Linux namespace utilities

create-ns.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sched.h>
#include <sys/wait.h>
#include <errno.h>
#include <linux/limits.h>
#include <inttypes.h>
#include <sys/stat.h>

#define STACKSIZE (1024*1024)
static char _child_stack[STACKSIZE];

struct clone_args {
    char **argv;
};

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

static int _child_exec(void *stuff);
static void _print_mnt_namespace_id(void);

int main(int argc, char **argv)
{
    struct clone_args args;
    int clone_flags = CLONE_NEWNS | SIGCHLD;
    pid_t pid;

    if (argc < 2) {
        fprintf(stderr, "\n");
        fprintf(stderr, "%s <cmd>\n", argv[0]);
        fprintf(stderr, "\n");
        fprintf(stderr, "Example: %s /bin/bash\n", argv[0]);
        fprintf(stderr, "\n");
        exit(EXIT_FAILURE);
    }

    args.argv = &argv[1];

    _print_mnt_namespace_id();

    pid = clone(_child_exec, _child_stack + STACKSIZE, clone_flags, &args);
    if (pid < 0) {
        errExit("clone");
    }

    if (waitpid(pid, NULL, 0) == -1) {
        errExit("waitpid");
    }

    return EXIT_SUCCESS;
}

static int _child_exec(void *stuff)
{
    struct clone_args *args = (struct clone_args *)stuff;

    _print_mnt_namespace_id();

    if (execvp(args->argv[0], args->argv) != 0) {
        errExit("execvp");
    }
    abort();
}

static void _print_mnt_namespace_id(void)
{
    char proc_path[PATH_MAX];
    int n;
    struct stat sb;

    n = snprintf(proc_path, sizeof(proc_path), "/proc/%d/ns/mnt", getpid());
    if (n < 0 || (size_t)n >= sizeof(proc_path)) {
        errExit("snprintf");
    }

    if (stat(proc_path, &sb)) {
        errExit("stat");
    }

    printf("mnt %" PRIu32 "\n", sb.st_ino);
}

enter-ns.c

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <linux/limits.h>
#include <asm-generic/unistd.h>

/* CentOS 6 */
// #define setns(fd, nstype) syscall(__NR_setns, fd, nstype);

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

static void _print_mnt_namespace_id(void);
static int _mnt_namespace_fd(pid_t pid);

int main(int argc, char *argv[])
{
    int fd;

    if (argc < 3) {
        fprintf(stderr, "\n");
        fprintf(stderr, "%s <pid> <cmd> <args>...\n", argv[0]);
        fprintf(stderr, "\n");
        fprintf(stderr, "pid\tPID of process to join the mnt namespace\n");
        fprintf(stderr, "cmd\tCommand to execute after joining namespace\n");
        fprintf(stderr, "args\tCommand arguments\n");
        fprintf(stderr, "\n");
        fprintf(stderr, "Example: %s $(pidof NetworkManager) /bin/bash\n", argv[0]);
        fprintf(stderr, "\n");
        exit(EXIT_FAILURE);
    }

    /* Get file descriptor for namespace */
    fd =  _mnt_namespace_fd(atoi(argv[1]));
    if (fd == -1) {
        errExit("open");
    }

    _print_mnt_namespace_id();

    /* Join that namespace */
    if (setns(fd, 0) == -1) {  
        errExit("setns");
    }

    _print_mnt_namespace_id();

    /* Execute a command in namespace */
    execvp(argv[2], &argv[2]);
    errExit("execvp");
}

static void _print_mnt_namespace_id(void)
{
    char proc_path[PATH_MAX];
    int n;
    struct stat sb;

    n = snprintf(proc_path, sizeof(proc_path), "/proc/%d/ns/mnt", getpid());
    if (n < 0 || (size_t)n >= sizeof(proc_path)) {
        errExit("snprintf");
    }

    if (stat(proc_path, &sb)) {
        errExit("stat");
    }

    printf("mnt %" PRIu32 "\n", sb.st_ino);
}

static int _mnt_namespace_fd(pid_t pid)
{
    char proc_path[PATH_MAX];
    int n;

    n = snprintf(proc_path, sizeof(proc_path), "/proc/%d/ns/mnt", pid);
    if (n < 0 || (size_t)n >= sizeof(proc_path)) {
        errExit("snprintf");
    }

    return open(proc_path, O_RDONLY);
}