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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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pivot_root(2) System Calls Manual pivot_root(2)
pivot_root - change the root mount
Standard C library (libc, -lc)
#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).
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.
On success, zero is returned. On error, -1 is returned, and
errno is set to indicate the error.
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.
Linux.
Linux 2.3.41.
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.
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);
}
chdir(2), chroot(2), mount(2), stat(2), initrd(4),
mount_namespaces(7), pivot_root(8), switch_root(8)
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Linux man-pages 6.9.1 2024-05-02 pivot_root(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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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