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pivot_root(2)
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 sys‐
tem mounts a temporary root filesystem (e.g., an initrd), then mounts
the real root filesystem, and eventually turns the latter into the cur‐
rent 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 recom‐
mended 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 cur‐
rent root.
- put_old must be underneath new_root, that is, adding a nonzero num‐
ber 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 propa‐
gation 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 every‐
thing in rootfs, overmount rootfs with the new root, attach stdin/std‐
out/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 direc‐
tory 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)