pivot_root

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



NAME
       pivot_root - change the root filesystem

SYNOPSIS
       int pivot_root(const char *new_root, const char *put_old);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       pivot_root() moves the root filesystem of the calling process to the
       directory put_old and makes new_root the new root filesystem of the
       calling process.

       The typical use of pivot_root() is during system startup, when the
       system mounts a temporary root filesystem (e.g., an initrd), then
       mounts the real root filesystem, and eventually turns the latter into
       the current root of all relevant processes or threads.

       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().

       The paragraph above is intentionally vague because the implementation
       of pivot_root() may change in the future.  At the time of writing,
       pivot_root() changes root and current working directory of each process
       or thread to new_root if they point to the old root directory.  This is
       necessary in order to prevent kernel threads from keeping the old root
       directory busy with their root and current working directory, even if
       they never access the filesystem in any way.  In the future, there may
       be a mechanism for kernel threads to explicitly relinquish any access
       to the filesystem, such that this fairly intrusive mechanism can be
       removed from pivot_root().

       Note that this also applies to the calling process: pivot_root() may or
       may not affect its current working directory.  It is therefore
       recommended to call chdir("/") immediately after pivot_root().

       The following restrictions apply to new_root and put_old:

       -  They must be directories.

       -  new_root and put_old must not be on the same filesystem as the
          current root.

       -  put_old must be underneath new_root, that is, adding a nonzero
          number of /.. to the string pointed to by put_old must yield the
          same directory as new_root.

       -  No other filesystem may be mounted on put_old.

       See also pivot_root(8) for additional usage examples.

       If the current root is not a mount point (e.g., after chroot(2) or
       pivot_root(), see also below), not the old root directory, but the
       mount point of that filesystem is mounted on put_old.

       new_root must be a mount point.  (If it is not otherwise a mount point,
       it suffices to bind mount new_root on top of itself.)

       The propagation type of new_root and its parent mount must not be
       MS_SHARED; similarly, if put_old is an existing mount point, its
       propagation type must not be MS_SHARED.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

ERRORS
       pivot_root() may return (in errno) any of the errors returned by
       stat(2).  Additionally, it may return:

       EBUSY  new_root or put_old are on the current root filesystem, or a
              filesystem is already mounted on put_old.

       EINVAL new_root is not a mount point.

       EINVAL put_old is not underneath new_root.

       EINVAL The current root is on the rootfs (initial ramfs) filesystem.

       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
       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).

       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).

BUGS
       pivot_root() should not have to change root and current working
       directory of all other processes in the system.

       Some of the more obscure uses of pivot_root() may quickly lead to
       insanity.

SEE ALSO
       chdir(2), chroot(2), mount(2), stat(2), initrd(4), pivot_root(8),
       switch_root(8)

COLOPHON
       This page is part of release 5.02 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                             2019-08-02                     PIVOT_ROOT(2)