.32-431.el6.x86_64.img | cpio -idmv

However, it does not work when I try to extract the contents of initramfs image on RHEL7 as shown below:

# zcat /boot/initrd-$(uname -r).img | cpio -idmv
gzip: /boot/initrd-4.18.0-80.el8.x86_64.img.gz: No such file or directory
cpio: premature end of archive

So we get cpio premature end of archive error with file format of initramfs is ASCII

# file /boot/initramfs-$(uname -r).img
/boot/initramfs-4.18.0-80.el8.x86_64.img: ASCII cpio archive (SVR4 with no CRC)

To fix cpio premature end of archive error and to extract initramfs we must use a separate command to extract the initramfs content
First, create a temporary work directory and switch into it. This will be the location where the initramfs contents will be viewed:

# mkdir /tmp/initramfs

# cd /tmp/initramfs

Uncompress and extract the contents of the image in the /boot/ directory:

# /usr/lib/dracut/skipcpio /boot/initramfs-$(uname -r).img | gunzip -c | cpio -idmv

To view the content of initramfs

# ls -l
total 44
lrwxrwxrwx.  1 root root    7 Jul 14 09:48 bin -> usr/bin
drwxr-xr-x.  2 root root 4096 Jul 14 09:48 dev
drwxr-xr-x. 12 root root 4096 Jul 14 09:48 etc
lrwxrwxrwx.  1 root root   23 Jul 14 09:48 init -> usr/lib/systemd/systemd
lrwxrwxrwx.  1 root root    7 Jul 14 09:48 lib -> usr/lib
lrwxrwxrwx.  1 root root    9 Jul 14 09:48 lib64 -> usr/lib64
drwxr-xr-x.  2 root root 4096 Jan 15 22:19 proc
drwxr-xr-x.  2 root root 4096 Jan 15 22:19 root
drwxr-xr-x.  2 root root 4096 Jan 15 22:19 run
lrwxrwxrwx.  1 root root    8 Jul 14 09:48 sbin -> usr/sbin
-rwxr-xr-x.  1 root root 3126 Oct  8  2018 shutdown
drwxr-xr-x.  2 root root 4096 Jan 15 22:19 sys
drwxr-xr-x.  2 root root 4096 Jan 15 22:19 sysroot
drwxr-xr-x.  2 root root 4096 Jan 15 22:19 tmp
drwxr-xr-x.  7 root root 4096 Jul 14 09:48 usr
drwxr-xr-x.  3 root root 4096 Jul 14 09:48 var

When viewing this combined image file on a booted system, it is necessary to use skipcpio to “skip past” the CPIO-compressed CPU microcode to access the boot image.

 

List initramfs content

You can also list initramfs content without actually extracting it, assuming your requirement is to only view the content of initramfs image file. This can be achieved using lsinitrd as shown below:

# lsinitrd /boot/initramfs-4.18.0-80.el8.x86_64.img

 

Lastly I hope the steps from the article to fix cpio premature end of archive, List initramfs using lsinitrd & Extract initramfs in RHEL Linux was helpful. So, let me know your suggestions and feedback using the comment section.

Leave a Reply

Your email address will not be published. Required fields are marked *