SETNS(2)                  (2020-08-13)                   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
          The setns() system call allows the calling thread to move
          into different namespaces.  The fd argument is one of the
          following:

          +o a file descriptor referring to one of the magic links in a
            /proc/[pid]/ns/ directory (or a bind mount to such a
            link);

          +o 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() reasso-
          ciates 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.

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          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. Spec-
          ifying 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 reassociat-
          ing 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, regard-
               less of its user and group IDs.

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               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 capabili-
               ties 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 names-
               pace, the caller must have the CAP_SYS_ADMIN capability
               both in its own user namespace and in the user names-
               pace 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 names-
               pace 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, grand-
               child, 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

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               In order to reassociate itself with a new cgroup names-
               pace, the caller must have the CAP_SYS_ADMIN capability
               both in its own user namespace and in the user names-
               pace 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 names-
               pace.

     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, grandpar-
               ent, 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 par-
               ticular, 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).

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          ENOMEM
               Cannot allocate sufficient memory to change the speci-
               fied 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 exe-
          cutes 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

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              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)

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     COLOPHON
          This page is part of release 5.10 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/.

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