How OpenVZ boot OS?

I bought VPS on OpenVZ virtualization and enabled Debian guest OS.

Guest /boot dir is empty. /etc/inittab is empty.

How is the OS initialization process performed?

What is the meaning of reboot for OpenVZ container?

Answer

After research I clone vzctl tool as main job about starting/stopping done here:

git clone https://github.com/OpenVZ/vzctl.git
git clone https://github.com/OpenVZ/libvzctl.git

vzctl uses libvzctl where you can find https://github.com/OpenVZ/libvzctl/blob/master/lib/env.c#L783:

int exec_init(struct start_param *param)
{
   char cid[STR_SIZE];
   char *argv[] = {"init", "-z", "      ", NULL};
   char *envp[] = {"HOME=/", "TERM=linux", cid, NULL};
   char **env;
   int errcode = 0;
   logger(1, 0, "Starting init");

   if (stat_file("/sbin/init") == 0 &&
                   stat_file("/ertc/init") == 0  &&
                   stat_file("/bin/init") == 0)
           errcode = VZCTL_E_BAD_TMPL;

   if (write(param->err_p[1], &errcode, sizeof(errcode)) == -1)
           logger(-1, errno, "exec_init: write(param->err_p[1]");

   snprintf(cid, sizeof(cid), "container="SYSTEMD_CTID_FMT, EID(param->h));
   env = makeenv(envp, &param->h->env_param->misc->ve_env);
   if (env == NULL)
           return VZCTL_E_NOMEM;

   execve("/sbin/init", argv, env);
   execve("/etc/init", argv, env);
   execve("/bin/init", argv, env);
   free_ar_str(env);
   free(env);

   return VZCTL_E_BAD_TMPL;
}

Stop done by https://github.com/OpenVZ/libvzctl/blob/master/lib/env.c#L103:

int real_env_stop(int stop_mode)
{
  logger(10, 0, "* stop mode %d", stop_mode);
  close_fds(1, -1);
  /* Disable fsync. The fsync will be done by umount() */
  configure_sysctl("/proc/sys/fs/fsync-enable", "0");
  switch (stop_mode) {
  case M_HALT: {
          char *argv[] = {"halt", NULL};
          char *argv_init[] = {"init", "0", NULL};
          execvep(argv[0], argv, NULL);
          execvep(argv_init[0], argv_init, NULL);
          break;
  }
  case M_REBOOT: {
          char *argv[] = {"reboot", NULL};
          execvep(argv[0], argv, NULL);
          break;
  }
  case M_KILL:
          return syscall(__NR_reboot, LINUX_REBOOT_MAGIC1,
                  LINUX_REBOOT_MAGIC2,
                  LINUX_REBOOT_CMD_POWER_OFF, NULL);
  }
  return -1;
}

Before calling /sbin/init vzctl make some checks (like is system already run or mounted, some file present, etc), halt if required, mount fs, make some isolated analog of chroot and do /sbin/init.

CONCLUSION OpenVZ doesn’t use grub/linux-image/initrd from guest OS and do direct call to first that find among:

"/sbin/init"
"/etc/init"
"/bin/init"

in guest OS. In order to stop it uses one of

halt
init 0
reboot

from guest OS. Container inicialization (security, isolation, mounts, etc) isn’t interested from user point view for guest OS booting process.