pivot_root moves the root file system of the current process to the
directory put_old and makes new_root the new root file system of
the current process.
The typical use of pivot_root is during system startup, when the system
mounts a temporary root file system (e.g. an initrd), then mounts the
real root file system, 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 (cwd) of any processes or threads which use the old root directory.
The caller of pivot_root must ensure that processes with root or cwd at
the old root operate correctly in either case. An easy way to ensure this is
to change their root and cwd to new_root before invoking
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 cwd 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 cwd, even if they never access the file system in any way. In
the future, there may be a mechanism for kernel threads to explicitly
relinquish any access to the file system, such that this fairly intrusive
mechanism can be removed from pivot_root.
Note that this also applies to the current process: pivot_root may or may
not affect its cwd. 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
file system as the current root.
put_old must be underneath new_root, i.e.
adding a non-zero 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.
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 file system is mounted on put_old.