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wmakowski
n00b
Joined: 27 Jan 2006
Posts: 33
Location: Ohio, USA
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 Posted: Wed Feb 01, 2006 7:48 pm Post subject: Contents of initramfs |
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I wanted to take a look at what is in initramfs-genkernel-x86-2.6.12-gentoo-r6 included on the LiveCD. So I unzipped it and then used cpio to extract the archive. The command used to extract was:
| Code: | | cpio -idv < initramfs-genkernel-x86-2.6.12-gentoo-r6 |
After extracting I found a basic root directory structure with only a few files.
| Code: | drwxr-xr-x 11 root root 0 Aug 9 02:18 .
drwxr-xr-x 2 root root 0 Aug 9 02:18 sys
drwxr-xr-x 2 root root 0 Aug 9 02:18 dev
crw------- 1 root root 4, 1 Aug 9 02:18 dev/tty1
crw-rw---- 1 root root 5, 1 Aug 9 02:18 dev/console
crw-rw---- 1 root root 1, 3 Aug 9 02:18 dev/null
drwxr-xr-x 3 root root 0 Aug 9 02:18 var
drwxr-xr-x 3 root root 0 Aug 9 02:18 var/lock
drwxr-xr-x 2 root root 0 Aug 9 02:18 var/lock/dmraid
drwxr-xr-x 4 root root 0 Aug 9 02:18 usr
drwxr-xr-x 2 root root 0 Aug 9 02:18 usr/bin
drwxr-xr-x 2 root root 0 Aug 9 02:18 usr/sbin
drwxr-xr-x 2 root root 0 Aug 9 02:18 bin
drwxr-xr-x 2 root root 0 Aug 9 02:18 temp
drwxr-xr-x 2 root root 0 Aug 9 02:18 sbin
drwxr-xr-x 2 root root 0 Aug 9 02:18 etc
-rw-r--r-- 1 root root 93 Aug 9 02:18 etc/fstab
drwxr-xr-x 2 root root 0 Aug 9 02:18 proc |
The initramfs-genkernel-x86-2.6.12-gentoo-r6 file is roughly 28MB unzipped and I was expecting to find a lot more content. Does anyone see an error in how I extracted the archive?
Bill |
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Antimatter
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Joined: 11 Aug 2003
Posts: 463
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| Posted: Fri Feb 03, 2006 12:02 am Post subject: |
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I was actually wondering the very same thing myself, i managed to get my initramfs open and got the very same results as you did, from 2 different initramfs, one was 14 MB, other was 28 MB so i have no idea what's going on here.
hey i think i've found the answer for both of our questions here Unpacking Genkernel or gentoo.igz initramfs | Strange result |
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wmakowski
n00b
Joined: 27 Jan 2006
Posts: 33
Location: Ohio, USA
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| Posted: Fri Feb 03, 2006 12:51 am Post subject: |
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Antimatter,
An excellent find, and it explains the layout of the archive. Thanks for sharing.
Bill |
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widan
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Joined: 07 Jun 2005
Posts: 1512
Location: Paris, France
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| Posted: Fri Feb 03, 2006 1:49 am Post subject: |
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Another script to unpack an initramfs:
| Code: | #!/bin/bash
if [[ $# -ne 2 ]]; then
echo "Usage: $0 initramfs directory"
exit 1
fi
mkdir -p "$2"
if file "$1" | grep gzip > /dev/null 2>&1; then
echo "Initramfs is gzip compressed, unpacking first"
cp "$1" .temp.gz
gunzip .temp.gz
else
cp "$1" .temp
fi
if file .temp | grep ext2 > /dev/null 2>&1; then
echo
echo "This is an initrd (ext2 filesystem), not an initramfs"
echo
echo "Those are ext2 filesystem images, that can be mounted on loopback:"
echo
echo " # mount -o loop $1-unpacked $2"
echo
mv .temp $1-unpacked
exit
fi
compno=1
while [[ -s .temp ]]; do
blocks=$(cpio -t -H newc < .temp 2>&1 > /dev/null | sed "s/[^0-9]*//g")
dd if=.temp of=.$compno.cpio bs=512 count=$blocks 2> /dev/null
dd if=.temp of=.scratch bs=512 skip=$blocks 2> /dev/null
mv .scratch .temp
echo "Extracting component #$compno ($blocks blocks)"
pushd "$2" > /dev/null 2>&1
cpio -i -m -H newc < ../.$compno.cpio > /dev/null 2>&1
popd > /dev/null 2>&1
rm .$compno.cpio
compno=$((compno+1))
done
rm .temp |
And to rebuild it when you're done modifying it, you just need to do that:
| Code: | # cd irfs-dir
# find . -print | cpio -o -H newc > ../initramfs
# cd ..
(if you want)
# gzip initramfs |
Edit: Added detection of old-style initrd images, that don't need this script as they are just gzipped ext2 filesystems.
