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 .sp
  17 .BI "int pivot_root(const char *" new_root ", const char *" put_old );
  18 .SH DESCRIPTION
  19 \fBpivot_root\fP() moves the root file system of the current process to the
  20 directory \fIput_old\fP and makes \fInew_root\fP the new root file system
  21 of the current process.
  22 .\"
  23 .\" The
  24 .\" .B CAP_SYS_ADMIN
  25 .\" capability is required.
  26
  27 The typical use of \fBpivot_root\fP() is during system startup, when the
  28 system mounts a temporary root file system (e.g. an \fBinitrd\fP), then
  29 mounts the real root file system, and eventually turns the latter into
  30 the current root of all relevant processes or threads.
  31
  32 \fBpivot_root\fP() may or may not change the current root and the current
  33 working directory (cwd) of any processes or threads which use the old
  34 root directory. The caller of \fBpivot_root\fP()
  35 must ensure that processes with root or cwd at the old root operate
  36 correctly in either case. An easy way to ensure this is to change their
  37 root and cwd to \fInew_root\fP before invoking \fBpivot_root\fP().
  38
  39 The paragraph above is intentionally vague because the implementation
  40 of \fBpivot_root\fP() may change in the future. At the time of writing,
  41 \fBpivot_root\fP() changes root and cwd of each process or
  42 thread to \fInew_root\fP if they point to the old root directory. This
  43 is necessary in order to prevent kernel threads from keeping the old
  44 root directory busy with their root and cwd, even if they never access
  45 the file system in any way. In the future, there may be a mechanism for
  46 kernel threads to explicitly relinquish any access to the file system,
  47 such that this fairly intrusive mechanism can be removed from
  48 \fBpivot_root\fP().
  49
  50 Note that this also applies to the current process: \fBpivot_root\fP() may
  51 or may not affect its cwd. It is therefore recommended to call
  52 \fBchdir("/")\fP immediately after \fBpivot_root\fP().
  53
  54 The following restrictions apply to \fInew_root\fP and \fIput_old\fP:
  55 .IP \- 3
  56 They must be directories.
  57 .IP \- 3
  58 \fInew_root\fP and \fIput_old\fP must not be on the same file system as
  59 the current root.
  60 .IP \- 3
  61 \fIput_old\fP must be underneath \fInew_root\fP, i.e. adding a non-zero
  62 number of \fB/..\fP to the string pointed to by \fIput_old\fP must yield
  63 the same directory as \fInew_root\fP.
  64 .IP \- 3
  65 No other file system may be mounted on \fIput_old\fP.
  66 .PP
  67 See also \fBpivot_root(8)\fP for additional usage examples.
  68
  69 If the current root is not a mount point (e.g. after \fBchroot(2)\fP or
  70 \fBpivot_root\fP(), see also below), not the old root directory, but the
  71 mount point of that file system is mounted on \fIput_old\fP.
  72 .SH NOTES
  73 \fInew_root\fP does not have to be a mount point. In this case,
  74 \fB/proc/mounts\fP will show the mount point of the file system containing
  75 \fInew_root\fP as root (\fB/\fP).
  76 .SH "RETURN VALUE"
  77 On success, zero is returned.  On error, \-1 is returned, and
  78 \fIerrno\fP is set appropriately.
  79 .SH ERRORS
  80 \fBpivot_root\fP() may return (in \fIerrno\fP) any of the errors returned by
  81 \fBstat(2)\fP. Additionally, it may return:
  82
  83 .TP
  84 .B EBUSY
  85 \fInew_root\fP or \fIput_old\fP are on the current root file system,
  86 or a file system is already mounted on \fIput_old\fP.
  87 .TP
  88 .B EINVAL
  89 \fIput_old\fP is not underneath \fInew_root\fP.
  90 .TP
  91 .B ENOTDIR
  92 \fInew_root\fP or \fIput_old\fP is not a directory.
  93 .TP
  94 .B EPERM
  95 The current process does not have the
  96 .B CAP_SYS_ADMIN
  97 capability.
  98 .SH BUGS
  99 \fBpivot_root\fP() should not have to change root and cwd of all other
 100 processes in the system.
 101
 102 Some of the more obscure uses of \fBpivot_root\fP() may quickly lead to
 103 insanity.
 104 .SH "CONFORMING TO"
 105 \fBpivot_root\fP() is Linux-specific and hence is not portable.
 106 .SH HISTORY
 107 \fBpivot_root\fP() was introduced in Linux 2.3.41.
 108 .SH "SEE ALSO"
 109 .BR chdir (2),
 110 .BR chroot (2),
 111 .BR stat (2),
 112 .BR initrd (4),
 113 .BR pivot_root (8)