Re: pivot_root alternative

From: Kyle Moffett
Date: Sun Jun 24 2007 - 19:29:53 EST


On Jun 24, 2007, at 17:27:34, Salvatore De Paolis wrote:
i built the kernel with a busybox initramfs which runs from a usb stick. While i boot i need to mount the real root and i'm using pivot_root. Pivot_root . old-root return to me an "Invalid argument" error and googling i found here http://lkml.org/lkml/ 2005/10/8/17 that pivot_root should be avoided. I want to ask now what i should do to switch to a real root?

One interesting fact about the mount() syscall is this: when overmounting a directory tree, a process whose current working directory is within said directory tree will see the old vfsmounts until it does an absolute chdir(). Shells tend not to be terribly useful for this because they use absolute paths more than necessary, but I think you can make it work. For instance, if you run these commands after mounting the root filesystem:

mount -t proc proc /realroot/proc
cd /
mount --move /realroot /
find /proc/$$/cwd/ -mindepth 1 -maxdepth 1 -print0 \
| xargs -0 rm -rf -- '{}' ';'
cd /

This will set up proc on the new root filesystem, move /realroot to the root directory, and delete the contents of the old initramfs using the old current-working-directory link of your shell. The amusing thing is that you are using the "find" and "rm" commands from the newly mounted root filesystem to remove the contents of the completely hidden initramfs. The only reason it's actually possible is because you have the tenuous thread of the working directory.

Cheers,
Kyle Moffett

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