man2/setns.2

   1 .\" Copyright (C) 2011, Eric Biederman <ebiederm@xmission.com>
   2 .\" and Copyright (C) 2011, 2012, Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" %%%LICENSE_START(GPLv2_ONELINE)
   5 .\" Licensed under the GPLv2
   6 .\" %%%LICENSE_END
   7 .\"
   8 .TH SETNS 2 2016-03-15 "Linux" "Linux Programmer's Manual"
   9 .SH NAME
  10 setns \- reassociate thread with a namespace
  11 .SH SYNOPSIS
  12 .nf
  13 .BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
  14 .B #include <sched.h>
  15 .sp
  16 .BI "int setns(int " fd ", int " nstype );
  17 .fi
  18 .SH DESCRIPTION
  19 Given a file descriptor referring to a namespace,
  20 reassociate the calling thread with that namespace.
  21
  22 The
  23 .I fd
  24 argument is a file descriptor referring to one of the namespace entries in a
  25 .I /proc/[pid]/ns/
  26 directory; see
  27 .BR namespaces (7)
  28 for further information on
  29 .IR /proc/[pid]/ns/ .
  30 The calling thread will be reassociated with the corresponding namespace,
  31 subject to any constraints imposed by the
  32 .I nstype
  33 argument.
  34
  35 The
  36 .I nstype
  37 argument specifies which type of namespace
  38 the calling thread may be reassociated with.
  39 This argument can have one of the following values:
  40 .TP
  41 .BR 0
  42 Allow any type of namespace to be joined.
  43 .TP
  44 .BR CLONE_NEWCGROUP " (since Linux 4.6)"
  45 .I fd
  46 must refer to a cgroup namespace.
  47 .TP
  48 .BR CLONE_NEWIPC " (since Linux 3.0)"
  49 .I fd
  50 must refer to an IPC namespace.
  51 .TP
  52 .BR CLONE_NEWNET " (since Linux 3.0)"
  53 .I fd
  54 must refer to a network namespace.
  55 .TP
  56 .BR CLONE_NEWNS " (since Linux 3.8)"
  57 .I fd
  58 must refer to a mount namespace.
  59 .TP
  60 .BR CLONE_NEWPID " (since Linux 3.8)"
  61 .I fd
  62 must refer to a descendant PID namespace.
  63 .TP
  64 .BR CLONE_NEWUSER " (since Linux 3.8)"
  65 .I fd
  66 must refer to a user namespace.
  67 .TP
  68 .BR CLONE_NEWUTS " (since Linux 3.0)"
  69 .I fd
  70 must refer to a UTS namespace.
  71 .PP
  72 Specifying
  73 .I nstype
  74 as 0 suffices if the caller knows (or does not care)
  75 what type of namespace is referred to by
  76 .IR fd .
  77 Specifying a nonzero value for
  78 .I nstype
  79 is useful if the caller does not know what type of namespace is referred to by
  80 .IR fd
  81 and wants to ensure that the namespace is of a particular type.
  82 (The caller might not know the type of the namespace referred to by
  83 .IR fd
  84 if the file descriptor was opened by another process and, for example,
  85 passed to the caller via a UNIX domain socket.)
  86
  87 .B CLONE_NEWPID
  88 behaves somewhat differently from the other
  89 .I nstype
  90 values:
  91 reassociating the calling thread with a PID namespace changes only
  92 the PID namespace that child processes of the caller will be created in;
  93 it does not change the PID namespace of the caller itself.
  94 Reassociating with a PID namespace is allowed only if the
  95 PID namespace specified by
  96 .IR fd
  97 is a descendant (child, grandchild, etc.)
  98 of the PID namespace of the caller.
  99 For further details on PID namespaces, see
 100 .BR pid_namespaces (7).
 101
 102 A process reassociating itself with a user namespace must have the
 103 .B CAP_SYS_ADMIN
 104 .\" See kernel/user_namespace.c:userns_install() [3.8 source]
 105 capability in the target user namespace.
 106 Upon successfully joining a user namespace,
 107 a process is granted all capabilities in that namespace,
 108 regardless of its user and group IDs.
 109 A multithreaded process may not change user namespace with
 110 .BR setns ().
 111 It is not permitted to use
 112 .BR setns ()
 113 to reenter the caller's current user namespace.
 114 This prevents a caller that has dropped capabilities from regaining
 115 those capabilities via a call to
 116 .BR setns ().
 117 For security reasons,
 118 .\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
 119 .\" https://lwn.net/Articles/543273/
 120 a process can't join a new user namespace if it is sharing
 121 filesystem-related attributes
 122 (the attributes whose sharing is controlled by the
 123 .BR clone (2)
 124 .B CLONE_FS
 125 flag) with another process.
 126 For further details on user namespaces, see
 127 .BR user_namespaces (7).
 128
 129 A process may not be reassociated with a new mount namespace if it is
 130 multithreaded.
 131 .\" Above check is in fs/namespace.c:mntns_install() [3.8 source]
 132 Changing the mount namespace requires that the caller possess both
 133 .B CAP_SYS_CHROOT
 134 and
 135 .BR CAP_SYS_ADMIN
 136 capabilities in its own user namespace and
 137 .BR CAP_SYS_ADMIN
 138 in the target mount namespace.
 139 See
 140 .BR user_namespaces (7)
 141 for details on the interaction of user namespaces and mount namespaces.
