Unfortunately it's not safe as the kernel can still write to (what it thinks is) the old filesystem on the device, which will introduce corruption to the new disk image.
However a fun fact is that you can (do not actually do this!) boot a qemu VM from /dev/sda. You have to use an overlay (eg. qemu -drive snapshot=on flag) so that qemu won't write through to /dev/sda. I use this trick in supernested, a script I wrote that runs nested within nested within nested VMs ad infinitum until your hypervisor crashes. http://git.annexia.org/?p=supernested.git;a=blob;f=run-super...
I used to dual-boot windows, but I was too lazy to actually reboot, so naturally I had Virtualbox just boot the physical Windows partition while Linux was running. Which is totally fine!
It's not a real dual boot if you don't boot both partitions at the same time.
As long as you don't install guest VBox drivers, those would make it hang when it boots as the host on physical hardware, since there's no longer someone above to answer the hypercalls.
I think Windows refused to do that at some point? So I booted the physical Linux partition from Windows if I needed both at the same time. That's on a laptop that otherwise almost always ran Linux.
Yeah. That is a valid use. I mean, this is how I installed Windows to begin with, from Linux via QEMU, onto my other hard drive. I did reboot and test it out, and it worked just fine.
What if we remount the filesystem(s) at /dev/sda as read-only first? Then make a small ramfs with statically-linked curl in it and exec it. Hmm. Ideally, you'd also want to call reboot(2) after it's done...
One bit of magic you may be interested in is pivot_root, which allows another filesystem to take the place of the root filesystem (e.g. / and /mnt become /old and /). It's usually used during startup, to allow the "real" root filesystem to take the place of the initrd, but could have other uses.
Last time I tried to use it though I just could not get it to let go of the main filesystem even after repeatedly killing the processes I could and restarting the rest.
Taking control at the initrd stage, as in the second page of the article, is significantly more reliable.
But have busybox in your initrd so you don't have to suffer. It takes up 0.5% of the size of my initrd file.
You also don't want to do this under any kind of memory pressure, because the kernel will happily drop read-only pages from memory if it thinks they can be re-read from disk when needed.
Oh, I see, the posts got published in the reversed order.
On the topic itself: wow, what a journey. And I personally fully support "come on, you should totally be able to just dump the system image onto your disk and reboot/exec it!"
in most cases you could just drop back into the initramfs that is included in most distros
Or if you have access to the boot command line you can also usually stop the boot process before pivot_root happens (hence you’ll be left running in the initramfs environment)
On Fedora/EL it would be done by putting `rd.break` in the kernel command line
depending on the size of your disk image and your uefi+boot partitions it's still possible to safely pull off.
unmount the efi and boot partitions, write your image to the head of the disk, power cycle, then grow the last filesystem from the image to cover the rest of the disk.
you might get lucky and have all three of uefi/boot/swap to work with.
of course with the advent of uefi, you could instead just drop an installer image directly into the efi parition and boot that.
To be honest, even this has plenty of room to go down. I get the feeling I could have squeezed a couple more MB off if I had actually cut things off of the default Nixpkgs busybox, and possibly also cut a couple of kernel drivers out.
Reminds me of the first company I worked for out of school.
We had a big drive with the source of truth image used to boot all our machines on it, and we added rsync to the init image. When each machine booted init would rsync everything from the storage box to the local machine. We'd keep the storage machine up to date and when we wanted to update other machines in the fleet we'd just do a reboot and it would sync up the latest files (provisioning for whatever each machine was supposed to do happened later, can't remember how that was handled now). The storage machine was running ZFS so we also took a snapshot before doing any rolling reboots, so if anything did go wrong you could just revert to the previous snapshot and reboot again as long as you didn't break the init image.
Sounds jank saying it out loud, but I don't remember it ever causing us any problems.
> How do you unmount your OS’s disk while keeping the OS running to be able to overwrite itself?
I went down a similar rabbit-hole myself, with the goal of safely replacing the Linux installation on a disk that a machine is already running from (e.g. replace a VPS's setup image with one of your own) without needing a KVM-style remote access tool to the console.
