setns(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

SETNS(2)                  Linux Programmer's Manual                 SETNS(2)

NAME         top

       setns - reassociate thread with a namespace

SYNOPSIS         top

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

       int setns(int fd, int nstype);

DESCRIPTION         top

       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 5.6)
              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(2) 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 names‐
              pace 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 subse‐
              quently 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 tar‐
              get PID namespace is a descendant (child, grandchild, etc.)
              of, or is the same as, the current PID namespace of the call‐
              er.

              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 capa‐
              bility both in its own user namespace and in the user names‐
              pace that owns the target namespace.

RETURN VALUE         top

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

ERRORS         top

       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         top

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

CONFORMING TO         top

       The setns() system call is Linux-specific.

NOTES         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.08 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
       https://www.kernel.org/doc/man-pages/.

Linux                            2020-08-13                         SETNS(2)

Pages that refer to this page: nsenter(1)clone2(2)__clone2(2)clone(2)clone3(2)pidfd_open(2)setns(2)syscalls(2)unshare(2)proc(5)procfs(5)systemd.exec(5)capabilities(7)cgroup_namespaces(7)ipc_namespaces(7)mount_namespaces(7)namespaces(7)pid_namespaces(7)time_namespaces(7)user_namespaces(7)uts_namespaces(7)ip-netns(8)