Ubuntu Manpage: pivot_root - change the root filesystem

NAME

       pivot_root - change the root filesystem

SYNOPSIS

       int pivot_root(const char *new_root, const char *put_old);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

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 (/).

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       pivot_root()  may  return (in errno) any of the errors returned by stat(2).  Additionally,
       it may return:

       EBUSY  new_root or put_old are on the current root filesystem, or a filesystem is  already
              mounted on put_old.

       EINVAL put_old is not underneath new_root.

       ENOTDIR
              new_root or put_old is not a directory.

       EPERM  The calling process does not have the CAP_SYS_ADMIN capability.

VERSIONS

       pivot_root() was introduced in Linux 2.3.41.

CONFORMING TO

       pivot_root() is Linux-specific and hence is not portable.

NOTES

       Glibc does not provide a wrapper for this system call; call it using syscall(2).

BUGS

       pivot_root()  should  not  have  to change root and current working directory of all other
       processes in the system.

       Some of the more obscure uses of pivot_root() may quickly lead to insanity.

COLOPHON

       This page is part of release 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.