change initramfs by hand?

frankie06 · #1 (**permalink**) 12-14-2008

What's the correct way to change the initramfs file that's used during boot?
I know that it's a gzipped cpio archive, but when I gunzip, extract, re-archive (without changing any files), and gzip, then the result is that the system does not boot any more. And I even set the cpio archive type.

Do I need special tools for the initramfs? There are progs like update-initramfs but the point is that I need to put in files that were *not* put in by the tools that originally created the initramfs.

otheus · #2 (**permalink**) 12-15-2008

Not special tools, per se... First you gunzip it, like you said. What method did you use to extract/archive? Usually, one would do:

Code:

mkdir /tmp/initrd
cd /tmp/initrd
gunzip -c Initrd.img.gz | cpio -id 

# make changes to /tmp/initrd directory
find . | cpio -o | gzip -c >/boot/initrd.new.img

frankie06 · #3 (**permalink**) 12-15-2008

Thanks otheus! I tried that (without making changes to the files in the /tmp/initrd directory), and I changed the link in /boot to point to initrd.new.img.
Now I get the following error during boot:

Code:

RAMDISK: Compressed image found at block 0
RAMDISK: incomplete write (-28 != 32768) 8388608
List of all partitions:
No filesystem could mount root, tried:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

And there the computer hangs. Not even Ctrl-Alt-Del reboots it. Poweroff is needed.

otheus · #4 (**permalink**) 12-15-2008

The method I described works as of Linux 2.6.13 (or before FC3) If you are using an older version (uname -k) perhaps you have to create a filesystem. See "Manually building a custom initial RAM disk" at Linux initial RAM disk (initrd) overview.

Code:

$ mkdir /tmp/initrd.tmp ; cd /tmp/initrd.tmp
$ <extract>
$ du -sk . 
$ # use size reported above for: $blocks
$ dd if=/dev/zero of=/tmp/initrd.img bs=1024 count=$blocks
$ /sbin/mke2fs -F -m 0 -b 1024 /tmp/initrd.img $blocks
$ mount /tmp/initrd.img /mnt/initrd -t ext2 -o loop=/dev/loop0

If that works, copy the files from /tmp/initrd.tmp into /mnt/initrd. Afterwards:

Code:

$ umount /mnt/initrd
$ gzip /tmp/initrd.img

Now move it to /boot. Good luck!

frankie06 · #5 (**permalink**) 12-15-2008

Thanks for the help otheus. I appreciate.

Quote:

Originally Posted by otheus

The method I described works as of Linux 2.6.13 (or before FC3) If you are using an older version (uname -k) perhaps you have to create a filesystem.

I have kernel 2.6.26.

The ibm web page (initrd overview) is really well made, but it's partially obsolete (as they say themselves: "up to FC3"; it's also 2.5 years old).

I tried the code you suggested anyway; every step of the initrd creation process works well, but the system won't boot. With both methods (this and the one you suggested earlier) I get the following error during boot:

Code:

RAMDISK: Compressed image found at block 0
RAMDISK: incomplete write (-28 != 32768) 8388608
List of all partitions:
No filesystem could mount root, tried:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

Which is probably an improvement over the previous error message which I got before posting here for the first time:

Code:

Failed to execute /init
Kernel panic - not syncing: No init found. Try passing init= option to kernel.

So I guess I'm closer, but not there yet...

otheus · #6 (**permalink**) 12-16-2008

OK, so a modern kernel, so the first method should be enough.

Could you post your grub.conf file please?

frankie06 · #7 (**permalink**) 12-16-2008

Quote:

Originally Posted by otheus

Could you post your grub.conf file please?

Sure, but I don't think the grub.conf matters too much; it boots the original initrd just fine, and I don't modify any grub files when trying the new initrd.

BTW where is grub.conf? The only grub files I see are in /boot/grub, and the only human-readable ones are "default", "device.map" and "menu.lst". The first is empty except comments, the second only contains the lines

(hd0) /dev/hda
(hd1) /dev/sda

but seems to be unimportant as the unmodified initrd boots just the same when switching this to

(hd0) /dev/sda
(hd1) /dev/hda

and the third contains the usual kernel and initrd config, but it boots the unmodified initrd fine and I never modify it. Instead I change the link initrd.img in /boot to point to the modified initrd. There's still a grub entry using an initrd called initrd.img-2.6.26-1-686 which I never modify, and that always boots. Given that initrd.img-2.6.26-1-686 and the unmodified initrd.img are binary equal, the file menu.lst probably isn't the problem either.

Something else occured to me: do I need to specify the --format option in cpio? E.g. change from "bin" to "newc" format? Because when I look at the (gunzipped) initrd in boot with "file", it says:

"ASCII cpio archive (SVR4 with no CRC)".

But this is not the default which cpio creates.

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