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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 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 pid_namespaces(7).

       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.

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...\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 */
           errExit("execvp");
       }

SEE ALSO

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

COLOPHON

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