pivot_root(2) — Linux manual page


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

NAME         top

       pivot_root - change the root mount

SYNOPSIS         top

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

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

DESCRIPTION         top

       pivot_root() changes the root mount in the mount namespace of the
       calling 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
       directory 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
          nonnegative 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;
          similarly, 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

       -  The current root directory must be a mount point.

RETURN VALUE         top

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

ERRORS         top

       pivot_root() may fail with any of the same errors as stat(2).
       Additionally, 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

              new_root or put_old is not a directory.

       EPERM  The calling process does not have the CAP_SYS_ADMIN

VERSIONS         top

       pivot_root() was introduced in Linux 2.3.41.

CONFORMING TO         top

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

NOTES         top

       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 easily.)

       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
       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).

   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
       directory refer to the new root mount point (new_root).  During the
       subsequent 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
              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().

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

EXAMPLES         top

       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)


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.07 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                            2020-06-09                    PIVOT_ROOT(2)

Pages that refer to this page: chroot(2)mount(2)syscalls(2)initrd(4)capabilities(7)mount_namespaces(7)pivot_root(8)