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 call-
       ing 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 further
       information on /proc/[pid]/ns/.	The calling thread will	 be  reassoci-
       ated  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
       following values:

       0      Allow any type of namespace to be joined.

       CLONE_NEWCGROUP (since Linux 4.6)
	      fd must refer to a cgroup namespace.

       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.	 Reassociating
       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_names-
       paces(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 names-
       pace.  This prevents  a	caller	that  has  dropped  capabilities  from
       regaining  those capabilities via a call to setns().  For security rea-
       sons, 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  fur-
       ther 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.

       Using setns() to change the caller's cgroup namespace does  not	change
       the caller's cgroup memberships.

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	speci-
	      fied in nstype.

       EINVAL There  is	 problem with reassociating the thread with the speci-
	      fied 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	names-
	      pace.

       ENOMEM Cannot allocate sufficient memory to change the specified names-
	      pace.

       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  cre-
       ated 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	 (com-
       piled  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 back-
       ground.	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...\n", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   fd = open(argv[1], O_RDONLY);  /* Get file 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)

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-03-15			      SETNS(2)