1SETNS(2)                   Linux Programmer's Manual                  SETNS(2)
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NAME

6       setns - reassociate thread with a namespace
7

SYNOPSIS

9       #define _GNU_SOURCE             /* See feature_test_macros(7) */
10       #include <sched.h>
11
12       int setns(int fd, int nstype);
13

DESCRIPTION

15       Given a file descriptor referring to a namespace, reassociate the call‐
16       ing thread with that namespace.
17
18       The fd argument is a file descriptor referring to one of the  namespace
19       entries  in  a /proc/[pid]/ns/ directory; see namespaces(7) for further
20       information on /proc/[pid]/ns/.  The calling thread will  be  reassoci‐
21       ated  with  the  corresponding  namespace,  subject  to any constraints
22       imposed by the nstype argument.
23
24       The nstype argument specifies  which  type  of  namespace  the  calling
25       thread  may  be  reassociated  with.  This argument can have one of the
26       following values:
27
28       0      Allow any type of namespace to be joined.
29
30       CLONE_NEWCGROUP (since Linux 4.6)
31              fd must refer to a cgroup namespace.
32
33       CLONE_NEWIPC (since Linux 3.0)
34              fd must refer to an IPC namespace.
35
36       CLONE_NEWNET (since Linux 3.0)
37              fd must refer to a network namespace.
38
39       CLONE_NEWNS (since Linux 3.8)
40              fd must refer to a mount namespace.
41
42       CLONE_NEWPID (since Linux 3.8)
43              fd must refer to a descendant PID namespace.
44
45       CLONE_NEWUSER (since Linux 3.8)
46              fd must refer to a user namespace.
47
48       CLONE_NEWUTS (since Linux 3.0)
49              fd must refer to a UTS namespace.
50
51       Specifying nstype as 0 suffices if the caller knows (or does not  care)
52       what  type  of  namespace  is  referred to by fd.  Specifying a nonzero
53       value for nstype is useful if the caller does not  know  what  type  of
54       namespace  is  referred to by fd and wants to ensure that the namespace
55       is of a particular type.  (The caller might not know the  type  of  the
56       namespace  referred  to  by  fd  if  the  file descriptor was opened by
57       another process and, for example, passed  to  the  caller  via  a  UNIX
58       domain socket.)
59
60   Details for specific namespace types
61       Note  the  following  details  and restrictions when reassociating with
62       specific namespace types:
63
64       User namespaces
65              A process reassociating itself with a user namespace  must  have
66              the  CAP_SYS_ADMIN  capability  in  the  target  user namespace.
67              (This necessarily implies that it is only  possible  to  join  a
68              descendant  user  namespace.)   Upon successfully joining a user
69              namespace, a process is granted all capabilities in that  names‐
70              pace, regardless of its user and group IDs.
71
72              A  multithreaded  process  may  not  change  user namespace with
73              setns().
74
75              It is not permitted to use setns() to reenter the caller's  cur‐
76              rent  user  namespace.   This prevents a caller that has dropped
77              capabilities from regaining those capabilities  via  a  call  to
78              setns().
79
80              For  security reasons, a process can't join a new user namespace
81              if it is sharing filesystem-related attributes  (the  attributes
82              whose  sharing is controlled by the clone(2) CLONE_FS flag) with
83              another process.
84
85              For further details on user namespaces, see user_namespaces(7).
86
87       Mount namespaces
88              Changing the mount namespace requires that  the  caller  possess
89              both  CAP_SYS_CHROOT  and  CAP_SYS_ADMIN capabilities in its own
90              user namespace and CAP_SYS_ADMIN in the user namespace that owns
91              the target mount namespace.
92
93              A  process  can't  join  a  new mount namespace if it is sharing
94              filesystem-related attributes (the attributes whose  sharing  is
95              controlled by the clone(2) CLONE_FS flag) with another process.
96
97              See  user_namespaces(7)  for  details on the interaction of user
98              namespaces and mount namespaces.
99
100       PID namespaces
101              In order to reassociate itself with a  new  PID  namespace,  the
102              caller  must  have  the CAP_SYS_ADMIN capability both in its own
103              user namespace and in the user namespace that  owns  the  target
104              PID namespace.
105
106              If fd refers to a PID namespace, the semantics are somewhat dif‐
107              ferent from other namespace  types:  reassociating  the  calling
108              thread  with a PID namespace changes only the PID namespace that
109              subsequently created child  processes  of  the  caller  will  be
110              placed  in;  it  does not change the PID namespace of the caller
111              itself.
112
113              Reassociating with a PID namespace is allowed only  if  the  PID
114              namespace  specified  by  fd is a descendant (child, grandchild,
115              etc.)  of the PID namespace of the caller.
116
117              For further details on PID namespaces, see pid_namespaces(7).
118
119       Cgroup namespaces
120              In order to reassociate itself with a new cgroup namespace,  the
121              caller  must  have  the CAP_SYS_ADMIN capability both in its own
122              user namespace and in the user namespace that  owns  the  target
123              cgroup namespace.
124
125              Using  setns()  to change the caller's cgroup namespace does not
126              change the caller's cgroup memberships.
127
128       Network, IPC, and UTS namespaces
129              In order to reassociate itself with a new network, IPC,  or  UTS
130              namespace,  the  caller  must  have the CAP_SYS_ADMIN capability
131              both in its own user namespace and in the  user  namespace  that
132              owns the target namespace.
133

