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