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@gmx.net>
   6 .\"
   7 .TH PIVOT_ROOT 2 2004-06-17 "Linux 2.6.7" "Linux Programmer's Manual"
   8 .SH NAME
   9 pivot_root \- change the root file system
  10 .SH SYNOPSIS
  11 .B #include <linux/unistd.h>
  12 .br
  13 .B #include <errno.h>
  14 .sp
  15 .B _syscall2(int,pivot_root,const char *,new_root,const char *,put_old)
  16           /* Using \fBsyscall\fP(2) may be preferable; see \fBintro\fP(2) */
  17 .sp
  18 .BI "int pivot_root(const char *" new_root ", const char *" put_old );
  19 .SH DESCRIPTION
  20 .BR pivot_root ()
  21 moves the root file system of the current process to the
  22 directory \fIput_old\fP and makes \fInew_root\fP the new root file system
  23 of the current process.
  24 .\"
  25 .\" The
  26 .\" .B CAP_SYS_ADMIN
  27 .\" capability is required.
  28
  29 The typical use of
  30 .BR pivot_root ()
  31 is during system startup, when the
  32 system mounts a temporary root file system (e.g. an \fBinitrd\fP), then
  33 mounts the real root file system, and eventually turns the latter into
  34 the current root of all relevant processes or threads.
  35
  36 .BR pivot_root ()
  37 may or may not change the current root and the current
  38 working directory (cwd) of any processes or threads which use the old
  39 root directory.
  40 The caller of
  41 .BR pivot_root ()
  42 must ensure that processes with root or cwd at the old root operate
  43 correctly in either case.
  44 An easy way to ensure this is to change their
  45 root and cwd to \fInew_root\fP before invoking
  46 .BR pivot_root ().
  47
  48 The paragraph above is intentionally vague because the implementation
  49 of
  50 .BR pivot_root ()
  51 may change in the future.
  52 At the time of writing,
  53 .BR pivot_root ()
  54 changes root and cwd of each process or
  55 thread to \fInew_root\fP if they point to the old root directory.
  56 This
  57 is necessary in order to prevent kernel threads from keeping the old
  58 root directory busy with their root and cwd, even if they never access
  59 the file system in any way.
  60 In the future, there may be a mechanism for
  61 kernel threads to explicitly relinquish any access to the file system,
  62 such that this fairly intrusive mechanism can be removed from
  63 .BR pivot_root ().
  64
  65 Note that this also applies to the current process:
  66 .BR pivot_root ()
  67 may
  68 or may not affect its cwd.
  69 It is therefore recommended to call
  70 \fBchdir("/")\fP immediately after
  71 .BR pivot_root ().
  72
  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 file system as
  78 the current root.
  79 .IP \- 3
  80 \fIput_old\fP must be underneath \fInew_root\fP, i.e. adding a non-zero
  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 No other file system may be mounted on \fIput_old\fP.
  85 .PP
  86 See also
  87 .BR pivot_root (8)
  88 for additional usage examples.
  89
  90 If the current root is not a mount point (e.g. after
  91 .BR chroot (2)
  92 or
  93 .BR pivot_root (),
  94 see also below), not the old root directory, but the
  95 mount point of that file system is mounted on \fIput_old\fP.
  96 .SH NOTES
  97 \fInew_root\fP does not have to be a mount point.
  98 In this case,
  99 \fI/proc/mounts\fP will show the mount point of the file system containing
 100 \fInew_root\fP as root (\fI/\fP).
 101 .SH "RETURN VALUE"
 102 On success, zero is returned.
 103 On error, \-1 is returned, and
 104 \fIerrno\fP is set appropriately.
 105 .SH ERRORS
 106 .BR pivot_root ()
 107 may return (in \fIerrno\fP) any of the errors returned by
 108 .BR stat (2).
 109 Additionally, it may return:
 110 .TP
 111 .B EBUSY
 112 \fInew_root\fP or \fIput_old\fP are on the current root file system,
 113 or a file system is already mounted on \fIput_old\fP.
 114 .TP
 115 .B EINVAL
 116 \fIput_old\fP is not underneath \fInew_root\fP.
 117 .TP
 118 .B ENOTDIR
 119 \fInew_root\fP or \fIput_old\fP is not a directory.
 120 .TP
 121 .B EPERM
 122 The current process does not have the
 123 .B CAP_SYS_ADMIN
 124 capability.
 125 .SH BUGS
 126 .BR pivot_root ()
 127 should not have to change root and cwd of all other
 128 processes in the system.
 129
 130 Some of the more obscure uses of
 131 .BR pivot_root ()
 132 may quickly lead to
 133 insanity.
 134 .SH "CONFORMING TO"
 135 .BR pivot_root ()
 136 is Linux specific and hence is not portable.
 137 .SH VERSIONS
 138 .BR pivot_root ()
 139 was introduced in Linux 2.3.41.
 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)