RETURN VALUE

135       On success, setns() returns 0.  On failure, -1 is returned and errno is
136       set to indicate the error.
137

ERRORS

139       EBADF  fd is not a valid file descriptor.
140
141       EINVAL fd refers to a namespace whose type does not match  that  speci‐
142              fied in nstype.
143
144       EINVAL There  is  problem with reassociating the thread with the speci‐
145              fied namespace.
146
147       EINVAL The caller tried to join an ancestor (parent,  grandparent,  and
148              so on) PID namespace.
149
150       EINVAL The  caller  attempted to join the user namespace in which it is
151              already a member.
152
153       EINVAL The caller shares filesystem (CLONE_FS)  state  (in  particular,
154              the root directory) with other processes and tried to join a new
155              user namespace.
156
157       EINVAL The caller is multithreaded and tried to join a new user  names‐
158              pace.
159
160       ENOMEM Cannot allocate sufficient memory to change the specified names‐
161              pace.
162
163       EPERM  The calling thread did not have the required capability for this
164              operation.
165

VERSIONS

167       The  setns() system call first appeared in Linux in kernel 3.0; library
168       support was added to glibc in version 2.14.
169

CONFORMING TO

171       The setns() system call is Linux-specific.
172

NOTES

174       Not all of the attributes that can be shared when a new thread is  cre‐
175       ated using clone(2) can be changed using setns().
176

EXAMPLE

178       The  program  below  takes  two  or more arguments.  The first argument
179       specifies  the  pathname  of  a   namespace   file   in   an   existing
180       /proc/[pid]/ns/  directory.   The remaining arguments specify a command
181       and its arguments.  The program opens the namespace  file,  joins  that
182       namespace using setns(), and executes the specified command inside that
183       namespace.
184
185       The following shell session demonstrates the use of this program  (com‐
186       piled  as  a binary named ns_exec) in conjunction with the CLONE_NEWUTS
187       example program in the clone(2) man page (complied as  a  binary  named
188       newuts).
189
190       We  begin  by  executing  the  example program in clone(2) in the back‐
191       ground.  That program creates a child in a separate UTS namespace.  The
192       child  changes  the  hostname in its namespace, and then both processes
193       display the hostnames in their UTS namespaces, so that we can see  that
194       they are different.
195
196           $ su                   # Need privilege for namespace operations
197           Password:
198           # ./newuts bizarro &
199           [1] 3549
200           clone() returned 3550
201           uts.nodename in child:  bizarro
202           uts.nodename in parent: antero
203           # uname -n             # Verify hostname in the shell
204           antero
205
206       We  then  run  the  program  shown  below, using it to execute a shell.
207       Inside that shell, we verify that the hostname is the one  set  by  the
208       child created by the first program:
209
210           # ./ns_exec /proc/3550/ns/uts /bin/bash
211           # uname -n             # Executed in shell started by ns_exec
212           bizarro
213
214   Program source
215       #define _GNU_SOURCE
216       #include <fcntl.h>
217       #include <sched.h>
218       #include <unistd.h>
219       #include <stdlib.h>
220       #include <stdio.h>
221
222       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
223                               } while (0)
224
225       int
226       main(int argc, char *argv[])
227       {
228           int fd;
229
230           if (argc < 3) {
231               fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\n", argv[0]);
232               exit(EXIT_FAILURE);
233           }
234
235           fd = open(argv[1], O_RDONLY); /* Get file descriptor for namespace */
236           if (fd == -1)
237               errExit("open");
238
239           if (setns(fd, 0) == -1)       /* Join that namespace */
240               errExit("setns");
241
242           execvp(argv[2], &argv[2]);    /* Execute a command in namespace */
243           errExit("execvp");
244       }
245

SEE ALSO

247       nsenter(1),  clone(2),  fork(2),  unshare(2),  vfork(2), namespaces(7),
248       unix(7)
249

COLOPHON

251       This page is part of release 5.04 of the Linux  man-pages  project.   A
252       description  of  the project, information about reporting bugs, and the
253       latest    version    of    this    page,    can     be     found     at
254       https://www.kernel.org/doc/man-pages/.
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258Linux                             2019-10-10                          SETNS(2)
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