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NAME

       pivot_root - change the root file system

SYNOPSIS

       #include <linux/unistd.h>
       #include <errno.h>

       _syscall2(int,pivot_root,const char *,new_root,const char *,put_old)

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

DESCRIPTION

       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.

NOTES

       new_root does not have to be a mount point. In this case,  /proc/mounts
       will  show  the  mount  point of the file system 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 file  system,  or  a
              file system 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  current process does not have the CAP_SYS_ADMIN capability.

BUGS

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

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

CONFORMING TO

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

HISTORY

       pivot_root() was introduced in Linux 2.3.41.

SEE ALSO

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