mirror of https://github.com/mkerrisk/man-pages
142 lines
4.1 KiB
Groff
142 lines
4.1 KiB
Groff
.\" Copyright (C) 2000 by Werner Almesberger
|
|
.\" May be distributed under GPL
|
|
.\"
|
|
.\" Written 2000-02-23 by Werner Almesberger
|
|
.\" Modified 2004-06-17 Michael Kerrisk <mtk.manpages@gmail.com>
|
|
.\"
|
|
.TH PIVOT_ROOT 2 2007-06-01 "Linux" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
pivot_root \- change the root file system
|
|
.SH SYNOPSIS
|
|
.BI "int pivot_root(const char *" new_root ", const char *" put_old );
|
|
.SH DESCRIPTION
|
|
.BR pivot_root ()
|
|
moves the root file system of the current process to the
|
|
directory \fIput_old\fP and makes \fInew_root\fP the new root file system
|
|
of the current process.
|
|
.\"
|
|
.\" The
|
|
.\" .B CAP_SYS_ADMIN
|
|
.\" capability is required.
|
|
|
|
The typical use of
|
|
.BR pivot_root ()
|
|
is during system startup, when the
|
|
system mounts a temporary root file system (e.g., an \fBinitrd\fP), then
|
|
mounts the real root file system, and eventually turns the latter into
|
|
the current root of all relevant processes or threads.
|
|
|
|
.BR pivot_root ()
|
|
may or may not change the current root and the current
|
|
working directory (cwd) of any processes or threads which use the old
|
|
root directory.
|
|
The caller of
|
|
.BR pivot_root ()
|
|
must ensure that processes with root or cwd at the old root operate
|
|
correctly in either case.
|
|
An easy way to ensure this is to change their
|
|
root and cwd to \fInew_root\fP before invoking
|
|
.BR pivot_root ().
|
|
|
|
The paragraph above is intentionally vague because the implementation
|
|
of
|
|
.BR pivot_root ()
|
|
may change in the future.
|
|
At the time of writing,
|
|
.BR pivot_root ()
|
|
changes root and cwd of each process or
|
|
thread to \fInew_root\fP 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 cwd, even if they never access
|
|
the file system in any way.
|
|
In the future, there may be a mechanism for
|
|
kernel threads to explicitly relinquish any access to the file system,
|
|
such that this fairly intrusive mechanism can be removed from
|
|
.BR pivot_root ().
|
|
|
|
Note that this also applies to the current process:
|
|
.BR pivot_root ()
|
|
may
|
|
or may not affect its cwd.
|
|
It is therefore recommended to call
|
|
\fBchdir("/")\fP immediately after
|
|
.BR pivot_root ().
|
|
|
|
The following restrictions apply to \fInew_root\fP and \fIput_old\fP:
|
|
.IP \- 3
|
|
They must be directories.
|
|
.IP \- 3
|
|
\fInew_root\fP and \fIput_old\fP must not be on the same file system as
|
|
the current root.
|
|
.IP \- 3
|
|
\fIput_old\fP must be underneath \fInew_root\fP, that is, adding a non-zero
|
|
number of \fI/..\fP to the string pointed to by \fIput_old\fP must yield
|
|
the same directory as \fInew_root\fP.
|
|
.IP \- 3
|
|
No other file system may be mounted on \fIput_old\fP.
|
|
.PP
|
|
See also
|
|
.BR pivot_root (8)
|
|
for additional usage examples.
|
|
|
|
If the current root is not a mount point (e.g., after
|
|
.BR chroot (2)
|
|
or
|
|
.BR pivot_root (),
|
|
see also below), not the old root directory, but the
|
|
mount point of that file system is mounted on \fIput_old\fP.
|
|
|
|
\fInew_root\fP does not have to be a mount point.
|
|
In this case,
|
|
\fI/proc/mounts\fP will show the mount point of the file system containing
|
|
\fInew_root\fP as root (\fI/\fP).
|
|
.SH "RETURN VALUE"
|
|
On success, zero is returned.
|
|
On error, \-1 is returned, and
|
|
\fIerrno\fP is set appropriately.
|
|
.SH ERRORS
|
|
.BR pivot_root ()
|
|
may return (in \fIerrno\fP) any of the errors returned by
|
|
.BR stat (2).
|
|
Additionally, it may return:
|
|
.TP
|
|
.B EBUSY
|
|
\fInew_root\fP or \fIput_old\fP are on the current root file system,
|
|
or a file system is already mounted on \fIput_old\fP.
|
|
.TP
|
|
.B EINVAL
|
|
\fIput_old\fP is not underneath \fInew_root\fP.
|
|
.TP
|
|
.B ENOTDIR
|
|
\fInew_root\fP or \fIput_old\fP is not a directory.
|
|
.TP
|
|
.B EPERM
|
|
The current process does not have the
|
|
.B CAP_SYS_ADMIN
|
|
capability.
|
|
.SH VERSIONS
|
|
.BR pivot_root ()
|
|
was introduced in Linux 2.3.41.
|
|
.SH "CONFORMING TO"
|
|
.BR pivot_root ()
|
|
is Linux specific and hence is not portable.
|
|
.SH NOTES
|
|
Glibc does not provide a wrapper for this system call; call it using
|
|
.BR syscall (2).
|
|
.SH BUGS
|
|
.BR pivot_root ()
|
|
should not have to change root and cwd of all other
|
|
processes in the system.
|
|
|
|
Some of the more obscure uses of
|
|
.BR pivot_root ()
|
|
may quickly lead to
|
|
insanity.
|
|
.SH "SEE ALSO"
|
|
.BR chdir (2),
|
|
.BR chroot (2),
|
|
.BR stat (2),
|
|
.BR initrd (4),
|
|
.BR pivot_root (8)
|