pivot_root(2) - Linux manual page (original) (raw)
pivotroot(2) System Calls Manual pivotroot(2)
NAME top
pivot_root - change the root mountLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/syscall.h>** /* Definition of **SYS_*** constants */
**#include <unistd.h>**
**int syscall(SYS_pivot_root, const char ***_newroot_**, const char ***_putold_**);**
_Note_: glibc provides no wrapper for **pivot_root**(), necessitating
the use of [syscall(2)](../man2/syscall.2.html).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 _putold_ and makes _newroot_ 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
_newroot_ 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:
• _newroot_ and _putold_ must be directories.
• _newroot_ and _putold_ must not be on the same mount as the
current root.
• _putold_ must be at or underneath _newroot_; that is, adding some
nonnegative number of "_/.._" suffixes to the pathname pointed to
by _putold_ must yield the same directory as _newroot_.
• _newroot_ 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 _newroot_ and the
parent mount of the current root directory must not be
**MS_SHARED**; similarly, if _putold_ 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](../man3/errno.3.html)_
is set to indicate the error.ERRORS top
**pivot_root**() may fail with any of the same errors as [stat(2)](../man2/stat.2.html).
Additionally, it may fail with the following errors:
**EBUSY** _newroot_ or _putold_ is on the current root mount. (This
error covers the pathological case where _newroot_ is _"/"_.)
**EINVAL** _newroot_ is not a mount point.
**EINVAL** _putold_ is not at or underneath _newroot_.
**EINVAL** The current root directory is not a mount point (because of
an earlier [chroot(2)](../man2/chroot.2.html)).
**EINVAL** The current root is on the rootfs (initial ramfs) mount;
see NOTES.
**EINVAL** Either the mount point at _newroot_, or the parent mount of
that mount point, has propagation type **MS_SHARED**.
**EINVAL** _putold_ is a mount point and has the propagation type
**MS_SHARED**.
**ENOTDIR**
_newroot_ or _putold_ is not a directory.
**EPERM** The calling process does not have the **CAP_SYS_ADMIN**
capability.STANDARDS top
Linux.HISTORY top
Linux 2.3.41.NOTES top
A command-line interface for this system call is provided by
[pivot_root(8)](../man8/pivot%5Froot.8.html).
**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 _newroot_ 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)](../man4/initrd.4.html)), 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)](../man1/init.1.html). Helper programs for this process exist; see
[switch_root(8)](../man8/switch%5Froot.8.html).pivot_root(".", ".") newroot and putold 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 (_newroot_).
During the subsequent **umount**() call, resolution of _"."_ starts with
_newroot_ 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 _newroot_ 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)](../man2/clone.2.html). After pivoting to
the root directory named in the program's first command-line
argument, the child created by [clone(2)](../man2/clone.2.html) 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 worldProgram 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);
}SEE ALSO top
[chdir(2)](../man2/chdir.2.html), [chroot(2)](../man2/chroot.2.html), [mount(2)](../man2/mount.2.html), [stat(2)](../man2/stat.2.html), [initrd(4)](../man4/initrd.4.html),
[mount_namespaces(7)](../man7/mount%5Fnamespaces.7.html), [pivot_root(8)](../man8/pivot%5Froot.8.html), [switch_root(8)](../man8/switch%5Froot.8.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 pivotroot(2)
Pages that refer to this page:chroot(2), mount(2), syscalls(2), initrd(4), systemd.exec(5), capabilities(7), mount_namespaces(7), pivot_root(8)