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

       pivot_root - change the root mount

       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() changes the root mount in the mount namespace of the call-
       ing process.  More precisely, it moves the root mount to the  directory
       put_old  and  makes  new_root  the new root mount.  The calling process
       must have the CAP_SYS_ADMIN capability in the user namespace that  owns
       the caller's mount namespace.

       pivot_root()  changes the root directory and the current working direc-
       tory of each process or thread in the same mount namespace to  new_root
       if  they  point  to  the old root directory.  (See also NOTES.)  On the
       other hand, pivot_root() does not change the caller's  current  working
       directory  (unless it is on the old root directory), and thus it should
       be followed by a chdir("/") call.

       The following restrictions apply:

       -  new_root and put_old must be directories.

       -  new_root and put_old must not be on the same mount  as  the  current

       -  put_old must be at or underneath new_root; that is, adding some non-
          negative number of "/.." prefixes to  the  pathname  pointed  to  by
          put_old must yield the same directory as new_root.

       -  new_root  must be a path to a mount point, but can't be "/".  A path
          that is not already a mount point can be converted into one by  bind
          mounting the path onto itself.

       -  The  propagation type of the parent mount of new_root and the parent
          mount of the current root directory must  not  be  MS_SHARED;  simi-
          larly,  if  put_old is an existing mount point, its propagation type
          must not be MS_SHARED.  These restrictions ensure that  pivot_root()
          never propagates any changes to another mount namespace.

       -  The current root directory must be a mount point.

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

       pivot_root() may fail with any of the same errors  as  stat(2).   Addi-
       tionally, it may fail with the following errors:

       EBUSY  new_root  or  put_old is on the current root mount.  (This error
              covers the pathological case where new_root is "/".)

       EINVAL new_root is not a mount point.

       EINVAL put_old is not at or underneath new_root.

       EINVAL The current root directory is not a mount point (because  of  an
              earlier chroot(2)).

       EINVAL The  current  root  is  on the rootfs (initial ramfs) mount; see

       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.

              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

       A  command-line  interface  for  this  system  call  is   provided   by

       pivot_root() allows the caller to switch to a new root filesystem while
       at the same time placing  the  old  root  mount  at  a  location  under
       new_root  from  where it can subsequently be unmounted.  (The fact that
       it moves all processes that have a root directory  or  current  working
       directory  on the old root directory to the new root frees the old root
       directory of users, allowing the old root mount to  be  unmounted  more

       One  use  of  pivot_root()  is  during  system startup, when the system
       mounts a temporary root filesystem (e.g., an  initrd(4)),  then  mounts
       the real root filesystem, and eventually turns the latter into the root
       directory of all relevant processes and threads.  A modern  use  is  to
       set up a root filesystem during the creation of a container.

       The  fact  that  pivot_root() modifies process root and current working
       directories in the manner noted in DESCRIPTION is necessary in order to
       prevent  kernel threads from keeping the old root mount busy with their
       root and current working directories, even if  they  never  access  the
       filesystem in any way.

       The  rootfs  (initial ramfs) cannot be pivot_root()ed.  The recommended
       method of changing the root filesystem in this case is to delete every-
       thing  in rootfs, overmount rootfs with the new root, attach stdin/std-
       out/stderr to the new /dev/console, and exec the new  init(1).   Helper
       programs for this process exist; see switch_root(8).

   pivot_root(".", ".")
       new_root  and  put_old  may  be the same directory.  In particular, the
       following sequence allows a pivot-root  operation  without  needing  to
       create and remove a temporary directory:

           pivot_root(".", ".");
           umount2(".", MNT_DETACH);

       This  sequence  succeeds  because  the pivot_root() call stacks the old
       root mount point on top of the new root mount  point  at  /.   At  that
       point,  the calling process's root directory and current working direc-
       tory refer to the new root mount point (new_root).  During  the  subse-
       quent  umount()  call, resolution of "."  starts with new_root and then
       moves up the list of mounts stacked at /, with the result that old root
       mount point is unmounted.

   Historical notes
       For many years, this manual page carried the following text:

              pivot_root() may or may not change the current root and the cur-
              rent 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().

       This  text,  written before the system call implementation was even fi-
       nalized in the kernel, was probably intended to warn users at that time
       that  the  implementation  might change before final release.  However,
       the behavior stated in DESCRIPTION has remained consistent  since  this
       system call was first implemented and will not change now.

       The  program  below demonstrates the use of pivot_root() inside a mount
       namespace that is created using clone(2).  After pivoting to  the  root
       directory named in the program's first command-line argument, the child
       created by clone(2) then executes the program named  in  the  remaining
       command-line arguments.

       We  demonstrate  the program by creating a directory that will serve as
       the new root filesystem and placing a copy of the  (statically  linked)
       busybox(1) executable in that directory.

           $ mkdir /tmp/rootfs
           $ ls -id /tmp/rootfs    # Show inode number of new root directory
           319459 /tmp/rootfs
           $ cp $(which busybox) /tmp/rootfs
           $ PS1='bbsh$ ' sudo ./pivot_root_demo /tmp/rootfs /busybox sh
           bbsh$ PATH=/
           bbsh$ busybox ln busybox ln
           bbsh$ ln busybox echo
           bbsh$ ln busybox ls
           bbsh$ ls
           busybox  echo     ln       ls
           bbsh$ ls -id /          # Compare with inode number above
           319459 /
           bbsh$ echo 'hello world'
           hello world

   Program source

       /* pivot_root_demo.c */

       #define _GNU_SOURCE
       #include <sched.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <sys/wait.h>
       #include <sys/syscall.h>
       #include <sys/mount.h>
       #include <sys/stat.h>
       #include <limits.h>
       #include <sys/mman.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static int
       pivot_root(const char *new_root, const char *put_old)
           return syscall(SYS_pivot_root, new_root, put_old);

       #define STACK_SIZE (1024 * 1024)

       static int              /* Startup function for cloned child */
       child(void *arg)
           char **args = arg;
           char *new_root = args[0];
           const char *put_old = "/oldrootfs";
           char path[PATH_MAX];

           /* Ensure that 'new_root' and its parent mount don't have
              shared propagation (which would cause pivot_root() to
              return an error), and prevent propagation of mount
              events to the initial mount namespace */

           if (mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) == 1)

           /* Ensure that 'new_root' is a mount point */

           if (mount(new_root, new_root, NULL, MS_BIND, NULL) == -1)

           /* Create directory to which old root will be pivoted */

           snprintf(path, sizeof(path), "%s/%s", new_root, put_old);
           if (mkdir(path, 0777) == -1)

           /* And pivot the root filesystem */

           if (pivot_root(new_root, path) == -1)

           /* Switch the current working directory to "/" */

           if (chdir("/") == -1)

           /* Unmount old root and remove mount point */

           if (umount2(put_old, MNT_DETACH) == -1)
           if (rmdir(put_old) == -1)

           /* Execute the command specified in argv[1]... */

           execv(args[1], &args[1]);

       main(int argc, char *argv[])
           /* Create a child process in a new mount namespace */

           char *stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE,
                              MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
           if (stack == MAP_FAILED)

           if (clone(child, stack + STACK_SIZE,
                       CLONE_NEWNS | SIGCHLD, &argv[1]) == -1)

           /* Parent falls through to here; wait for child */

           if (wait(NULL) == -1)


       chdir(2), chroot(2), mount(2), stat(2), initrd(4), mount_namespaces(7),
       pivot_root(8), switch_root(8)

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       description  of  the project, information about reporting bugs, and the
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Linux                             2019-11-19                     PIVOT_ROOT(2)
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