man2/unshare.2

   1 .\" (C) 2006, Janak Desai <janak@us.ibm.com>
   2 .\" (C) 2006, Michael Kerrisk <mtk-manpages@gmx.ne>
   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 .\"
  20 .TH UNSHARE 2 2007-07-26 "Linux" "Linux Programmer's Manual"
  21 .SH NAME
  22 unshare \- disassociate parts of the process execution context
  23 .SH SYNOPSIS
  24 .nf
  25 .B #define _GNU_SOURCE
  26 .\" Actually _BSD_SOURCE || _SVID_SOURCE
  27 .\" See http://sources.redhat.com/bugzilla/show_bug.cgi?id=4749
  28 .B #include <sched.h>
  29 .sp
  30 .BI "int unshare(int " flags );
  31 .fi
  32 .SH DESCRIPTION
  33 .BR unshare ()
  34 allows a process to disassociate parts of its execution
  35 context that are currently being shared with other processes.
  36 Part of the execution context, such as the namespace, is shared
  37 implicitly when a new process is created using
  38 .BR fork (2)
  39 or
  40 .BR vfork (2),
  41 while other parts, such as virtual memory, may be
  42 shared by explicit request when creating a process using
  43 .BR clone (2).
  44
  45 The main use of
  46 .BR unshare ()
  47 is to allow a process to control its
  48 shared execution context without creating a new process.
  49
  50 The
  51 .I flags
  52 argument is a bit mask that specifies which parts of
  53 the execution context should be unshared.
  54 This argument is specified by ORing together zero or more
  55 of the following constants:
  56 .TP
  57 .B CLONE_FILES
  58 Reverse the effect of the
  59 .BR clone (2)
  60 .B CLONE_FILES
  61 flag.
  62 Unshare the file descriptor table, so that the calling process
  63 no longer shares its file descriptors with any other process.
  64 .TP
  65 .B CLONE_FS
  66 Reverse the effect of the
  67 .BR clone (2)
  68 .B CLONE_FS
  69 flag.
  70 Unshare file system attributes, so that the calling process
  71 no longer shares its root directory, current directory,
  72 or umask attributes with any other process.
  73 .BR chroot (2),
  74 .BR chdir (2),
  75 or
  76 .BR umask (2)
  77 .TP
  78 .B CLONE_NEWNS
  79 .\" These flag name are inconsistent:
  80 .\" CLONE_NEWNS does the same thing in clone(), but CLONE_VM,
  81 .\" CLONE_FS, and CLONE_FILES reverse the action of the clone()
  82 .\" flags of the same name.
  83 This flag has the
  84 .I same
  85 effect as the
  86 .BR clone (2)
  87 .B CLONE_NEWNS
  88 flag.
  89 Unshare the namespace, so that the calling process has a private copy of
  90 its namespace which is not shared with any other process.
  91 Specifying this flag automatically implies
  92 .B CLONE_FS
  93 as well.
  94 .\" As at 2.6.16, the following forced implications also apply,
  95 .\" although the relevant flags are not yet implemented.
  96 .\" If CLONE_THREAD is set force CLONE_VM.
  97 .\" If CLONE_VM is set, force CLONE_SIGHAND.
  98 .\" If CLONE_SIGHAND is set and signals are also being shared
  99 .\" (i.e., current->signal->count > 1), force CLONE_THREAD.
 100 .\"
 101 .\" FIXME . CLONE_VM is not (yet, as at 2.6.16) implemented.
 102 .\" .TP
 103 .\" .B CLONE_VM
 104 .\" Reverse the effect of the
 105 .\" .BR clone (2)
 106 .\" .B CLONE_VM
 107 .\" flag.
 108 .\" .RB ( CLONE_VM
 109 .\" is also implicitly set by
 110 .\" .BR vfork (2),
 111 .\" and can be reversed using this
 112 .\" .BR unshare ()
 113 .\" flag.)
 114 .\" Unshare virtual memory, so that the calling process no
 115 .\" longer shares its virtual address space with any other process.
 116 .PP
 117 If
 118 .I flags
 119 is specified as zero, then
 120 .BR unshare ()
 121 is a no-op;
 122 no changes are made to the calling process's execution context.
 123 .SH RETURN VALUE
 124 On success, zero returned.
 125 On failure, \-1 is returned and
 126 .I errno
 127 is set to indicate the error.
 128 .SH ERRORS
 129 .TP
 130 .B EPERM
 131 .I flags
 132 specified
 133 .B CLONE_NEWNS
 134 but the calling process was not privileged (did not have the
 135 .B CAP_SYS_ADMIN
 136 capability).
 137 .TP
 138 .B ENOMEM
 139 Cannot allocate sufficient memory to copy parts of caller's
 140 context that need to be unshared.
 141 .TP
 142 .B EINVAL
 143 An invalid bit was specified in
 144 .IR flags .
 145 .SH VERSIONS
 146 The
 147 .BR unshare ()
 148 system call was added to Linux in kernel 2.6.16.
 149 .SH CONFORMING TO
 150 The
 151 .BR unshare ()
 152 system call is Linux specific.
 153 .SH NOTES
 154 Not all of the process attributes that can be shared when
 155 a new process is created using
 156 .BR clone (2)
 157 can be unshared using
 158 .BR unshare ().
 159 In particular, as at kernel 2.6.16,
 160 .BR unshare ()
 161 does not implement flags that reverse the effects of
 162 .BR CLONE_SIGHAND ,
 163 .\" However, we can do unshare(CLONE_SIGHAND) if CLONE_SIGHAND
 164 .\" was not specified when doing clone(); i.e., unsharing
 165 .\" signal handlers is permitted if we are not actually
 166 .\" sharing signal handlers.   mtk
 167 .BR CLONE_SYSVSEM ,
 168 .BR CLONE_THREAD ,
 169 or
 170 .\" FIXME . check future kernel versions (e.g., 2.6.17)
 171 .\" to see if CLONE_VM gets implemented.
 172 .BR CLONE_VM .
 173 .\" However, as at 2.6.16, we can do unshare(CLONE_VM) if CLONE_VM
 174 .\" was not specified when doing clone(); i.e., unsharing
 175 .\" virtual memory is permitted if we are not actually
 176 .\" sharing virtual memory.   mtk
 177 Such functionality may be added in the future, if required.
 178 .\"
 179 .\"9) Future Work
 180 .\"--------------
 181 .\"The current implementation of unshare does not allow unsharing of
 182 .\"signals and signal handlers. Signals are complex to begin with and
 183 .\"to unshare signals and/or signal handlers of a currently running
 184 .\"process is even more complex. If in the future there is a specific
 185 .\"need to allow unsharing of signals and/or signal handlers, it can
 186 .\"be incrementally added to unshare without affecting legacy
 187 .\"applications using unshare.
 188 .\"
 189 .SH SEE ALSO
 190 .BR clone (2),
 191 .BR fork (2),
 192 .BR vfork (2),
 193 Documentation/unshare.txt