man2/pivot_root.2

   1 .\" Copyright (C) 2000 by Werner Almesberger
   2 .\"
   3 .\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
   4 .\" May be distributed under GPL
   5 .\" %%%LICENSE_END
   6 .\"
   7 .\" Written 2000-02-23 by Werner Almesberger
   8 .\" Modified 2004-06-17 Michael Kerrisk <mtk.manpages@gmail.com>
   9 .\"
  10 .TH PIVOT_ROOT 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  11 .SH NAME
  12 pivot_root \- change the root filesystem
  13 .SH SYNOPSIS
  14 .BI "int pivot_root(const char *" new_root ", const char *" put_old );
  15 .PP
  16 .IR Note :
  17 There is no glibc wrapper for this system call; see NOTES.
  18 .SH DESCRIPTION
  19 .BR pivot_root ()
  20 moves the root filesystem of the calling process to the
  21 directory \fIput_old\fP and makes \fInew_root\fP the new root filesystem
  22 of the calling process.
  23 .\"
  24 .\" The
  25 .\" .B CAP_SYS_ADMIN
  26 .\" capability is required.
  27 .PP
  28 The typical use of
  29 .BR pivot_root ()
  30 is during system startup, when the
  31 system mounts a temporary root filesystem (e.g., an \fBinitrd\fP), then
  32 mounts the real root filesystem, and eventually turns the latter into
  33 the current root of all relevant processes or threads.
  34 .PP
  35 .BR pivot_root ()
  36 may or may not change the current root and the current
  37 working directory of any processes or threads which use the old
  38 root directory.
  39 The caller of
  40 .BR pivot_root ()
  41 must ensure that processes with root or current working directory
  42 at the old root operate correctly in either case.
  43 An easy way to ensure this is to change their
  44 root and current working directory to \fInew_root\fP before invoking
  45 .BR pivot_root ().
  46 .PP
  47 The paragraph above is intentionally vague because the implementation of
  48 .BR pivot_root ()
  49 may change in the future.
  50 At the time of writing,
  51 .BR pivot_root ()
  52 changes root and current working directory of each process or
  53 thread to \fInew_root\fP if they point to the old root directory.
  54 This is necessary in order to prevent kernel threads from keeping the old
  55 root directory busy with their root and current working directory,
  56 even if they never access
  57 the filesystem in any way.
  58 In the future, there may be a mechanism for
  59 kernel threads to explicitly relinquish any access to the filesystem,
  60 such that this fairly intrusive mechanism can be removed from
  61 .BR pivot_root ().
  62 .PP
  63 Note that this also applies to the calling process:
  64 .BR pivot_root ()
  65 may 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 .PP
  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 filesystem 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 filesystem may be mounted on \fIput_old\fP.
  82 .PP
  83 See also
  84 .BR pivot_root (8)
  85 for additional usage examples.
  86 .PP
  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 filesystem is mounted on \fIput_old\fP.
  93 .PP
  94 .I new_root
  95 must be a mount point.
  96 (If it is not otherwise a mount point, it suffices to bind mount
  97 .I new_root
  98 on top of itself.)
  99 .PP
 100 The propagation type of
 101 .I new_root
 102 and its parent mount must not be
 103 .BR MS_SHARED ;
 104 similarly, if
 105 .I put_old
 106 is an existing mount point, its propagation type must not be
 107 .BR MS_SHARED .
 108 .SH RETURN VALUE
 109 On success, zero is returned.
 110 On error, \-1 is returned, and
 111 \fIerrno\fP is set appropriately.
 112 .SH ERRORS
 113 .BR pivot_root ()
 114 may return (in \fIerrno\fP) any of the errors returned by
 115 .BR stat (2).
 116 Additionally, it may return:
 117 .TP
 118 .B EBUSY
 119 \fInew_root\fP or \fIput_old\fP are on the current root filesystem,
 120 or a filesystem is already mounted on \fIput_old\fP.
 121 .TP
 122 .B EINVAL
 123 .I new_root
 124 is not a mount point.
 125 .TP
 126 .B EINVAL
 127 \fIput_old\fP is not underneath \fInew_root\fP.
 128 .TP
 129 .B EINVAL
 130 The current root is on the rootfs (initial ramfs) filesystem.
 131 .TP
 132 .B EINVAL
 133 Either the mount point at
 134 .IR new_root ,
 135 or the parent mount of that mount point,
 136 has propagation type
 137 .BR MS_SHARED .
 138 .TP
 139 .B EINVAL
 140 .I put_old
 141 is a mount point and has the propagation type
 142 .BR MS_SHARED .
 143 .TP
 144 .B ENOTDIR
 145 \fInew_root\fP or \fIput_old\fP is not a directory.
 146 .TP
 147 .B EPERM
 148 The calling process does not have the
 149 .B CAP_SYS_ADMIN
 150 capability.
 151 .SH VERSIONS
 152 .BR pivot_root ()
 153 was introduced in Linux 2.3.41.
 154 .SH CONFORMING TO
 155 .BR pivot_root ()
 156 is Linux-specific and hence is not portable.
 157 .SH NOTES
 158 Glibc does not provide a wrapper for this system call; call it using
 159 .BR syscall (2).
 160 .PP
 161 The rootfs (initial ramfs) cannot be
 162 .BR pivot_root ()ed.
 163 The recommended method of changing the root filesystem in this case is
 164 to delete everything in rootfs, overmount rootfs with the new root, attach
 165 .IR stdin / stdout / stderr
 166 to the new
 167 .IR /dev/console ,
 168 and exec the new
 169 .BR init (1).
 170 Helper programs for this process exist; see
 171 .BR switch_root (8).
 172 .SH BUGS
 173 .BR pivot_root ()
 174 should not have to change root and current working directory of all other
 175 processes in the system.
 176 .PP
 177 Some of the more obscure uses of
 178 .BR pivot_root ()
 179 may quickly lead to
 180 insanity.
 181 .SH SEE ALSO
 182 .BR chdir (2),
 183 .BR chroot (2),
 184 .BR mount (2),
 185 .BR stat (2),
 186 .BR initrd (4),
 187 .BR pivot_root (8),
 188 .BR switch_root (8)