pivot_root() moves the root filesystem of the calling process to the directory put_old and makes new_root the new root filesystem of the calling process.
The typical use of pivot_root() is during system startup, when the system mounts a temporary root filesystem (e.g., an initrd), then mounts the real root filesystem, and eventually turns the latter into the current root of all relevant processes or threads.
pivot_root() may or may not change the current root and the current working directory of any processes or threads which use the old root directory. The caller of pivot_root() must ensure that processes with root or current working directory at the old root operate correctly in either case. An easy way to ensure this is to change their root and current working directory to new_root before invoking pivot_root().
The paragraph above is intentionally vague because the implementation of pivot_root() may change in the future. At the time of writing, pivot_root() changes root and current working directory of each process or thread to new_root if they point to the old root directory. This is necessary in order to prevent kernel threads from keeping the old root directory busy with their root and current working directory, even if they never access the filesystem in any way. In the future, there may be a mechanism for kernel threads to explicitly relinquish any access to the filesystem, such that this fairly intrusive mechanism can be removed from pivot_root().
Note that this also applies to the calling process: pivot_root() may or may not affect its current working directory. It is therefore recommended to call chdir("/") immediately after pivot_root().
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 filesystem 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 filesystem may be mounted on put_old.
See also pivot_root(8) for additional usage examples.
If the current root is not a mount point (e.g., after chroot(2) or pivot_root(), see also below), not the old root directory, but the mount point of that filesystem is mounted on put_old.
new_root does not have to be a mount point. In this case, /proc/mounts will show the mount point of the filesystem containing new_root as root ( /).
Glibc does not provide a wrapper for this system call; call it using syscall(2).
The rootfs (initial ramfs) cannot be pivot_root()ed. The recommended method of changing the root filesystem in this case is to delete everything in rootfs, overmount rootfs with the new root, attach stdin/stdout/stderr to the new /dev/console, and exec the new init(1). Helper programs for this process exist; see switch_root(8).