That part of the manpage is misleading. Generally you want a different ordering, as described in man 8 pivot_root
.
cd new_root # chdir(new_root);
pivot_root . put_old # pivot_root(".", put_old);
exec chroot . # chroot(".");
This seems to be yet another subtle detail with pivot_root
. Although the point of pivot_root
is to rearrange the mount namespace, the kernel code seems to say that the root filesystem that it moves is determined by the per-process root, which is what chroot
sets.
As a result, we hit the error "new_root or put_old are on the current root filesystem".
This subtle detail of pivot_root
is necessary in order for it to work at all on modern Linux. If it was defined to work on the root mount of the mount namespace, it would try to move the special rootfs
filesystem which you normally can't see. But this is not allowed, because rootfs must always be the root mount of the namespace.
We can confirm pivot_root
works this way, by continuing the example as follows.
# unshare -m
# mount --bind / /mnt
# cd /mnt
# chroot /mnt
# pivot_root . mnt
pivot_root: failed to change root from `.' to `mnt': Device or resource busy
# exit # leave chroot
# mount --bind . mnt
# cd mnt
# mount --bind /proc proc
# findmnt | grep mnt
└─/mnt /dev/mapper/alan_dell_2016-fedora ext4 rw,relatime,seclabel
└─/mnt /dev/mapper/alan_dell_2016-fedora ext4 rw,relatime,seclabel
└─/mnt/proc proc proc rw,nosuid,nodev,noexec,relatime
# chroot /mnt # re-enter chroot
# cd /mnt
# pivot_root . mnt # this one works
# exit # leave chroot
# findmnt | grep mnt
└─/mnt /dev/mapper/alan_dell_2016-fedora ext4 rw,relatime,seclabel
├─/mnt/mnt /dev/mapper/alan_dell_2016-fedora ext4 rw,relatime,seclabel
└─/mnt/proc /dev/mapper/alan_dell_2016-fedora[/proc] ext4 rw,relatime,seclabel
The second pivot_root
call works. But it didn't have any effect on the root of the mount namespace. Looking from outside the chroot
, it swapped /mnt
and /mnt/mnt
.