[SOLVED] How to properly boot a custom initramfs?

cami · n00b Joined: 15 Jan 2005 Posts: 36

I've been trying to figure out how to boot with initramfs for weeks, but I always end up with a kernel panicking in mount_root. The thing is I don't want it to attempt mounting the (real) root filesystem, I want it to mount initramfs and run its magic. I can't find any documentation on the correct bootloader / kernel command line for initramfs, only old initrd stuff which doesn't work.

- Should I compile initramfs into the kernel or use an external image?
- Do I need "initrd path/to/external/image" in grub.cfg if I compiled initramfs into bzImage?
- Can grub2 load a kernel from a RAID-1 volume?
- What's the correct root= parameter?
- Do I need rootfstype?
- Do I need rootdelay/rootwait?

Final goal is the following setup:
- (OK) BIOS assembles RAID1 mirror "Volume0_0" on SATA disks sdb and sdc (full-disk raid)
- (OK) BIOS loads grub2 from "Volume0"
- (OK?) Grub2 loads kernel from sdb1 (or sdc1 - or Volume0_0p1?)
- (FAIL) Kernel loads initramfs
- (OK*) initramfs assembles /dev/Volume0_0
- (OK*) initramfs mounts /dev/Volume0_0p3 over / using xfs
- (?) initramfs runs /sbin/init

*) only tested using chroot

NeddySeagoon · Posted: Tue Jun 14, 2016 3:47 pm Post subject:

cami,

cami · n00b Joined: 15 Jan 2005 Posts: 36

NeddySeagoon · Posted: Wed Jun 15, 2016 12:06 pm Post subject:

cami,

The init script is normally a shell script.

Building the initrd into the kernel makes for time consuming updates.
To change anything you need to rebuild the kernel.

You will have Intel Storage Manager fakeraid.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

cami · n00b Joined: 15 Jan 2005 Posts: 36

Thanks for the warning. I change the kernel much more frequently than the initramfs, however. It's only there to assemble the RAID.

I finally got initramfs to boot - it seems the /dev/console node needs to be present in initramfs (maybe initramfs was even executed and I just couldn't see it). I set root=/dev/Volume0_0p3 for now.

I know that init is a script normally, but at least for now, I am using an executable so I dont need to install a shell in initramfs (one less source of problems). This might change in the future.
The system gets very far booting, only the final call to mount fails due to lack of /dev/md/Volume0_0p3. I'd expect it to be there when mdadm exits, maybe I need to insert a brief wait or sync?

The messages from initramfs /init don't appear in netconsole, I guess because they are sent directly to the terminal and not to the kernel logger, and using klogctl() instead of printf() will make them appear?

NeddySeagoon · Posted: Wed Jun 15, 2016 7:19 pm Post subject:

cami,

There are three nodes needed in /dev to get started but you get them for free if you choose DEVTMPFS and mount DEVTMPFS in your kernel config.
You need console, null and one other, I forget.

To get net console, you may need a few items on the kernel command line.
Did you find /usr/src/linux/Documentation/networking/netconsole.txt ?
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

cami · n00b Joined: 15 Jan 2005 Posts: 36

Yes, I have netconsole running, I can see all the kernel messages, just not the init messages. Im not sure but I think netconsole only forwards the kernel messages, not process outputs. Thats the reasond for my plan to use klogctl.

Maybe /dev/zero or /dev/u?random. Atm it doesnt seem as if theyre necessary. The default initramfs only contains /dev/console.

szatox · Advocate Joined: 27 Aug 2013 Posts: 3137

cami · n00b Joined: 15 Jan 2005 Posts: 36

Not using genkernel or busybox (yet). The more automation, the more possible sources of oddities. Thanks for the hint anyway, busybox is definitely my #1 candidate for a more standard early init

NeddySeagoon · Posted: Wed Jun 15, 2016 10:36 pm Post subject:

cami,

With full disk raid you cannot use raid metadata 0.9 and kernel autoassembly.
Kernel autoassembly depends on the partition type being fd in the partition table.
With full disk raid, you don't have a partition table, so its not an option for you.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

cami · n00b Joined: 15 Jan 2005 Posts: 36

Thanks for the hint. I think i read that somewhere, too, that's why I went for mdadm / initramfs.

So, funny story, I made it work by accident on Friday evening! As you have certainly noticed, the RAID mirror uses disks sdb and sdc. You might also have guessed already that sda is the "old" system. It might also be worth noting that I installed Grub 0.97 on sda and normally used it for booting the old system. So on Friday, I was running some commands in GRUB2 (which is on the RAID), and noticing that they wouldn't work, decided not to attempt to boot the RAID. I also thought it'd be quicker to boot sda directly from GRUB2 instead of a full reboot. The system came up nicely, but when I tried to mount the RAID it failed - already mounted. What? Well, it appeared that in GRUB2, (hd0) is the FakeRAID, not sda, and I had booted the RAID successfully by accident. Oh, the irony! I'd have noticed that long before if (hd1) would have been sda, but it isn't. It is something weird that looks like sdb/sdc but produces odd kernel behavior.

Lesson learned: don't rely on device numbers. That's why we have labels and UUID. I was planning to use them in a later stage, oh well.

For reference, some additional complications I encountered:

Used the Linux socket API to send error messages as UDP datagrams in addition to printing them out, so I could see them like the netconsole messages. Had to use bind() to enforce the same source port, otherwise netcat only shows the first message.
Can't use the same ports for init messages (socket API) and kernel (netconsole). Well, not really surprising.
Read-write access to the array required mdmon. I decided to add it to the initramfs, because once the main filesystem is mounted the array is already being used and cannot be reassembled.
I added mdmon --takeover --all to /etc/init.d/mdraid. It runs successfully, but the initramfs mdmon doesn't go away. I shrugged and let it live.
Over-mounting / with xfs on /dev/md/Volume0_0p3 failed for reasons I didn't bother to investigate. I was planning to use the switch_root utility from sys-apps/util-linux anyway, as it is specialized for this task and does some other nice things. Now mounting /dev/md/Volume0_0p3 on /mnt and letting switch_root do all the remaining work, resulting in a more standard init process.

If interested, you can find my current init.c and file list for gen_initramfs_list.sh on pastebin. The socket code has already been removed, If anyone wants to see it just poke me. And sorry, I didn't bother to install a shell yet, but an equivalent init script would be: