Making a Gentoo LiveCD - problems with initrd/initramfs

[bob doe]

Hello,

I've been more or less following the Gentoo-wiki on building a LiveCD from scratch available here. The problem with that wiki is that its confusing to go through, and seems to be half-assed because of all the comments put in. It also uses Genkernel. I've been using Gentoo for nearly three years now, and when I first started, I was told Genkernel was the devil, so I never touched it with a 10 foot pole.

This wiki doesnt offer a non-genkernel alternative which is kinda lame, so Im left trying to figure things out on my own. My problem is, my ISO image wont boot. At first I was getting kernel panics because it couldnt mount a file system. Likely the cause of this because I dont have an initrd/initramfs (are these terms the same thing?)

The wiki says to create an initrd by running 'genkernel initrd'. This fails for me as the kernel on my host Gentoo and the kernel Im building the LiveCD with are not the same. And the chroot environment uses the kernel and modules from the host.

I tried to build my own initrd using the contents of /lib/modules/ (650mb) in the chroot but this gives me an initrd of about 250mb. Compare this to other LiveCDs which are max 10mb I've seen. The new initrd I built also hangs my ISO booting at some CPU check.

So alas, my question is. Can anyone clear up this initrd thing for me? What am I supposed to put in it?

Thanks,

[BradN]

Well first of all, be careful with /lib/modules because there might be old modules left over from other kernel versions.

Also, you've got a couple options for getting the kernel/modules built on your host system... I believe you can use INSTALL_MOD_PATH to tell the kernel a different place to put the modules (it uses lib/modules within whatever you specify). Make sure to back up your .config file.

The other option (that might play nicer with genkernel) is making a chroot stage 3 install (just uncompress a stage 3 image somewhere and chroot into it), and install kernel sources in there, and build your livecd kernel/initrd with that. This is probably the way I would do it.

And yeah, initrd / initramfs are the same thing (initrd = init ramdisk)

[nativemad]

Hi

When you are building the kernel for the livecd in the chroot with genkernels help, then it will build the initramfs for you. Just get sure you mounted proc in the chroot!
Something like "genkernel --no-clean --menuconfig all" will do all things needed... the file will then be in /boot/initramfs-genkernel-something.

Over the grub's boot parameters, genkernel get advised to use the CD aus root and will search for it!

As far as i know, initrd isn't exactly the same as initramfs... but its initramfs nowadays with newer kernels. (http://en.wikipedia.org/wiki/Initrd)

There was (sometime ago) an entry in that wiki-page for building a linuxrc-script and the initrd around it from scratch... Maybe you can find it in the history? But genkernel does basically the same thing (and alot more) without hassling!

[bob doe]

I found an old post dating back to 2004 which seems to be the origin to the wiki I posted above. In it, it describes how to create and initramfs without genkernel. I did that and now I get this kernel panic while booting the livecd:

[BradN]

/lib/modules is where the kernel modules are stored, in subfolders by the kernel versions. /lib is for very basic system libraries needed to boot the system. /usr/lib is for system libraries that aren't strictly necessary to get the machine running (things like KDE, X, most extra programs you install put their libraries here). Just to make things more confusing, /usr/lib/modules is where X's drivers used to be stored.

One way of thinking of it, is that /usr has many of the same folders as /, but they're not as essential (this makes it nice to put /usr on a separate partition, because the system can boot initially without it, and then mount it).

Anyway, to your initrd problem... can you post the commands or steps you used to generate it? Especially things relating to how you finally packaged it as a file.

[bob doe]

Sure thing. Here's the entire process A to Z:

[bob doe]

Someone on #gentoo told me to use initramfs rather than initrd and I was able to construct the following commands, I havent tested it yet, but perhaps you can tell me if its worth anything?

[nativemad]

Why don't you just add "lib" in the first line?

Don't you also need a linuxrc script?

[bob doe]

See thats the thing... I dont know Im treading in unknown waters here

I'll add the lib folder to that first line.

[BradN]

Ok, one thing you need to be careful of is the size of the initrd/initramfs/whatever. There's a limit set in the kernel config (4MB by default) that you'll need to change (it's found in device drivers > block) to make the first example you posted work. I know that limit applies to initrd, but I'm not sure about cpio initramfs. Also, pay attention to the filesystem used (ext2 in the first example, cpio archive in the 2nd) - so for the first case, you will need to make sure ext2 filesystem support is built into the kernel (not as a module), for cpio, I'm not sure what if any options are needed.

Personally I like using squashfs if the ramdisk doesn't need to be writable, since this offers compression for the initrd even after it's loaded, and reduces memory pressure.

And as an added bonus, here some appropriate reading material, straight from the kernel's Documentation/early-userspace/README:

[bob doe]

I had my kernel set to allocate 8192 for the initrd, which is 1024*8

Did my original commands (the red ones) fit the bill though or did I miss something?

[BradN]

[bob doe]

Well I created the ext2 initrd and gziped it and got a kernel panic this time saying 'no cpio magic' which is kinda retarded. I guess initramfs is the only thing I can do. Or so I was told in #gentoo.

[BradN]

Don't gzip the initrd. What does your grub.conf look like for this?

[bob doe]

Its actually menu.lst but it looks like this:

[BradN]

Weird... here's a working grub config and script to generate the initrd...

[bob doe]

Hmm I'll give yours a try now. I had started with the initramfs as above (made a few corrections)

[BradN]

I would think init=/linuxrc would make more sense because it's relative to the position inside the initrd, not where the initrd itself is located.

[bob doe]

I tried the method you suggested with the initrd and squashfs. The system hangs trying to load the kernel though, right after Grub.

Editing the boot line for the kernel and changing to init=/linuxrc still gives the same kernel panic error. Though I noticed two other failures that I didnt notice before (though likely they occurred before too)

[BradN]

Hmm, I can't think of many other suggestions unless you want to look at the full "recovery partition" project I've got... probably somewhere in the neighborhood of 10MB. BTW, I strongly recommend using qemu to debug boot CDs... it beats burning and rebooting and all of that nonsense. In fact, it will load a kernel and initrd straight without even using grub, etc.

[bob doe]

I just toss the ISO onto an SD card, then plug it into the back of my desktop, and fire up VMware. Heh, no way I'd be wasting disks on this!

Just an off-topic question.. the command update-etc. My response is usually always -5 (as in, copy all updated configs). Now in my scripts, they hang until I press -5 [enter]. Is there a way to add that value into the script? I tried 'update-etc -5' but it gave me an invalid option error.

[BradN]

echo -5 | etc-update

is probably what you want

Also, I can extract the kernel config from the working kernel from this project if you'd like. However, it's very stripped down and intended to be non-modular.

[bob doe]

Sure I can always take a look at it.

Curious would bad file permissions or bad scripting in either of /boot/initramfs/init and /boot/initramfs/linuxrc be the cause of my kernel panic?

[nativemad]

[bob doe]

That test fails.

Here is an output of the /boot directory structure: