#include <iostream>
#include <sstream>

#include <cerrno>
#include <cstring>

#include <sys/stat.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sched.h>

#include <cstdio>
#include <cstdlib>

#include <unistd.h>

#define OLD_ROOT "oldroot"

using namespace std;

int main(int argc, char* argv[]) {
  if(argc != 2) {
    cerr << "First argument is the template for the root" << endl;
    return -1;
  }
  string template_root(argv[1]);

  // Generate tmpfs directory
  stringstream ss;
  ss << "tmpfs_" << template_root;
  string tmpfs = ss.str();

  struct stat st;
  if(stat(tmpfs.c_str(),&st) == 0) {
    cerr << tmpfs << " already exists!" << endl;
    return -1;
  }

  if(mkdir(tmpfs.c_str(),644) != 0) {
    perror("mkdir");
    return -1;
  }

  // Mount tmpfs
  if(mount("none", tmpfs.c_str(), "tmpfs", 0, "mode=0644,uid=65534,size=10M") != 0) {
    perror("mount");
    return 1;
  }

  // Copy content
  ss.str("");
  ss.clear();
  ss << "cp -r " << template_root << "/* " << tmpfs;
  system(ss.str().c_str());

  // Create mount namespace
  if(unshare(CLONE_NEWNS) < 0) {
    perror("unshare");
    return 1;
  }

  // Make old root MS_SLAVE
  // Using the syscall directly is not easy since it requires
  // reading the /etc/fstab
  if(system("mount --make-rslave /") != 0) {
    perror("mount");
    return 1;
  }

  // Change directory
  chdir(tmpfs.c_str());

  // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
  // This command leads to EBUSY of the pivot_root.
  // Without it, everything works fine
  chroot(".");

  // Move to new root file system
  if(mkdir(OLD_ROOT,644) != 0) {
    perror("mkdir");
    return -1;
  }
  if(syscall(SYS_pivot_root,".",OLD_ROOT) < 0) {
    perror("pivot_root");
    return 1;
  }

  // Remove old mount
  if(umount2(OLD_ROOT,MNT_DETACH) != 0) {
    perror("umount");
    return 1;
  }
  if(rmdir(OLD_ROOT) != 0) {
    perror("rmdir");
    return 1;
  }

  // Enter shell - only for testing
  execl("/bin/bash","-i",NULL);

  return 0;
}