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 2019-08-02 "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 changes the root filesystem in the mount namespace of the calling process.
  21 More precisely, it moves the root filesystem to the
  22 directory \fIput_old\fP and makes \fInew_root\fP the new root filesystem.
  23 The calling process must have the
  24 .B CAP_SYS_ADMIN
  25 capability in the user namespace that owns the caller's mount namespace.
  26 .PP
  27 The typical use of
  28 .BR pivot_root ()
  29 is during system startup, when the
  30 system mounts a temporary root filesystem (e.g., an \fBinitrd\fP), then
  31 mounts the real root filesystem, and eventually turns the latter into
  32 the current root of all relevant processes or threads.
  33 .PP
  34 .BR pivot_root ()
  35 may or may not change the current root and the current
  36 working directory of any processes or threads that
  37 use the old root directory and which are in
  38 the same mount namespace as the caller of
  39 .BR pivot_root ().
  40 The caller of
  41 .BR pivot_root ()
  42 must ensure that processes with root or current working directory
  43 at the old root operate correctly in either case.
  44 An easy way to ensure this is to change their
  45 root and current working directory to \fInew_root\fP before invoking
  46 .BR pivot_root ().
  47 .PP
  48 The paragraph above is intentionally vague because the implementation of
  49 .BR pivot_root ()
  50 may change in the future.
  51 Currently,
  52 .BR pivot_root ()
  53 changes the root directory and the current working directory
  54 of each process or thread in the same mount namespace to
  55 .I new_root
  56 if they point to the old root directory.
  57 This is necessary in order to prevent kernel threads from keeping the old
  58 root directory busy with their root and current working directory,
  59 even if they never access
  60 the filesystem in any way.
  61 In the future, there may be a mechanism for
  62 kernel threads to explicitly relinquish any access to the filesystem,
  63 such that this fairly intrusive mechanism can be removed from
  64 .BR pivot_root ().
  65 .PP
  66 Note that this also applies to the calling process:
  67 .BR pivot_root ()
  68 may or may not affect its current working directory.
  69 It is therefore recommended to call
  70 \fBchdir("/")\fP immediately after
  71 .BR pivot_root ().
  72 .PP
  73 The following restrictions apply to \fInew_root\fP and \fIput_old\fP:
  74 .IP \- 3
  75 They must be directories.
  76 .IP \- 3
  77 \fInew_root\fP and \fIput_old\fP must not be on the same filesystem as
  78 the current root.
  79 .IP \- 3
  80 \fIput_old\fP must be underneath \fInew_root\fP, that is, adding a nonzero
  81 number of \fI/..\fP to the string pointed to by \fIput_old\fP must yield
  82 the same directory as \fInew_root\fP.
  83 .IP \- 3
  84 .I new_root
  85 must be a mount point.
  86 (If it is not otherwise a mount point, it suffices to bind mount
  87 .I new_root
  88 on top of itself.)
  89 .IP \- 3
  90 The propagation type of
  91 .I new_root
  92 and its parent mount must not be
  93 .BR MS_SHARED ;
  94 similarly, if
  95 .I put_old
  96 is an existing mount point, its propagation type must not be
  97 .BR MS_SHARED .
  98 .IP \- 3
  99 No other filesystem may be mounted on \fIput_old\fP.
 100 .PP
 101 See also
 102 .BR pivot_root (8)
 103 for additional usage examples.
 104 .PP
 105 If the current root is not a mount point (e.g., after an earlier
 106 .BR chroot (2)
 107 or
 108 .BR pivot_root ()),
 109 then the mount point of the filesystem containing the current root directory
 110 (i.e., not the directory itself) is mounted on \fIput_old\fP.
 111 .SH RETURN VALUE
 112 On success, zero is returned.
 113 On error, \-1 is returned, and
 114 \fIerrno\fP is set appropriately.
 115 .SH ERRORS
 116 .BR pivot_root ()
 117 may fail with any of the same errors as
 118 .BR stat (2).
 119 Additionally, it may fail with the following errors:
 120 .TP
 121 .B EBUSY
 122 \fInew_root\fP or \fIput_old\fP are on the current root filesystem,
 123 or a filesystem is already mounted on \fIput_old\fP.
 124 .TP
 125 .B EINVAL
 126 .I new_root
 127 is not a mount point.
 128 .TP
 129 .B EINVAL
 130 \fIput_old\fP is not underneath \fInew_root\fP.
 131 .TP
 132 .B EINVAL
 133 The current root is on the rootfs (initial ramfs) filesystem.
 134 .TP
 135 .B EINVAL
 136 Either the mount point at
 137 .IR new_root ,
 138 or the parent mount of that mount point,
 139 has propagation type
 140 .BR MS_SHARED .
 141 .TP
 142 .B EINVAL
 143 .I put_old
 144 is a mount point and has the propagation type
 145 .BR MS_SHARED .
 146 .TP
 147 .B ENOTDIR
 148 \fInew_root\fP or \fIput_old\fP is not a directory.
 149 .TP
 150 .B EPERM
 151 The calling process does not have the
 152 .B CAP_SYS_ADMIN
 153 capability.
 154 .SH VERSIONS
 155 .BR pivot_root ()
 156 was introduced in Linux 2.3.41.
 157 .SH CONFORMING TO
 158 .BR pivot_root ()
 159 is Linux-specific and hence is not portable.
 160 .SH NOTES
 161 Glibc does not provide a wrapper for this system call; call it using
 162 .BR syscall (2).
 163 .PP
 164 The rootfs (initial ramfs) cannot be
 165 .BR pivot_root ()ed.
 166 The recommended method of changing the root filesystem in this case is
 167 to delete everything in rootfs, overmount rootfs with the new root, attach
 168 .IR stdin / stdout / stderr
 169 to the new
 170 .IR /dev/console ,
 171 and exec the new
 172 .BR init (1).
 173 Helper programs for this process exist; see
 174 .BR switch_root (8).
 175 .SH BUGS
 176 .BR pivot_root ()
 177 should not have to change root and current working directory of other
 178 processes in the system.
 179 .PP
 180 Some of the more obscure uses of
 181 .BR pivot_root ()
 182 may quickly lead to
 183 insanity.
 184 .SH SEE ALSO
 185 .BR chdir (2),
 186 .BR chroot (2),
 187 .BR mount (2),
 188 .BR stat (2),
 189 .BR initrd (4),
 190 .BR mount_namespaces (7),
 191 .BR pivot_root (8),
 192 .BR switch_root (8)