setns

SETNS(2)                   Linux Programmer's Manual                  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
       Given a file descriptor referring to a namespace, reassociate the
       calling thread with that namespace.

       The fd argument is a file descriptor referring to one of the namespace
       entries in a /proc/[pid]/ns/ directory; see namespaces(7) for further
       information on /proc/[pid]/ns/.  The calling thread will be
       reassociated with the corresponding namespace, subject to any
       constraints imposed by the nstype argument.

       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_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.)

   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.

              If fd refers to a PID namespace, the semantics are 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 PID
              namespace specified by fd is a descendant (child, grandchild,
              etc.)  of the 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, and UTS namespaces
              In order to reassociate itself with a new network, IPC, 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.

       ENOMEM Cannot allocate sufficient memory to change the specified
              namespace.

       EPERM  The calling thread did not have the required capability for this
              operation.

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
       Not all of the attributes that can be shared when a new thread is
       created using clone(2) can be changed using setns().

EXAMPLE
       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);
           }

           fd = open(argv[1], O_RDONLY); /* Get file descriptor for namespace */
           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
       This page is part of release 5.03 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                             2019-10-10                          SETNS(2)