pivot_root(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

       pivot_root - change the root mount

SYNOPSIS         top

       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

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

       Note: glibc provides no wrapper for pivot_root(), necessitating
       the use of syscall(2).

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 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         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

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 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         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

       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         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)
               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);
       }

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 5.12 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                          2021-03-22                  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)