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Re: notify_no_senders oddity

From: Thomas Bushnell BSG
Subject: Re: notify_no_senders oddity
Date: Mon, 01 Jan 2007 15:26:36 -0800

On Mon, 2007-01-01 at 23:05 +0100, Samuel Thibault wrote:
> Ok, that I can understand, but the code snippet I showed doesn't pass
> through MiG, it's just a function call:
> kern/ipc_kobject.c:ipc_kobject_notify():
>         ipc_port_t port = (ipc_port_t) request_header->msgh_remote_port;
>       ...
>         switch (ip_kotype(port)) {
>                 case IKOT_DEVICE:
>                 return ds_notify(request_header);
> and i386/i386at/i386at_ds_routines.c:ds_notify():
> ds_notify (mach_msg_header_t *msg)
> {
> ...
>         dev = (device_t) msg->msgh_remote_port;
>       if (dev->emul_ops->no_senders)
> > > Both types are used for the same port.
> > 
> > Yes, so?
> They're of incompatible type (neither can cast to the other one), while
> ip_kotype(port) wants ipc_port_t and dev->emul_ops wants device_t.
> Just to be really clear: the code snippet above, given some msg, first
> considers its msgh_remote_port member as an ipc_port_t, and _uses_ it
> as such (reading its ip_object member), and then calls ds_notify, which
> itself considers msgh_remote_port as a device_t, and _uses_ it as such
> (reading its emul_ops member).  I fail to see how this can ever work.

My source (have things changed here in some material way?) does not have
i386/i386at/i386at_ds_routines.c.  It has oskit/ds_routines.c, which

ds_notify (mach_msg_header_t *msg)
  if (msg->msgh_id == MACH_NOTIFY_NO_SENDERS)
      device_t dev;
      mach_no_senders_notification_t *ns;

      ns = (mach_no_senders_notification_t *) msg;
      dev = dev_port_lookup ((ipc_port_t)
      assert (dev);

      /* Extant send rights held one ref on the device object,
         and we just got another one from dev_port_lookup.  */
      device_deallocate (dev);
      device_deallocate (dev);

      return TRUE;
  printf ("ds_notify: strange notification %d\n", msg->msgh_id);
  return FALSE;

Notice that this routine does not simply cast the remote point, but
looks it up in the table of device ports.


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