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 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 reassoci‐
       ated 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 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 names‐
       pace  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 only changes 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 only allowed 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  capabil‐
       ity  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_names‐
       paces(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 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 names‐
       pace, 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
       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2014-09-21                                   SETNS(2)