# setns

## NAME



           setns - reassociate thread with a namespace
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#define _GNU_SOURCE** /* See feature_test_macros(7) */
           **#include <sched.h>**
    
           **int setns(int** _fd_**, int** _nstype_**);**
    


## DESCRIPTION



           The **setns**() system call allows the calling thread to move into
           different namespaces.  The _fd_ argument is one of the following:
    
           •  a file descriptor referring to one of the magic links in a
              _/proc/_ pid _/ns/_ directory (or a bind mount to such a link);
    
           •  a PID file descriptor (see pidfd_open(2)).
    
           The _nstype_ argument is interpreted differently in each case.
    
       **fd refers to a _/proc/_ pid _/ns/_ link**
           If _fd_ refers to a _/proc/_ pid _/ns/_ link, then **setns**() reassociates
           the calling thread with the namespace associated with that link,
           subject to any constraints imposed by the _nstype_ argument.  In
           this usage, each call to **setns**() changes just one of the caller's
           namespace memberships.
    
           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_NEWTIME**(since Linux 5.8)
                  _fd_ must refer to a time 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.)
    
       **fd is a PID file descriptor**
           Since Linux 5.8, _fd_ may refer to a PID file descriptor obtained
           from pidfd_open(2) or clone(2).  In this usage, **setns**() atomically
           moves the calling thread into one or more of the same namespaces
           as the thread referred to by _fd_.
    
           The _nstype_ argument is a bit mask specified by ORing together _one_
           _or more_ of the **CLONE_NEW*** namespace constants listed above.  The
           caller is moved into each of the target thread's namespaces that
           is specified in _nstype_ ; the caller's memberships in the remaining
           namespaces are left unchanged.
    
           For example, the following code would move the caller into the
           same user, network, and UTS namespaces as PID 1234, but would
           leave the caller's other namespace memberships unchanged:
    
               int fd = pidfd_open(1234, 0);
               setns(fd, CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWUTS);
    
       **Details for specific namespace types**
           Note the following details and restrictions when reassociating
           with specific namespace types:
    
           User namespaces
                  A process reassociating itself with a user namespace must
                  have the **CAP_SYS_ADMIN** capability in the target user
                  namespace.  (This necessarily implies that it is only
                  possible to join a descendant 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).
    
           Mount namespaces
                  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 user
                  namespace that owns the target mount namespace.
    
                  A process can't join a new mount namespace if it is sharing
                  filesystem-related attributes (the attributes whose sharing
                  is controlled by the clone(2) **CLONE_FS** flag) with another
                  process.
    
                  See user_namespaces(7) for details on the interaction of
                  user namespaces and mount namespaces.
    
           PID namespaces
                  In order to reassociate itself with a new PID namespace,
                  the caller must have the **CAP_SYS_ADMIN** capability both in
                  its own user namespace and in the user namespace that owns
                  the target PID namespace.
    
                  Reassociating the PID namespace has somewhat different from
                  other namespace types.  Reassociating the calling thread
                  with a PID namespace changes only the PID namespace that
                  subsequently created child processes of the caller will be
                  placed in; it does not change the PID namespace of the
                  caller itself.
    
                  Reassociating with a PID namespace is allowed only if the
                  target PID namespace is a descendant (child, grandchild,
                  etc.)  of, or is the same as, the current PID namespace of
                  the caller.
    
                  For further details on PID namespaces, see
                  pid_namespaces(7).
    
           Cgroup namespaces
                  In order to reassociate itself with a new cgroup namespace,
                  the caller must have the **CAP_SYS_ADMIN** capability both in
                  its own user namespace and in the user namespace that owns
                  the target cgroup namespace.
    
                  Using **setns**() to change the caller's cgroup namespace does
                  not change the caller's cgroup memberships.
    
           Time namespaces
                  In order to reassociate itself with a new time namespace,
                  the caller must have the **CAP_SYS_ADMIN** capability both in
                  its own user namespace and in the user namespace that owns
                  the target namespace.
    
                  A multithreaded process may not change time namespace with
                  **setns**().
    
           Network, IPC, and UTS namespaces
                  In order to reassociate itself with a new network, IPC,
                  time, or UTS namespace, the caller must have the
                  **CAP_SYS_ADMIN** capability both in its own user namespace and
                  in the user namespace that owns the target namespace.
    


## 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.
    
           **EINVAL** _fd_ is a PID file descriptor and _nstype_ is invalid (e.g., it
                  is 0).
    
           **ENOMEM** Cannot allocate sufficient memory to change the specified
                  namespace.
    
           **EPERM** The calling thread did not have the required capability for
                  this operation.
    
           **ESRCH** _fd_ is a PID file descriptor but the process it refers to no
                  longer exists (i.e., it has terminated and been waited on).
    


## STANDARDS



           Linux.
    


## VERSIONS



           Linux 3.0, glibc 2.14.
    


## NOTES



           For further information on the _/proc/_ pid _/ns/_ magic links, see
           namespaces(7).
    
           Not all of the attributes that can be shared when a new thread is
           created using clone(2) can be changed using **setns**().
    


## EXAMPLES



           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 <err.h>
           #include <fcntl.h>
           #include <sched.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <unistd.h>
    
           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);
               }
    
               /* Get file descriptor for namespace; the file descriptor is opened
                  with O_CLOEXEC so as to ensure that it is not inherited by the
                  program that is later executed. */
    
               fd = open(argv[1], O_RDONLY | O_CLOEXEC);
               if (fd == -1)
                   err(EXIT_FAILURE, "open");
    
               if (setns(fd, 0) == -1)       /* Join that namespace */
                   err(EXIT_FAILURE, "setns");
    
               execvp(argv[2], &argv[2]);    /* Execute a command in namespace */
               err(EXIT_FAILURE, "execvp");
           }
    


* * *