pivot_root manpage

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

       pivot_root - change the root filesystem

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

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

       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 sys-
       tem mounts a temporary root filesystem (e.g., an initrd),  then	mounts
       the real root filesystem, and eventually turns the latter into the cur-
       rent 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 recom-
       mended 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  cur-
	  rent root.

       -  put_old  must be underneath new_root, that is, adding a nonzero num-
	  ber 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

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

       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.

	      new_root or put_old is not a directory.

       EPERM  The calling process does not have the CAP_SYS_ADMIN capability.

       pivot_root() was introduced in Linux 2.3.41.

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

       Glibc does not provide a wrapper for this system call;  call  it	 using

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

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

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

       This  page  is  part of release 4.10 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

Linux				  2012-07-13			 PIVOT_ROOT(2)