curl > /dev/sda: How I made a Linux distro that runs wget | dd
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Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#2[dead]
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#3From the article:
> The OS may stop you from unmounting /dev/sda1, but it won’t stop you from writing to /dev/sda1 or /dev/sda even if there’s something mounted!
Not always true. There's a kernel config option that allows it. CONFIG_BLK_DEV_WRITE_MOUNTED
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#4Unfortunately 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...
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#5Unfortunately 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…
The second part in the series deals with that by mounting it read-only from initrd.
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#6Unfortunately 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…
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...
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#7> 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.
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#8Reminded 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.
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#9> 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 whe…
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!
Re: curl > /dev/sda: How I made a Linux distro that runs wget | dd
#10> 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 whe…
> 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.
That's what I was expecting from the article.
Update: It's not obvious, but it turns out that this is a multipart article, and kexec is reserved for part 3: https://astrid.tech/2026/03/24/2/how-to-pass-secrets-between...