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 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 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.