Last edited by widan on Tue Feb 07, 2006 11:22 pm; edited 1 time in total |
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Antimatter
Guru
Joined: 11 Aug 2003
Posts: 463
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| Posted: Fri Feb 03, 2006 1:53 am Post subject: |
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| Thanks you! the c code that i linked to in the other thread end up not working at all, so i was about to post that it didn't work when i ran over the above script, and that script works great, so i finally can open the damn initramfs file so i can check out how its built and modify it if needed |
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modified_bessel
Tux's lil' helper
Joined: 16 Jul 2004
Posts: 101
Location: Cote d'Azur, France
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| Posted: Mon Feb 06, 2006 10:31 pm Post subject: |
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| Code: | | blocks=$(cpio -t -H newc < .temp 2>&1 > /dev/null | sed "s/[^0-9]*//g") |
returns a null string; any ideas??
Thanks! |
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widan
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Joined: 07 Jun 2005
Posts: 1512
Location: Paris, France
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| Posted: Mon Feb 06, 2006 11:51 pm Post subject: |
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What does that command return (you will need to add an exit command in the loop so you can get the .temp file) ?
| Code: | | cpio -t -H newc < .temp |
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modified_bessel
Tux's lil' helper
Joined: 16 Jul 2004
Posts: 101
Location: Cote d'Azur, France
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| Posted: Tue Feb 07, 2006 10:11 pm Post subject: |
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Code segment:
| Code: | compno=1
while [[ -s .temp ]]; do
test=$(cpio -t -H newc < .temp) #new
echo "test=$test" #new
exit #new
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Console response:
| Code: | localhost testplace # ./igz-unzipper.sh gentoo.igz irfs-dir
Initramfs is gzip compressed, unpacking first
cpio: premature end of file
test=
localhost testplace # |
wherein gentoo.igz (2472813 bytes) is the initrd image from the 2005.0 CD, irfs-dir is an empty directory in the same working directory. If the initrd is uncompressed into another image file before using the script, the script detects the new file (6746112 bytes) correctly, does not try to unzip it, but gives the same result for the above code(s).
Thanks for your previous prompt reply, and excuse my tardiness! |
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modified_bessel
Tux's lil' helper
Joined: 16 Jul 2004
Posts: 101
Location: Cote d'Azur, France
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| Posted: Tue Feb 07, 2006 11:14 pm Post subject: |
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Addendum....
.... the expansion work file .temp in both cases contains 6746112 bytes, as expected, but clearly not in the required format????
Thanks!
m_b |
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widan
Veteran
Joined: 07 Jun 2005
Posts: 1512
Location: Paris, France
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| Posted: Tue Feb 07, 2006 11:27 pm Post subject: |
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Some older images are really an initrd (compressed ext2 filesystems) and not an initramfs (cpio archive). These don't need this script and can be mounted as loopback filesystems after gunzipping. They can then be modified in place, there is no special procedure to rebuild them (except to remember to unmount the image, and gzip it again if you want).
The one on the 2005.0 x86 minimal LiveCD is clearly an initrd. I edited the post with the script to add a check for that kind of image. |
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modified_bessel
Tux's lil' helper
Joined: 16 Jul 2004
Posts: 101
Location: Cote d'Azur, France
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| Posted: Wed Feb 08, 2006 8:35 am Post subject: |
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Thanks for the very relevant observation; my problem has clearly been working on different CD's, all containing identical file names, but indeed different file types on each!
All the best!
m_b |
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Skandal
n00b
Joined: 28 Aug 2004
Posts: 12
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| Posted: Wed Feb 22, 2006 4:40 pm Post subject: |
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I'm working with the initramfs found on 2005.1-minimal-install-amd64
When I recompress the initramfs using
| Code: | # cd irfs-dir
# find . -print | cpio -o -H newc > ../initramfs
# cd ..
(if you want)
# gzip initramfs |
If I decompress the image using your script again the script no longer lists
several components, just one big one. The kernel panics if I use the image on my install medium.
I've been searching the internet for information about this, but haven't found any other compression method. |
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widan
Veteran
Joined: 07 Jun 2005
Posts: 1512
Location: Paris, France
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| Posted: Wed Feb 22, 2006 5:26 pm Post subject: |
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| Skandal wrote: | | If I decompress the image using your script again the script no longer lists several components, just one big one. |
This is normal. The original initramfs is a concatenation of several cpio archives. I rebuild it as only one cpio archive, but I don't think it matters for the kernel. The method given on the Linux kernel mailing list (this article, step 3) is exactly the same.
| Skandal wrote: | | The kernel panics if I use the image on my install medium. |
What kind of error message(s) do you have (the panic itself, and also error messages that happened just before) ? |
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Skandal
n00b
Joined: 28 Aug 2004
Posts: 12
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| Posted: Wed Feb 22, 2006 9:48 pm Post subject: |
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The kernel wasn't able to find /init the last time
but ofcourse it was all my fault 
I suspect that I must have decompressed the archives the wrong way myself the last time, and recompressed the archieve again.
Now I have a runaway loop modprobe problem instead.However, I have found a new release I didn't know about earlier, so I'm going to retry the procedure with the new release.
Thanks for your help btw=) |
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