SETNS(2)



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 call-
       ing 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  reassoci-
       ated  with  the corresponding namespace, subject to any constraints im-
       posed 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_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 an-
       other 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 de-
              scendant  user  namespace.)   Upon  successfully  joining a user
              namespace, a process is granted all capabilities in  that  name-
              space, 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  cur-
              rent  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 dif-
              ferent 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, 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  speci-
              fied in nstype.

       EINVAL There  is  problem with reassociating the thread with the speci-
              fied 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  name-
              space.

       ENOMEM Cannot  allocate sufficient memory to change the specified name-
              space.

       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  cre-
       ated 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  (com-
       piled  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 back-
       ground.  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.  In-
       side 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 5.07 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                             2020-04-11                          SETNS(2)

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