pivot_root

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



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.

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'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>

       #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 = malloc(STACK_SIZE);
           if (stack == NULL)
               errExit("malloc");

           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 5.03 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/.



Linux                             2019-08-02                     PIVOT_ROOT(2)