As found out during review for JDK-8292695, HotSpot installs SIGQUIT handler even if -XX:+AllowUserSignalHandlers is set.

This is because HotSpot installs SIGQUIT handler in a different code path via os::signal(). For other signals, their handlers are installed by set_signal_handler(int sig), which respects -XX:+AllowUserSignalHandlers:

  // Check for overwrite.
  struct sigaction oldAct;
  sigaction(sig, (struct sigaction*)NULL, &oldAct);

  // Query the current signal handler. Needs to be a separate operation
  // from installing a new handler since we need to honor AllowUserSignalHandlers.
  void* oldhand = get_signal_handler(&oldAct);
  if (!HANDLER_IS_IGN_OR_DFL(oldhand) &&
      !HANDLER_IS(oldhand, javaSignalHandler)) {
    if (AllowUserSignalHandlers) {
      // Do not overwrite; user takes responsibility to forward to us.
      return;
    }
...