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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 must be a mount point.  (If it is not otherwise a mount  point,  it  suffices  to
       bind mount new_root on top of itself.)

       The propagation type of new_root and its parent mount must not be MS_SHARED; similarly, if
       put_old is an existing mount point, its propagation type must not be MS_SHARED.

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 new_root is not a mount point.

       EINVAL put_old is not underneath new_root.

       EINVAL The current root is on the rootfs (initial ramfs) filesystem.

       EINVAL Either the mount point at new_root, or the parent mount of that  mount  point,  has
              propagation type MS_SHARED.

       EINVAL put_old is a mount point and has the propagation type MS_SHARED.

       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).

       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).

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), mount(2), stat(2), initrd(4), pivot_root(8), switch_root(8)

COLOPHON

       This  page  is  part of release 5.02 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 https://www.kernel.org/doc/man-pages/.