#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <sched.h> int setns(int fd, int nstype);
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:
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.)
CLONE_NEWPID behaves somewhat differently from the other nstype values: reassociating the calling thread with a PID namespace changes only the PID namespace that child processes of the caller will be created 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).
A process reassociating itself with a user namespace must have the CAP_SYS_ADMIN capability in the target 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).
A process may not be reassociated with a new mount namespace if it is multithreaded. 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 target mount namespace. See user_namespaces(7) for details on the interaction of user namespaces and mount namespaces.
Using setns() to change the caller's cgroup namespace does not change the caller's cgroup memberships.
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
#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"); }