takeover.sh

A script to completely take over a running Linux system remotely, allowing you to log into an in-memory rescue environment, unmount the original root filesystem, and do anything you want, all without rebooting. Replace one distro with another without touching a physical console.

WARNING WARNING WARNING WARNING

This is experimental. Do not use this script if you don't understand exactly how it works. Do not use this script on any system you care about. Do not use this script on any system you expect to be up. Do not run this script unless you can afford to get physical access to fix a botched takeover. If anything goes wrong, your system will most likely panic.

That said, this script will not (itself) make any permanent changes to your existing root filesystem (assuming you run it from a tmpfs), so as long as you can remotely reboot your box using an out-of-band mechanism, you should be OK. But don't blame me if it eats your dog.

This script does not have any provisions for exiting out of the new environment back into something sane. You will have to reboot when you're done. If you get anything wrong, your machine won't boot. Tough luck.

This is not a guide for newbies. I'm deliberately not giving you commands you can copy and paste. If you can't figure out what to do exactly without handholding, this script is not for you.

Compatibility

This script is designed for systems using sysvinit that support the telinit u command to reload /sbin/init. If your system uses something else, you will have to adapt it, or this might not work at all. You're on your own here.

You should always test this in a VM first. You can grab a tarball of your live root filesystem, extract it into a VM image, get your VM up and running (boot loader setup is left as an exercise for the reader), then try the process there and see if it works. Hint: mount --bind / /mnt will get you a view of your root filesystem on /mnt without any other filesystems that are mounted on top.

Usage

You need to decide on what rescue environment you want. I recommend SystemRescueCD, which comes with many useful tools (you have to loopmount the ISO and then use unsquashfs). Obviously, whatever you pick has to fit into free RAM, with room to spare. If your chosen rescue environment has /lib/modules, you may want to get rid of it to save space, as its kernel modules won't be useful on the host kernel anyway.

1. Create a directory /takeover on your target system and mount a tmpfs on it
2. Extract your rescue environment there. Make sure it works by chrooting into it and running a few commands. Make sure you do not bork filesystem permissions. Exit the chroot.
3. Grab a recent copy of busybox (statically linked) and put it in /takeover/busybox. You can find binaries here. Make sure it works by trying something like /takeover/busybox sh.
4. Copy the contents of this repository into /takeover.
5. Compile fakeinit.c. It must be compiled such that it works inside the takeover environment. If your rescue environment has gcc, you can just compile it inside the chroot: chroot /takeover gcc /fakeinit.c -o /fakeinit. Otherwise, you might want to statically link it.
6. Shut down as many services as you can on your host. takeover.sh will by default set up an SSHd listening on port 80, though you may edit this in the script.
7. Run sh /takeover/takeover.sh and follow the prompts.

If everything worked, congratulations! You may now use your new SSH session to kill any remaining old daemons (kill -9 is recommended to make sure they don't try to do anything silly during shutdown), and then unmount all filesystems under /old_root, including /old_root itself. You may want to first copy /old_root/lib/modules into your new tmpfs in case you need any old kernel modules.

You are now running entirely from RAM and should be able to do as you please. Note that you may still have to clean up LVM volumes (dmsetup is your friend) and similar before you can safely repartition your disk and install Gentoo Linux, which is of course the whole reason you're doing this crazy thing to begin with.

When you're done, unmount all filesystems, then reboot -f or echo b > /proc/sysrq-trigger and cross your fingers.

Further reading

I've been pointed to this StackExchange answer which details how to manually perform a similar process, but using a subset of the existing root filesystem instead of a rescue filesystem. This allows you to keep (a new copy of) the existing init system running, as well as essential daemons, and then go back to the original root filesystem once you're done. This is a more useful version if, for example, you want to resize the original root filesystem or re-configure disk partitions, but not actually install a different distro, and you want to avoid rebooting at all.

takeover.sh could be extended to support re-execing a new init once you're done. This could be used to switch to a new distro entirely without rebooting, as long as you're happy using the old kernel. If you're interested, pull requests welcome :-).