Section (2) pivot_root


Linux manual pages Section 2  

Name

pivot_root — change the root mount

Synopsis

int pivot_root( const char *new_root,
  const char *put_old);
 
[Note] Note
There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

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_zsingle_quotesz_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_zsingle_quotesz_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 root.

  • 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_zsingle_quotesz_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 namespace.

  • The current root directory must be a mount point.

RETURN VALUE

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

ERRORS

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 NOTES.

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.

ENOTDIR

new_root or put_old is not a directory.

EPERM

The calling process does not have the CAP_SYS_ADMIN capability.

VERSIONS

pivot_root() was introduced in Linux 2.3.41.

CONFORMING TO

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

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2).

A command-line interface for this system call is provided by pivot_root(8).

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:

chdir(new_root);
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_zsingle_quotesz_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. However, the behavior stated in DESCRIPTION has remained consistent since this system call was first implemented and will not change now.

EXAMPLE

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_zsingle_quotesz_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=_zsingle_quotesz_bbsh$ _zsingle_quotesz_ 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 _zsingle_quotesz_hello world_zsingle_quotesz_
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 _zsingle_quotesz_new_root_zsingle_quotesz_ and its parent mount don_zsingle_quotesz_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)
        errExit(mount−MS_PRIVATE);

    /* Ensure that _zsingle_quotesz_new_root_zsingle_quotesz_ is a mount point */

    if (mount(new_root, new_root, NULL, MS_BIND, NULL) == −1)
        errExit(mount−MS_BIND);

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

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

    /* And pivot the root filesystem */

    if (pivot_root(new_root, path) == −1)
        errExit(pivot_root);

    /* Switch the current working directory to / */

    if (chdir(/) == −1)
        errExit(chdir);

    /* Unmount old root and remove mount point */

    if (umount2(put_old, MNT_DETACH) == −1)
        perror(umount2);
    if (rmdir(put_old) == −1)
        perror(rmdir);

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

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

int
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)
        errExit(mmap);

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

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

    if (wait(NULL) == −1)
        errExit(wait);

    exit(EXIT_SUCCESS);
}

SEE ALSO

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

COLOPHON

This page is part of release 5.04 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 https://www.kernel.org/doc/man−pages/.


  Copyright (C) 2019 Michael Kerrisk <mtk.manpagesgmail.com>
A very few fragments remain from an earlier page written by
Werner Almesberger in 2000

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Section (8) pivot_root


Linux manual pages Section 8  

Name

pivot_root — change the root filesystem

Synopsis

pivot_root new_root 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. Since pivot_root(8) simply calls pivot_root(2), we refer to the man page of the latter for further details.

Note that, depending on the implementation of pivot_root, root and cwd of the caller may or may not change. The following is a sequence for invoking pivot_root that works in either case, assuming that pivot_root and chroot are in the current PATH:

cd new_root

pivot_root . put_old

exec chroot . command

Note that chroot must be available under the old root and under the new root, because pivot_root may or may not have implicitly changed the root directory of the shell.

Note that exec chroot changes the running executable, which is necessary if the old root directory should be unmounted afterwards. Also note that standard input, output, and error may still point to a device on the old root file system, keeping it busy. They can easily be changed when invoking chroot (see below; note the absence of leading slashes to make it work whether pivot_root has changed the shell_zsingle_quotesz_s root or not).

OPTIONS

−V, −−version

Display version information and exit.

−h, −−help

Display help text and exit.

EXAMPLES

Change the root file system to /dev/hda1 from an interactive shell:

mount /dev/hda1 /new-root
cd /new-root
pivot_root . old-root
exec chroot . sh <dev/console >dev/console 2>&1
umount /old-root

Mount the new root file system over NFS from 10.0.0.1:/my_root and run init:

ifconfig lo 127.0.0.1 up   # for portmap
# configure Ethernet or such
portmap   # for lockd (implicitly started by mount)
mount -o ro 10.0.0.1:/my_root /mnt
killall portmap   # portmap keeps old root busy
cd /mnt
pivot_root . old_root
exec chroot . sh -c _zsingle_quotesz_umount /old_root; exec /sbin/init_zsingle_quotesz_ 
  <dev/console >dev/console 2>&1

SEE ALSO

chroot(1), pivot_root(2), mount(8), switch_root(8), umount(8)

AVAILABILITY

The pivot_root command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.