setns

       #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_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
       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.

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().


       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 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 */

Linux                             2015-01-10                          SETNS(2)
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