man2/pivot_root.2

   1 .\" Copyright (C) 2000 by Werner Almesberger
   2 .\" May be distributed under GPL
   3 .\"
   4 .\" Written 2000-02-23 by Werner Almesberger
   5 .\" Modified 2004-06-17 Michael Kerrisk <mtk-manpages@gmx.net>
   6 .\"
   7 .TH PIVOT_ROOT 2 2007-06-01 "Linux" "Linux Programmer's Manual"
   8 .SH NAME
   9 pivot_root \- change the root file system
  10 .SH SYNOPSIS
  11 .BI "int pivot_root(const char *" new_root ", const char *" put_old );
  12 .SH DESCRIPTION
  13 .BR pivot_root ()
  14 moves the root file system of the current process to the
  15 directory \fIput_old\fP and makes \fInew_root\fP the new root file system
  16 of the current process.
  17 .\"
  18 .\" The
  19 .\" .B CAP_SYS_ADMIN
  20 .\" capability is required.
  21
  22 The typical use of
  23 .BR pivot_root ()
  24 is during system startup, when the
  25 system mounts a temporary root file system (e.g., an \fBinitrd\fP), then
  26 mounts the real root file system, and eventually turns the latter into
  27 the current root of all relevant processes or threads.
  28
  29 .BR pivot_root ()
  30 may or may not change the current root and the current
  31 working directory (cwd) of any processes or threads which use the old
  32 root directory.
  33 The caller of
  34 .BR pivot_root ()
  35 must ensure that processes with root or cwd at the old root operate
  36 correctly in either case.
  37 An easy way to ensure this is to change their
  38 root and cwd to \fInew_root\fP before invoking
  39 .BR pivot_root ().
  40
  41 The paragraph above is intentionally vague because the implementation
  42 of
  43 .BR pivot_root ()
  44 may change in the future.
  45 At the time of writing,
  46 .BR pivot_root ()
  47 changes root and cwd of each process or
  48 thread to \fInew_root\fP if they point to the old root directory.
  49 This
  50 is necessary in order to prevent kernel threads from keeping the old
  51 root directory busy with their root and cwd, even if they never access
  52 the file system in any way.
  53 In the future, there may be a mechanism for
  54 kernel threads to explicitly relinquish any access to the file system,
  55 such that this fairly intrusive mechanism can be removed from
  56 .BR pivot_root ().
  57
  58 Note that this also applies to the current process:
  59 .BR pivot_root ()
  60 may
  61 or may not affect its cwd.
  62 It is therefore recommended to call
  63 \fBchdir("/")\fP immediately after
  64 .BR pivot_root ().
  65
  66 The following restrictions apply to \fInew_root\fP and \fIput_old\fP:
  67 .IP \- 3
  68 They must be directories.
  69 .IP \- 3
  70 \fInew_root\fP and \fIput_old\fP must not be on the same file system as
  71 the current root.
  72 .IP \- 3
  73 \fIput_old\fP must be underneath \fInew_root\fP, that is, adding a non-zero
  74 number of \fI/..\fP to the string pointed to by \fIput_old\fP must yield
  75 the same directory as \fInew_root\fP.
  76 .IP \- 3
  77 No other file system may be mounted on \fIput_old\fP.
  78 .PP
  79 See also
  80 .BR pivot_root (8)
  81 for additional usage examples.
  82
  83 If the current root is not a mount point (e.g., after
  84 .BR chroot (2)
  85 or
  86 .BR pivot_root (),
  87 see also below), not the old root directory, but the
  88 mount point of that file system is mounted on \fIput_old\fP.
  89
  90 \fInew_root\fP does not have to be a mount point.
  91 In this case,
  92 \fI/proc/mounts\fP will show the mount point of the file system containing
  93 \fInew_root\fP as root (\fI/\fP).
  94 .SH "RETURN VALUE"
  95 On success, zero is returned.
  96 On error, \-1 is returned, and
  97 \fIerrno\fP is set appropriately.
  98 .SH ERRORS
  99 .BR pivot_root ()
 100 may return (in \fIerrno\fP) any of the errors returned by
 101 .BR stat (2).
 102 Additionally, it may return:
 103 .TP
 104 .B EBUSY
 105 \fInew_root\fP or \fIput_old\fP are on the current root file system,
 106 or a file system is already mounted on \fIput_old\fP.
 107 .TP
 108 .B EINVAL
 109 \fIput_old\fP is not underneath \fInew_root\fP.
 110 .TP
 111 .B ENOTDIR
 112 \fInew_root\fP or \fIput_old\fP is not a directory.
 113 .TP
 114 .B EPERM
 115 The current process does not have the
 116 .B CAP_SYS_ADMIN
 117 capability.
 118 .SH VERSIONS
 119 .BR pivot_root ()
 120 was introduced in Linux 2.3.41.
 121 .SH "CONFORMING TO"
 122 .BR pivot_root ()
 123 is Linux specific and hence is not portable.
 124 .SH NOTES
 125 Glibc does not provide a wrapper for this system call; call it using
 126 .BR syscall (2).
 127 .SH BUGS
 128 .BR pivot_root ()
 129 should not have to change root and cwd of all other
 130 processes in the system.
 131
 132 Some of the more obscure uses of
 133 .BR pivot_root ()
 134 may quickly lead to
 135 insanity.
 136 .SH "SEE ALSO"
 137 .BR chdir (2),
 138 .BR chroot (2),
 139 .BR stat (2),
 140 .BR initrd (4),
 141 .BR pivot_root (8)