# pivot_root

## NAME



           pivot_root - change the root mount
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#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



           **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 "_/.._ " suffixes 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 to indicate the error.
    


## 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.
    


## STANDARDS



           Linux.
    


## NOTES



           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 <err.h>
           #include <limits.h>
           #include <sched.h>
           #include <signal.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/mman.h>
           #include <sys/mount.h>
           #include <sys/stat.h>
           #include <sys/syscall.h>
           #include <sys/wait.h>
           #include <unistd.h>
    
           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        path[PATH_MAX];
               char        **args = arg;
               char        *new_root = args[0];
               const char  *put_old = "/oldrootfs";
    
               /* 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)
                   err(EXIT_FAILURE, "mount-MS_PRIVATE");
    
               /* Ensure that 'new_root' is a mount point. */
    
               if (mount(new_root, new_root, NULL, MS_BIND, NULL) == -1)
                   err(EXIT_FAILURE, "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)
                   err(EXIT_FAILURE, "mkdir");
    
               /* And pivot the root filesystem. */
    
               if (pivot_root(new_root, path) == -1)
                   err(EXIT_FAILURE, "pivot_root");
    
               /* Switch the current working directory to "/". */
    
               if (chdir("/") == -1)
                   err(EXIT_FAILURE, "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]);
               err(EXIT_FAILURE, "execv");
           }
    
           int
           main(int argc, char *argv[])
           {
               char *stack;
    
               /* Create a child process in a new mount namespace. */
    
               stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE,
                            MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
               if (stack == MAP_FAILED)
                   err(EXIT_FAILURE, "mmap");
    
               if (clone(child, stack + STACK_SIZE,
                         CLONE_NEWNS | SIGCHLD, &argv[1]) == -1)
                   err(EXIT_FAILURE, "clone");
    
               /* Parent falls through to here; wait for child. */
    
               if (wait(NULL) == -1)
                   err(EXIT_FAILURE, "wait");
    
               exit(EXIT_SUCCESS);
           }
    


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