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 5.8)
              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.

       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         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 5.11 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)clone(2)pidfd_open(2)syscalls(2)unshare(2)proc(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)