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

NAME

       setns - reassociate thread with a namespace

SYNOPSIS

       #define _GNU_SOURCE	       /* See feature_test_macros(7) */
       #include <sched.h>

       int setns(int fd, int nstype);

DESCRIPTION

       Given a file descriptor referring to a namespace, reassociate the calling thread with that namespace.

       The  fd	argument is a file descriptor referring to one of the namespace entries in a /proc/[pid]/ns/ directory; see namespaces(7) for fur-
       ther information on /proc/[pid]/ns/.  The calling thread will be reassociated with the corresponding namespace, subject to any  constraints
       imposed by the nstype argument.

       The  nstype argument specifies which type of namespace the calling thread may be reassociated with.  This argument can have one of the fol-
       lowing values:

       0      Allow any type of namespace to be joined.

       CLONE_NEWIPC (since Linux 3.0)
	      fd must refer to an IPC namespace.

       CLONE_NEWNET (since Linux 3.0)
	      fd must refer to a network namespace.

       CLONE_NEWNS (since Linux 3.8)
	      fd must refer to a mount namespace.

       CLONE_NEWPID (since Linux 3.8)
	      fd must refer to a descendant PID namespace.

       CLONE_NEWUSER (since Linux 3.8)
	      fd must refer to a user namespace.

       CLONE_NEWUTS (since Linux 3.0)
	      fd must refer to a UTS namespace.

       Specifying nstype as 0 suffices if the caller knows (or does not care) what type of namespace is referred to by fd.  Specifying	a  nonzero
       value  for  nstype is useful if the caller does not know what type of namespace is referred to by fd and wants to ensure that the namespace
       is of a particular type.  (The caller might not know the type of the namespace referred to by fd if  the  file  descriptor  was	opened	by
       another process and, for example, passed to the caller via a UNIX domain socket.)

       CLONE_NEWPID  behaves somewhat differently from the other nstype values: reassociating the calling thread with a PID namespace changes only
       the PID namespace that child processes of the caller will be created in; it does not change the PID namespace of the caller itself.   Reas-
       sociating  with	a PID namespace is allowed only if the PID namespace specified by fd is a descendant (child, grandchild, etc.)	of the PID
       namespace of the caller.  For further details on PID namespaces, see pid_namespaces(7).

       A process reassociating itself with a user namespace must have the CAP_SYS_ADMIN capability in the target user  namespace.   Upon  success-
       fully  joining  a  user namespace, a process is granted all capabilities in that namespace, regardless of its user and group IDs.  A multi-
       threaded process may not change user namespace with setns().  It is not permitted to use setns()  to  reenter  the  caller's  current  user
       namespace.   This  prevents  a  caller that has dropped capabilities from regaining those capabilities via a call to setns().  For security
       reasons, a process can't join a new user namespace if it is sharing filesystem-related attributes (the attributes  whose  sharing  is  con-
       trolled by the clone(2) CLONE_FS flag) with another process.  For further details on user namespaces, see user_namespaces(7).

       A process may not be reassociated with a new mount namespace if it is multithreaded.  Changing the mount namespace requires that the caller
       possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities in its own user namespace and CAP_SYS_ADMIN in the target mount namespace.   See
       user_namespaces(7) for details on the interaction of user namespaces and mount namespaces.

RETURN VALUE

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

ERRORS

       EBADF  fd is not a valid file descriptor.

       EINVAL fd refers to a namespace whose type does not match that specified in nstype.

       EINVAL There is problem with reassociating the thread with the specified namespace.

       EINVAL The caller tried to join an ancestor (parent, grandparent, and so on) PID namespace.

       EINVAL The caller attempted to join the user namespace in which it is already a member.

       EINVAL The  caller shares filesystem (CLONE_FS) state (in particular, the root directory) with other processes and tried to join a new user
	      namespace.

       EINVAL The caller is multithreaded and tried to join a new user namespace.

       ENOMEM Cannot allocate sufficient memory to change the specified namespace.

       EPERM  The calling thread did not have the required capability for this operation.

VERSIONS

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

CONFORMING TO

       The setns() system call is Linux-specific.

NOTES

       Not all of the attributes that can be shared when a new thread is created using clone(2) can be changed using setns().

EXAMPLE

       The program below takes two or  more  arguments.   The  first  argument	specifies  the	pathname  of  a  namespace  file  in  an  existing
       /proc/[pid]/ns/	directory.  The remaining arguments specify a command and its arguments.  The program opens the namespace file, joins that
       namespace using setns(), and executes the specified command inside that namespace.

       The following shell session demonstrates the use of this program (compiled as a binary named ns_exec) in conjunction with the  CLONE_NEWUTS
       example program in the clone(2) man page (complied as a binary named newuts).

       We  begin  by  executing the example program in clone(2) in the background.  That program creates a child in a separate UTS namespace.  The
       child changes the hostname in its namespace, and then both processes display the hostnames in their UTS namespaces, so that we can see that
       they are different.

	   $ su 		  # Need privilege for namespace operations
	   Password:
	   # ./newuts bizarro &
	   [1] 3549
	   clone() returned 3550
	   uts.nodename in child:  bizarro
	   uts.nodename in parent: antero
	   # uname -n		  # Verify hostname in the shell
	   antero

       We  then  run  the  program shown below, using it to execute a shell.  Inside that shell, we verify that the hostname is the one set by the
       child created by the first program:

	   # ./ns_exec /proc/3550/ns/uts /bin/bash
	   # uname -n		  # Executed in shell started by ns_exec
	   bizarro

   Program source
       #define _GNU_SOURCE
       #include <fcntl.h>
       #include <sched.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <stdio.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); 
			       } while (0)

       int
       main(int argc, char *argv[])
       {
	   int fd;

	   if (argc < 3) {
	       fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...
", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   fd = open(argv[1], O_RDONLY);  /* Get descriptor for namespace */
	   if (fd == -1)
	       errExit("open");

	   if (setns(fd, 0) == -1)	  /* Join that namespace */
	       errExit("setns");

	   execvp(argv[2], &argv[2]);	  /* Execute a command in namespace */
	   errExit("execvp");
       }

SEE ALSO

       clone(2), fork(2), unshare(2), vfork(2), namespaces(7), unix(7)

Linux								    2015-01-10								  SETNS(2)