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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

       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(3).   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.

       Network, IPC, time, 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).

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

       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 <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);
           }

           /* 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)
               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

       nsenter(1), clone(2), fork(2), unshare(2), vfork(2), namespaces(7), unix(7)

COLOPHON

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       found at https://www.kernel.org/doc/man-pages/.