pivot_root - change the root file system


SYNOPSIS

       #include <linux/unistd.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 pro­
       cess 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  sys­
          tem as the current root.
          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 administrator
              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