man2/unshare.2

   1 .\" Copyright (C) 2006, Janak Desai <janak@us.ibm.com>
   2 .\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\" Licensed under the GPL
   4 .\"
   5 .\" Patch Justification:
   6 .\" unshare system call is needed to implement, using PAM,
   7 .\" per-security_context and/or per-user namespace to provide
   8 .\" polyinstantiated directories. Using unshare and bind mounts, a
   9 .\" PAM module can create private namespace with appropriate
  10 .\" directories(based on user's security context) bind mounted on
  11 .\" public directories such as /tmp, thus providing an instance of
  12 .\" /tmp that is based on user's security context. Without the
  13 .\" unshare system call, namespace separation can only be achieved
  14 .\" by clone, which would require porting and maintaining all commands
  15 .\" such as login, and su, that establish a user session.
  16 .\"
  17 .\" FIXME Document CLONE_NEWIPC, which is new in 2.6.18
  18 .\" FIXME Document CLONE_NEWUTS, which is new in 2.6.19
  19 .\" FIXME Document CLONE_SYSVSEM, which is new in 2.6.26
  20 .\"
  21 .TH UNSHARE 2 2008-11-20 "Linux" "Linux Programmer's Manual"
  22 .SH NAME
  23 unshare \- disassociate parts of the process execution context
  24 .SH SYNOPSIS
  25 .nf
  26 .B #define _GNU_SOURCE
  27 .\" Actually _BSD_SOURCE || _SVID_SOURCE
  28 .\" See http://sources.redhat.com/bugzilla/show_bug.cgi?id=4749
  29 .B #include <sched.h>
  30 .sp
  31 .BI "int unshare(int " flags );
  32 .fi
  33 .SH DESCRIPTION
  34 .BR unshare ()
  35 allows a process to disassociate parts of its execution
  36 context that are currently being shared with other processes.
  37 Part of the execution context, such as the mount namespace, is shared
  38 implicitly when a new process is created using
  39 .BR fork (2)
  40 or
  41 .BR vfork (2),
  42 while other parts, such as virtual memory, may be
  43 shared by explicit request when creating a process using
  44 .BR clone (2).
  45
  46 The main use of
  47 .BR unshare ()
  48 is to allow a process to control its
  49 shared execution context without creating a new process.
  50
  51 The
  52 .I flags
  53 argument is a bit mask that specifies which parts of
  54 the execution context should be unshared.
  55 This argument is specified by ORing together zero or more
  56 of the following constants:
  57 .TP
  58 .B CLONE_FILES
  59 Reverse the effect of the
  60 .BR clone (2)
  61 .B CLONE_FILES
  62 flag.
  63 Unshare the file descriptor table, so that the calling process
  64 no longer shares its file descriptors with any other process.
  65 .TP
  66 .B CLONE_FS
  67 Reverse the effect of the
  68 .BR clone (2)
  69 .B CLONE_FS
  70 flag.
  71 Unshare file system attributes, so that the calling process
  72 no longer shares its root directory, current directory,
  73 or umask attributes with any other process.
  74 .BR chroot (2),
  75 .BR chdir (2),
  76 or
  77 .BR umask (2)
  78 .TP
  79 .B CLONE_NEWNS
  80 .\" These flag name are inconsistent:
  81 .\" CLONE_NEWNS does the same thing in clone(), but CLONE_VM,
  82 .\" CLONE_FS, and CLONE_FILES reverse the action of the clone()
  83 .\" flags of the same name.
  84 This flag has the
  85 .I same
  86 effect as the
  87 .BR clone (2)
  88 .B CLONE_NEWNS
  89 flag.
  90 Unshare the mount namespace,
  91 so that the calling process has a private copy of
  92 its namespace which is not shared with any other process.
  93 Specifying this flag automatically implies
  94 .B CLONE_FS
  95 as well.
  96 .\" As at 2.6.16, the following forced implications also apply,
  97 .\" although the relevant flags are not yet implemented.