The problem there is if you directly modify the disk when a filesystem is mounted on that disk then all bets are off in terms of corruption of the filesystem that's already on there and also the filesystem(s) you're writing over the top.
My solution was to kexec into a new kernel+initramfs which has a DHCP client and cURL in it - that effectively stops any filesystem access while the image is being written over the disk, then to just reboot.
> My solution was to kexec into a new kernel+initramfs which has a DHCP client and cURL in it - that effectively stops any filesystem access while the image is being written over the disk, then to just reboot.
I usually just move all the files to a new directory (/oldroot) and pivot_root -- any open files reference the new paths. Then install into the newly empty root directory of the filesystem, reboot and delete the /oldroot.
Don't you get any errors even if you race immediately to start pivot_root? pivot_root also won't modify all open file descriptors at once. Seems it's not fatal, but have you managed to do this over ssh and not be disconnected?
That sounds like the best way if keeping the filesystem is an option. In my case I wanted to also change filesystems and apply FDE, which is possible to do if the original filesystem supports online shrinking but many do not.
The gymnastics VPS providers force people to go through just so they can have some dumb "wizard" with a limited number of OS choices is maddening. Just allow people to upload an ISO!
I think it should be possible to make an image with many headers at different locations, so that it works on all types of disks at once, but I don't think any tools do it for you by default.
This reminds me of netbooting workflows from things like MaaS, Tinkerbell, and Dan's old Plunder tool.
They'd netboot.. not mount the disks, then download an ISO/IMG and write it directly to the primary boot disk.
If netbooting is a heavy lift, why not boot into a custom initramfs you built, with i.e. dd/curl installed, and flash the disk that way, without mounting / at all? Then kexec/chroot into it?
I'd much prefer this as a way to provision Raspberry Pis.
It's worth noting, though, that that config option was only introduced in kernel version 6.8! Before then the option didn't exist and you could write with impunity to mounted devices (as root, obviously).
If you have a swap partition, swapoff it and install there. Or at least a minimal kernel and initramfs. Set as default in grub and there you go.
Also, I once burned an iso straight from ftp using a fifo. I was low on disk space and really needed that CD. Worked fine because the Internet was already faster than the CDR.
Reminded me of how to install Alpine linux (which isn't available) on Oracle cloud over an ubuntu install. It uses dd and has the advantage of having a console.
I had found it in a github gist when I used it but here's a similar blog post.
Wait hold on, can you not simply just access the underlying volume/block device using an API? The VMs in OCI have a boot volume that is attached, so I reckon it's possible to "mount" this somehow and overwrite it with whatever data you want.
I am not sure. Maybe it's a thing about not being able to download the iso (no network on the console?) or not having space for it or something. I wouldn't know about the API thing. I am not a cloud user.
From what it sounds like, because you have a console and therefore aren't dependent on SSHD not getting overwritten, you can just dd the live running system here?
Like I said in the other comment I am not really familiar with the various clouds. I self host. The console is a weird web based thing that isn't the same environment as your VMs (or their hypervisor). It's a barebones shell where you can mount your volumes and such, not actually boot or enter the vm. (Edit: This was incorrect) And if I remember correctly I probably did have to do some mounting there to create device files and such. I didn't really have much use for the console after that either. I will try to find the actual gist I followed.
you may be in a restricted environment with no boot option selections, like on some VPS and dedi server providers.
i've seen similar techniques used to shove windows on "linux" VPS/dedis boxes by booting into rescue mode and then applying a raw Windows boot image that's preconfigured and rebooting back to the Windows install and hoping you stood the image up right.
good ol' days of getting Windows up on Kimsufi boxen.
Instead of applying some sense to the problem, and using a solution that actually allows you to kill all running processes of the original distro at runtime, incl. getting rid of the original init process, to be able to pivot_root somewhere else amd umount the original system's filesystems and free the block device for re-installation, this ridiculous approach gets promoted to a front page, lol.
I've been dd-ing A/B partitions for embedded yocto distributions for years and years. read-only-rootfs (/var/log is its own writable partition), dd the "other partition", sed fstab, reboot.
The neat part was the whole process kicked off when you scp'd the rootfs and inotifywait kicked off the whole process.