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       setns - reassociate thread with a namespace


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

       int setns(int fd, int nstype);


       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 further
       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
       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
       the   caller.    For   further   details   on   PID   namespaces,   see

       A process reassociating itself with a  user  namespace  must  have  the
       CAP_SYS_ADMIN   capability   in   the   target  user  namespace.   Upon
       successfully joining  a  user  namespace,  a  process  is  granted  all
       capabilities  in  that namespace, regardless of its user and group IDs.
       A multithreaded 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
       controlled 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.


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


       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

       ENOMEM Cannot  allocate  sufficient  memory  to  change  the  specified

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


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


       The setns() system call is Linux-specific.


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


       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

       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
           # ./newuts bizarro &
           [1] 3549
           clone() returned 3550
           uts.nodename in child:  bizarro
           uts.nodename in parent: antero
           # uname -n             # Verify hostname in the shell

       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

   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)

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

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

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

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

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


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


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