PIVOT_ROOT
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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 does not have to be a mount point.
In this case,
/proc/mounts will show the mount point of the filesystem containing
new_root as root (/).
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
-
put_old is not underneath new_root.
- 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).
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),
stat(2),
initrd(4),
pivot_root(8)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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