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@gmail.com>
   6 .\"
   7 .TH PIVOT_ROOT 2 2012-07-13 "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
  13 .IR Note :
  14 There is no glibc wrapper for this system call; see NOTES.
  15 .SH DESCRIPTION
  16 .BR pivot_root ()
  17 moves the root file system of the calling process to the
  18 directory \fIput_old\fP and makes \fInew_root\fP the new root file system
  19 of the calling process.
  20 .\"
  21 .\" The
  22 .\" .B CAP_SYS_ADMIN
  23 .\" capability is required.
  24
  25 The typical use of
  26 .BR pivot_root ()
  27 is during system startup, when the
  28 system mounts a temporary root file system (e.g., an \fBinitrd\fP), then
  29 mounts the real root file system, and eventually turns the latter into
  30 the current root of all relevant processes or threads.
  31
  32 .BR pivot_root ()
  33 may or may not change the current root and the current
  34 working directory of any processes or threads which use the old
  35 root directory.
  36 The caller of
  37 .BR pivot_root ()
  38 must ensure that processes with root or current working directory
  39 at the old root operate correctly in either case.
  40 An easy way to ensure this is to change their
  41 root and current working directory to \fInew_root\fP before invoking
  42 .BR pivot_root ().
  43
  44 The paragraph above is intentionally vague because the implementation
  45 of
  46 .BR pivot_root ()
  47 may change in the future.
  48 At the time of writing,
  49 .BR pivot_root ()
  50 changes root and current working directory of each process or
  51 thread to \fInew_root\fP if they point to the old root directory.
  52 This
  53 is necessary in order to prevent kernel threads from keeping the old
  54 root directory busy with their root and current working directory,
  55 even if they never access
  56 the file system in any way.
  57 In the future, there may be a mechanism for
  58 kernel threads to explicitly relinquish any access to the file system,
  59 such that this fairly intrusive mechanism can be removed from
  60 .BR pivot_root ().
  61
  62 Note that this also applies to the calling process:
  63 .BR pivot_root ()
  64 may
  65 or may not affect its current working directory.
  66 It is therefore recommended to call
  67 \fBchdir("/")\fP immediately after
  68 .BR pivot_root ().
  69
  70 The following restrictions apply to \fInew_root\fP and \fIput_old\fP:
  71 .IP \- 3
  72 They must be directories.
  73 .IP \- 3
  74 \fInew_root\fP and \fIput_old\fP must not be on the same file system as
  75 the current root.
  76 .IP \- 3
  77 \fIput_old\fP must be underneath \fInew_root\fP, that is, adding a nonzero
  78 number of \fI/..\fP to the string pointed to by \fIput_old\fP must yield
  79 the same directory as \fInew_root\fP.
  80 .IP \- 3
  81 No other file system may be mounted on \fIput_old\fP.
  82 .PP
  83 See also
  84 .BR pivot_root (8)
  85 for additional usage examples.
  86
  87 If the current root is not a mount point (e.g., after
  88 .BR chroot (2)
  89 or
  90 .BR pivot_root (),
  91 see also below), not the old root directory, but the
  92 mount point of that file system is mounted on \fIput_old\fP.
  93
  94 \fInew_root\fP does not have to be a mount point.
  95 In this case,
  96 \fI/proc/mounts\fP will show the mount point of the file system containing
  97 \fInew_root\fP as root (\fI/\fP).
  98 .SH RETURN VALUE
  99 On success, zero is returned.
 100 On error, \-1 is returned, and
 101 \fIerrno\fP is set appropriately.
 102 .SH ERRORS
 103 .BR pivot_root ()
 104 may return (in \fIerrno\fP) any of the errors returned by
 105 .BR stat (2).
 106 Additionally, it may return:
 107 .TP
 108 .B EBUSY
 109 \fInew_root\fP or \fIput_old\fP are on the current root file system,
 110 or a file system is already mounted on \fIput_old\fP.
 111 .TP
 112 .B EINVAL
 113 \fIput_old\fP is not underneath \fInew_root\fP.
 114 .TP
 115 .B ENOTDIR
 116 \fInew_root\fP or \fIput_old\fP is not a directory.
 117 .TP
 118 .B EPERM
 119 The calling process does not have the
 120 .B CAP_SYS_ADMIN
 121 capability.
 122 .SH VERSIONS
 123 .BR pivot_root ()
 124 was introduced in Linux 2.3.41.
 125 .SH CONFORMING TO
 126 .BR pivot_root ()
 127 is Linux-specific and hence is not portable.
 128 .SH NOTES
 129 Glibc does not provide a wrapper for this system call; call it using
 130 .BR syscall (2).
 131 .SH BUGS
 132 .BR pivot_root ()
 133 should not have to change root and current working directory of all other
 134 processes in the system.
 135
 136 Some of the more obscure uses of
 137 .BR pivot_root ()
 138 may quickly lead to
 139 insanity.
 140 .SH SEE ALSO
 141 .BR chdir (2),
 142 .BR chroot (2),
 143 .BR stat (2),
 144 .BR initrd (4),
 145 .BR pivot_root (8)