Ubuntu Manpage: pivot_root - change the root mount

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NAME

       pivot_root - change the root mount

SYNOPSIS

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

       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'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 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'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'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.

EXAMPLES

       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)
               errExit("mount-MS_PRIVATE");

           /* Ensure that 'new_root' 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);
       }

COLOPHON

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