man2/pivot_root.2

   1 .\" Copyright (C) 2000 by Werner Almesberger
   2 .\" and Copyright (C) 2019 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
   5 .\" May be distributed under GPL
   6 .\" %%%LICENSE_END
   7 .\"
   8 .\" Written 2000-02-23 by Werner Almesberger
   9 .\" Modified 2004-06-17 Michael Kerrisk <mtk.manpages@gmail.com>
  10 .\"
  11 .TH PIVOT_ROOT 2 2019-08-02 "Linux" "Linux Programmer's Manual"
  12 .SH NAME
  13 pivot_root \- change the root filesystem
  14 .SH SYNOPSIS
  15 .BI "int pivot_root(const char *" new_root ", const char *" put_old );
  16 .PP
  17 .IR Note :
  18 There is no glibc wrapper for this system call; see NOTES.
  19 .SH DESCRIPTION
  20 .BR pivot_root ()
  21 changes the root filesystem in the mount namespace of the calling process.
  22 More precisely, it moves the root filesystem to the
  23 directory \fIput_old\fP and makes \fInew_root\fP the new root filesystem.
  24 The calling process must have the
  25 .B CAP_SYS_ADMIN
  26 capability in the user namespace that owns the caller's mount namespace.
  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 that
  38 use the old root directory and which are in
  39 the same mount namespace as the caller of
  40 .BR pivot_root ().
  41 The caller of
  42 .BR pivot_root ()
  43 must ensure that processes with root or current working directory
  44 at the old root operate correctly in either case.
  45 An easy way to ensure this is to change their
  46 root and current working directory to \fInew_root\fP before invoking
  47 .BR pivot_root ().
  48 .PP
  49 The paragraph above is intentionally vague because the implementation of
  50 .BR pivot_root ()
  51 may change in the future.
  52 Currently,
  53 .BR pivot_root ()
  54 changes the root directory and the current working directory
  55 of each process or thread in the same mount namespace to
  56 .I new_root
  57 if they point to the old root directory.
  58 This is necessary in order to prevent kernel threads from keeping the old
  59 root directory busy with their root and current working directory,
  60 even if they never access
  61 the filesystem in any way.
  62 In the future, there may be a mechanism for
  63 kernel threads to explicitly relinquish any access to the filesystem,
  64 such that this fairly intrusive mechanism can be removed from
  65 .BR pivot_root ().
  66 .PP
  67 Note that this also applies to the calling process:
  68 .BR pivot_root ()
  69 may or may not affect its current working directory.
  70 It is therefore recommended to call
  71 \fBchdir("/")\fP immediately after
  72 .BR pivot_root ().
  73 .PP
  74 The following restrictions apply to \fInew_root\fP and \fIput_old\fP:
  75 .IP \- 3
  76 They must be directories.
  77 .IP \-
  78 \fInew_root\fP and \fIput_old\fP must not be on the same filesystem as
  79 the current root.
  80 .IP \-
  81 \fIput_old\fP must be underneath \fInew_root\fP, that is, adding a nonzero
  82 number of \fI/..\fP to the string pointed to by \fIput_old\fP must yield
  83 the same directory as \fInew_root\fP.
  84 .IP \-
  85 .I new_root
  86 must be a mount point.
  87 (If it is not otherwise a mount point, it suffices to bind mount
  88 .I new_root
  89 on top of itself.)
  90 .IP \-
  91 The propagation type of
  92 .I new_root
  93 and its parent mount must not be
  94 .BR MS_SHARED ;
  95 similarly, if
  96 .I put_old
  97 is an existing mount point, its propagation type must not be
  98 .BR MS_SHARED .
  99 .PP
 100 See also
 101 .BR pivot_root (8)
 102 for additional usage examples.
 103 .PP
 104 If the current root is not a mount point (e.g., after an earlier
 105 .BR chroot (2)
 106 or
 107 .BR pivot_root ()),
 108 then the mount point of the filesystem containing the current root directory
 109 (i.e., not the directory itself) is mounted on \fIput_old\fP.
 110 .SH RETURN VALUE
 111 On success, zero is returned.
 112 On error, \-1 is returned, and
 113 \fIerrno\fP is set appropriately.
 114 .SH ERRORS
 115 .BR pivot_root ()
 116 may fail with any of the same errors as
 117 .BR stat (2).
 118 Additionally, it may fail with the following errors:
 119 .TP
 120 .B EBUSY
 121 \fInew_root\fP or \fIput_old\fP are on the current root filesystem,
 122 or a filesystem is already mounted on \fIput_old\fP.
 123 .TP
 124 .B EINVAL
 125 .I new_root
 126 is not a mount point.
 127 .TP
 128 .B EINVAL
 129 \fIput_old\fP is not underneath \fInew_root\fP.
 130 .TP
 131 .B EINVAL
 132 The current root is on the rootfs (initial ramfs) filesystem.
 133 .TP
 134 .B EINVAL
 135 Either the mount point at
 136 .IR new_root ,
 137 or the parent mount of that mount point,
 138 has propagation type
 139 .BR MS_SHARED .
 140 .TP
 141 .B EINVAL
 142 .I put_old
 143 is a mount point and has the propagation type
 144 .BR MS_SHARED .
 145 .TP
 146 .B ENOTDIR
 147 \fInew_root\fP or \fIput_old\fP is not a directory.
 148 .TP
 149 .B EPERM
 150 The calling process does not have the
 151 .B CAP_SYS_ADMIN
 152 capability.
 153 .SH VERSIONS
 154 .BR pivot_root ()
 155 was introduced in Linux 2.3.41.
 156 .SH CONFORMING TO
 157 .BR pivot_root ()
 158 is Linux-specific and hence is not portable.
 159 .SH NOTES
 160 Glibc does not provide a wrapper for this system call; call it using
 161 .BR syscall (2).
 162 .PP
 163 The rootfs (initial ramfs) cannot be
 164 .BR pivot_root ()ed.
 165 The recommended method of changing the root filesystem in this case is
 166 to delete everything in rootfs, overmount rootfs with the new root, attach
 167 .IR stdin / stdout / stderr
 168 to the new
 169 .IR /dev/console ,
 170 and exec the new
 171 .BR init (1).
 172 Helper programs for this process exist; see
 173 .BR switch_root (8).
 174 .SH BUGS
 175 .BR pivot_root ()
 176 should not have to change root and current working directory of other
 177 processes in the system.
 178 .PP
 179 Some of the more obscure uses of
 180 .BR pivot_root ()
 181 may quickly lead to
 182 insanity.
 183 .SH SEE ALSO
 184 .BR chdir (2),
 185 .BR chroot (2),
 186 .BR mount (2),
 187 .BR stat (2),
 188 .BR initrd (4),
 189 .BR mount_namespaces (7),
 190 .BR pivot_root (8),
 191 .BR switch_root (8)