How to use/test pivot_root?

Question

I want to test the pivot_root command which moves the root file system of the current process to the directory put_old and makes new_root the new root file system.

But I always get the following error:

pivot_root: failed to change root from .' toold-root/': Invalid argument

I use fedora as base root, I have a Archlinux in my home folder

[root@localhost arch-root]# ls
bin boot dev etc home lib lib64 mnt old-root 
opt proc root run sbin srv sys tmp usr var
[root@localhost arch-root]# pivot_root . old-root/
pivot_root: failed to change root from .' toold-root/': Invalid argument

I also try to call linux function pivot_root("/chroot_test", "/chroot_test/old-root"); Got same error.

Any ideas about this ?

Update #1

I also try to test pivot_root in Docker. I mount this arch-root in to Docker container. But get the following error: Operation not permitted

root@00d871ce892b:/# cd test_root/
root@00d871ce892b:/test_root# ls
bin  boot  dev  etc  home  lib  lib64  mnt  old-root  opt  proc  root  run  sbin  srv     sys  test_pivot_root  test_pivot_root.c   tmp  usr  var
root@00d871ce892b:/test_root# pivot_root . tmp/
pivot_root: Operation not permitted

Solution

I have found the solution:

Run docker with --privileged=true

So we can test pivot_root in the docker container.

Answer 1

Are you really sure that arch-root is on a separate filesystem that can be mounted and umounted?

pivot_root, as well as the more current switch_root, works by juggling information about mounted filesystems in the kernel.

The new root file system must be "the root" of a file system, you cannot pass "." as the new root unless "." is the root directory of a mounted filesystem.

I believe the easiest way if you want to try pivot_root from your current setup (assuming arch-root is a subdirectory and not a root directory) is to create a tmpfs filesystem to switch to, and copy the required stuff there.

Something along these lines might get you started: (adjust 500M to fit du -sh arch-root)

mkdir /ramroot
mount -n -t tmpfs -o size=500M none /ramroot
cd arch-root # (containing the root filesystem contents)
find . -depth -xdev -print | cpio -pd --quiet /ramroot
cd /ramroot
mkdir oldroot
pivot_root . oldroot
exec chroot . bin/sh

Answer 2

From the man page, I believe this is your issue:

The following restrictions apply to new_root and put_old:

- They must be directories.

- new_root and put_old must not be on the same file system as the current
root.

- put_old must be underneath new_root, that is, adding a nonzero number of
/.. to the string pointed to by put_old must yield the same directory as
new_root.

- No other file system may be mounted on put_old.

According to the above the same filesystem cannot be used for both the put_old and the new_root.

References

• How to mount final root filesystem after loading initrd?