PIVOT_ROOT(2)                       Linux Programmer's Manual                       PIVOT_ROOT(2)



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  tempo‐
       rary  root filesystem (e.g., an initrd), then mounts the real root filesystem, and eventu‐
       ally 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 work‐
       ing directory of each process or thread to new_root if they point to the old  root  direc‐
       tory.   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 mecha‐
       nism 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.

SEE ALSO
       chdir(2), chroot(2), stat(2), initrd(4), pivot_root(8)

COLOPHON
       This page is part of release 3.74 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/.



Linux                                       2012-07-13                              PIVOT_ROOT(2)