  98 .\" If CLONE_THREAD is set force CLONE_VM.
  99 .\" If CLONE_VM is set, force CLONE_SIGHAND.
 100 .\" If CLONE_SIGHAND is set and signals are also being shared
 101 .\" (i.e., current->signal->count > 1), force CLONE_THREAD.
 102 .\"
 103 .\" FIXME . CLONE_VM is not (yet, as at 2.6.16) implemented.
 104 .\" .TP
 105 .\" .B CLONE_VM
 106 .\" Reverse the effect of the
 107 .\" .BR clone (2)
 108 .\" .B CLONE_VM
 109 .\" flag.
 110 .\" .RB ( CLONE_VM
 111 .\" is also implicitly set by
 112 .\" .BR vfork (2),
 113 .\" and can be reversed using this
 114 .\" .BR unshare ()
 115 .\" flag.)
 116 .\" Unshare virtual memory, so that the calling process no
 117 .\" longer shares its virtual address space with any other process.
 118 .PP
 119 If
 120 .I flags
 121 is specified as zero, then
 122 .BR unshare ()
 123 is a no-op;
 124 no changes are made to the calling process's execution context.
 125 .SH RETURN VALUE
 126 On success, zero returned.
 127 On failure, \-1 is returned and
 128 .I errno
 129 is set to indicate the error.
 130 .SH ERRORS
 131 .TP
 132 .B EINVAL
 133 An invalid bit was specified in
 134 .IR flags .
 135 .TP
 136 .B ENOMEM
 137 Cannot allocate sufficient memory to copy parts of caller's
 138 context that need to be unshared.
 139 .TP
 140 .B EPERM
 141 .I flags
 142 specified
 143 .B CLONE_NEWNS
 144 but the calling process was not privileged (did not have the
 145 .B CAP_SYS_ADMIN
 146 capability).
 147 .SH VERSIONS
 148 The
 149 .BR unshare ()
 150 system call was added to Linux in kernel 2.6.16.
 151 .SH CONFORMING TO
 152 The
 153 .BR unshare ()
 154 system call is Linux-specific.
 155 .SH NOTES
 156 Not all of the process attributes that can be shared when
 157 a new process is created using
 158 .BR clone (2)
 159 can be unshared using
 160 .BR unshare ().
 161 In particular, as at kernel 2.6.16,
 162 .BR unshare ()
 163 does not implement flags that reverse the effects of
 164 .BR CLONE_SIGHAND ,
 165 .\" However, we can do unshare(CLONE_SIGHAND) if CLONE_SIGHAND
 166 .\" was not specified when doing clone(); i.e., unsharing
 167 .\" signal handlers is permitted if we are not actually
 168 .\" sharing signal handlers.   mtk
 169 .BR CLONE_SYSVSEM ,
 170 .BR CLONE_THREAD ,
 171 or
 172 .\" FIXME . check future kernel versions (e.g., 2.6.17)
 173 .\" to see if CLONE_VM gets implemented.
 174 .BR CLONE_VM .
 175 .\" However, as at 2.6.16, we can do unshare(CLONE_VM) if CLONE_VM
 176 .\" was not specified when doing clone(); i.e., unsharing
 177 .\" virtual memory is permitted if we are not actually
 178 .\" sharing virtual memory.   mtk
 179 Such functionality may be added in the future, if required.
 180 .\"
 181 .\"9) Future Work
 182 .\"--------------
 183 .\"The current implementation of unshare does not allow unsharing of
 184 .\"signals and signal handlers. Signals are complex to begin with and
 185 .\"to unshare signals and/or signal handlers of a currently running
 186 .\"process is even more complex. If in the future there is a specific
 187 .\"need to allow unsharing of signals and/or signal handlers, it can
 188 .\"be incrementally added to unshare without affecting legacy
 189 .\"applications using unshare.
 190 .\"
 191 .SH SEE ALSO
 192 .BR clone (2),
 193 .BR fork (2),
 194 .BR vfork (2),
 195 Documentation/unshare.txt