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 .SH RETURN VALUE
 105 On success, zero is returned.
 106 On error, \-1 is returned, and
 107 \fIerrno\fP is set appropriately.
 108 .SH ERRORS
 109 .BR pivot_root ()
 110 may return (in \fIerrno\fP) any of the errors returned by
 111 .BR stat (2).
 112 Additionally, it may return:
 113 .TP
 114 .B EBUSY
 115 \fInew_root\fP or \fIput_old\fP are on the current root filesystem,
 116 or a filesystem is already mounted on \fIput_old\fP.
 117 .TP
 118 .B EINVAL
 119 .I new_root
 120 is not a mount point.
 121 .TP
 122 .B EINVAL
 123 \fIput_old\fP is not underneath \fInew_root\fP.
 124 .TP
 125 .B EINVAL
 126 The current root is on the rootfs (initial ramfs) filesystem.
 127 .TP
 128 .B EINVAL
 129 Either the mount point at
 130 .IR new_root ,
 131 or the parent mount of that mount point,
 132 has propagation type
 133 .BR MS_SHARED .
 134 .TP
 135 .B ENOTDIR
 136 \fInew_root\fP or \fIput_old\fP is not a directory.
 137 .TP
 138 .B EPERM
 139 The calling process does not have the
 140 .B CAP_SYS_ADMIN
 141 capability.
 142 .SH VERSIONS
 143 .BR pivot_root ()
 144 was introduced in Linux 2.3.41.
 145 .SH CONFORMING TO
 146 .BR pivot_root ()
 147 is Linux-specific and hence is not portable.
 148 .SH NOTES
 149 Glibc does not provide a wrapper for this system call; call it using
 150 .BR syscall (2).
 151 .PP
 152 The rootfs (initial ramfs) cannot be
 153 .BR pivot_root ()ed.
 154 The recommended method of changing the root filesystem in this case is
 155 to delete everything in rootfs, overmount rootfs with the new root, attach
 156 .IR stdin / stdout / stderr
 157 to the new
 158 .IR /dev/console ,
 159 and exec the new
 160 .BR init (1).
 161 Helper programs for this process exist; see
 162 .BR switch_root (8).
 163 .SH BUGS
 164 .BR pivot_root ()
 165 should not have to change root and current working directory of all other
 166 processes in the system.
 167 .PP
 168 Some of the more obscure uses of
 169 .BR pivot_root ()
 170 may quickly lead to
 171 insanity.
 172 .SH SEE ALSO
 173 .BR chdir (2),
 174 .BR chroot (2),
 175 .BR stat (2),
 176 .BR initrd (4),
 177 .BR pivot_root (8),
 178 .BR switch_root (8)