How OpenVZ boot OS?

Question

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 1

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.

Answer 2

An OpenVZ VPS runs, in essence, as its sandboxed process tree within a host system. As such, it does not have its own bootloader or kernel, and will typically not have any files in /boot.

OpenVZ VPSes are started up by having the host system run /bin/init in a special environment, such that it is isolated from the host system, and so that it believes itself to be PID 1. I'm not entirely sure of the details, as Parallels have not documented it in any great detail.

An empty or missing /etc/inittab is normal for a Linux system using systemd init rather than SysV init. Current versions of Debian use systemd by default; this behavior is not specific to OpenVZ.

I'm not entirely sure how running reboot in an OpenVZ VPS works, but I imagine that the host kernel must have some special handling in place for the reboot() system call within a container such that it causes the host kernel to stop or restart the container, rather than the entire system.