 142
 143 Using
 144 .BR setns ()
 145 to change the caller's cgroup namespace does not change
 146 the caller's cgroup memberships.
 147 .SH RETURN VALUE
 148 On success,
 149 .BR setns ()
 150 returns 0.
 151 On failure, \-1 is returned and
 152 .I errno
 153 is set to indicate the error.
 154 .SH ERRORS
 155 .TP
 156 .B EBADF
 157 .I fd
 158 is not a valid file descriptor.
 159 .TP
 160 .B EINVAL
 161 .I fd
 162 refers to a namespace whose type does not match that specified in
 163 .IR nstype .
 164 .TP
 165 .B EINVAL
 166 There is problem with reassociating
 167 the thread with the specified namespace.
 168 .TP
 169 .\" See kernel/pid_namespace.c::pidns_install() [kernel 3.18 sources]
 170 .B EINVAL
 171 The caller tried to join an ancestor (parent, grandparent, and so on)
 172 PID namespace.
 173 .TP
 174 .B EINVAL
 175 The caller attempted to join the user namespace
 176 in which it is already a member.
 177 .TP
 178 .B EINVAL
 179 .\" commit e66eded8309ebf679d3d3c1f5820d1f2ca332c71
 180 The caller shares filesystem
 181 .RB ( CLONE_FS )
 182 state (in particular, the root directory)
 183 with other processes and tried to join a new user namespace.
 184 .TP
 185 .B EINVAL
 186 .\" See kernel/user_namespace.c::userns_install() [kernel 3.15 sources]
 187 The caller is multithreaded and tried to join a new user namespace.
 188 .TP
 189 .B ENOMEM
 190 Cannot allocate sufficient memory to change the specified namespace.
 191 .TP
 192 .B EPERM
 193 The calling thread did not have the required capability
 194 for this operation.
 195 .SH VERSIONS
 196 The
 197 .BR setns ()
 198 system call first appeared in Linux in kernel 3.0;
 199 library support was added to glibc in version 2.14.
 200 .SH CONFORMING TO
 201 The
 202 .BR setns ()
 203 system call is Linux-specific.
 204 .SH NOTES
 205 Not all of the attributes that can be shared when
 206 a new thread is created using
 207 .BR clone (2)
 208 can be changed using
 209 .BR setns ().
 210 .SH EXAMPLE
 211 The program below takes two or more arguments.
 212 The first argument specifies the pathname of a namespace file in an existing
 213 .I /proc/[pid]/ns/
 214 directory.
 215 The remaining arguments specify a command and its arguments.
 216 The program opens the namespace file, joins that namespace using
 217 .BR setns (),
 218 and executes the specified command inside that namespace.
 219
 220 The following shell session demonstrates the use of this program
 221 (compiled as a binary named
 222 .IR ns_exec )
 223 in conjunction with the
 224 .BR CLONE_NEWUTS
 225 example program in the
 226 .BR clone (2)
 227 man page (complied as a binary named
 228 .IR newuts ).
 229
 230 We begin by executing the example program in
 231 .BR clone (2)
 232 in the background.
 233 That program creates a child in a separate UTS namespace.
 234 The child changes the hostname in its namespace,
 235 and then both processes display the hostnames in their UTS namespaces,
 236 so that we can see that they are different.
 237
 238 .nf
 239 .in +4n
 240 $ \fBsu\fP                   # Need privilege for namespace operations
 241 Password:
 242 # \fB./newuts bizarro &\fP
 243 [1] 3549
 244 clone() returned 3550
 245 uts.nodename in child:  bizarro
 246 uts.nodename in parent: antero
 247 # \fBuname \-n\fP             # Verify hostname in the shell
 248 antero
 249 .in
 250 .fi
 251
 252 We then run the program shown below,
 253 using it to execute a shell.
 254 Inside that shell, we verify that the hostname is the one
 255 set by the child created by the first program:
 256
 257 .nf
 258 .in +4n
 259 # \fB./ns_exec /proc/3550/ns/uts /bin/bash\fP
 260 # \fBuname \-n\fP             # Executed in shell started by ns_exec
 261 bizarro
 262 .in
 263 .fi
 264 .SS Program source
 265 .nf
 266 #define _GNU_SOURCE
 267 #include <fcntl.h>
 268 #include <sched.h>
 269 #include <unistd.h>
 270 #include <stdlib.h>
 271 #include <stdio.h>
 272
 273 #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \\
 274                         } while (0)
 275
 276 int
 277 main(int argc, char *argv[])
 278 {
 279     int fd;
 280
 281     if (argc < 3) {
 282         fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\\n", argv[0]);
 283         exit(EXIT_FAILURE);
 284     }
 285
 286     fd = open(argv[1], O_RDONLY);  /* Get file descriptor for namespace */
 287     if (fd == \-1)
 288         errExit("open");
 289
 290     if (setns(fd, 0) == \-1)        /* Join that namespace */
 291         errExit("setns");
 292
 293     execvp(argv[2], &argv[2]);     /* Execute a command in namespace */
 294     errExit("execvp");
 295 }
 296 .fi
 297 .SH SEE ALSO
 298 .BR clone (2),
 299 .BR fork (2),
 300 .BR unshare (2),
 301 .BR vfork (2),
 302 .BR namespaces (7),
 303 .BR unix (7)