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:

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:

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).

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.

/* 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

chdir(2), chroot(2), mount(2), stat(2), initrd(4), mount_namespaces(7), pivot_root(8), switch_root(8)