balloon: Separate out stat and balloon handling

Passing on '0' as ballooning target to indicate retrieval of stats is
bad API.  It also makes 'balloon 0' in the monitor cause a segfault.
Have two different functions handle the different functionality instead.

Detailed explanation from Markus's review:

1. do_info_balloon() is an info_async() method.  It receives a callback
   with argument, to be called exactly once (callback frees the
   argument).  It passes the callback via qemu_balloon_status() and
   indirectly through qemu_balloon_event to virtio_balloon_to_target().

   virtio_balloon_to_target() executes its balloon stats half.  It
   stores the callback in the device state.

   If it can't send a stats request, it resets stats and calls the
   callback right away.

   Else, it sends a stats request.  The device model runs the callback
   when it receives the answer.

   Works.

2. do_balloon() is a cmd_async() method.  It receives a callback with
   argument, to be called when the command completes.  do_balloon()
   calls it right before it succeeds.  Odd, but should work.

   Nevertheless, it passes the callback on via qemu_ballon() and
   indirectly through qemu_balloon_event to virtio_balloon_to_target().

   a. If the argument is non-zero, virtio_balloon_to_target() executes
      its balloon half, which doesn't use the callback in any way.

      Odd, but works.

   b. If the argument is zero, virtio_balloon_to_target() executes its
      balloon stats half, just like in 1.  It either calls the callback
      right away, or arranges for it to be called later.

      Thus, the callback runs twice: use after free and double free.

Test case: start with -S -device virtio-balloon, execute "balloon 0" in
human monitor.  Runs the callback first from virtio_balloon_to_target(),
then again from do_balloon().

Reported-by: Mike Cao <bcao@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>

balloon.c

@@ -32,30 +32,33 @@
 
 
 static QEMUBalloonEvent *balloon_event_fn;
+static QEMUBalloonStatus *balloon_stat_fn;
 static void *balloon_opaque;
 
-void qemu_add_balloon_handler(QEMUBalloonEvent *func, void *opaque)
+void qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
+                              QEMUBalloonStatus *stat_func, void *opaque)
 {
-    balloon_event_fn = func;
+    balloon_event_fn = event_func;
+    balloon_stat_fn = stat_func;
     balloon_opaque = opaque;
 }
 
-static int qemu_balloon(ram_addr_t target, MonitorCompletion cb, void *opaque)
+static int qemu_balloon(ram_addr_t target)
 {
     if (!balloon_event_fn) {
         return 0;
     }
     trace_balloon_event(balloon_opaque, target);
-    balloon_event_fn(balloon_opaque, target, cb, opaque);
+    balloon_event_fn(balloon_opaque, target);
     return 1;
 }
 
 static int qemu_balloon_status(MonitorCompletion cb, void *opaque)
 {
-    if (!balloon_event_fn) {
+    if (!balloon_stat_fn) {
         return 0;
     }
-    balloon_event_fn(balloon_opaque, 0, cb, opaque);
+    balloon_stat_fn(balloon_opaque, cb, opaque);
     return 1;
 }
 
@@ -135,7 +138,7 @@ int do_balloon(Monitor *mon, const QDict *params,
         return -1;
     }
 
-    ret = qemu_balloon(qdict_get_int(params, "value"), cb, opaque);
+    ret = qemu_balloon(qdict_get_int(params, "value"));
     if (ret == 0) {
         qerror_report(QERR_DEVICE_NOT_ACTIVE, "balloon");
         return -1;

balloon.h

@@ -16,10 +16,12 @@
 
 #include "monitor.h"
 
-typedef void (QEMUBalloonEvent)(void *opaque, ram_addr_t target,
-                                MonitorCompletion cb, void *cb_data);
+typedef void (QEMUBalloonEvent)(void *opaque, ram_addr_t target);
+typedef void (QEMUBalloonStatus)(void *opaque, MonitorCompletion cb,
+                                 void *cb_data);
 
-void qemu_add_balloon_handler(QEMUBalloonEvent *func, void *opaque);
+void qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
+                              QEMUBalloonStatus *stat_func, void *opaque);
 
 void monitor_print_balloon(Monitor *mon, const QObject *data);
 int do_info_balloon(Monitor *mon, MonitorCompletion cb, void *opaque);

hw/virtio-balloon.c

@@ -227,8 +227,7 @@ static void virtio_balloon_stat(void *opaque, MonitorCompletion cb,
     complete_stats_request(dev);
 }
 
-static void virtio_balloon_to_target(void *opaque, ram_addr_t target,
-                                     MonitorCompletion cb, void *cb_data)
+static void virtio_balloon_to_target(void *opaque, ram_addr_t target)
 {
     VirtIOBalloon *dev = opaque;
 
@@ -238,8 +237,6 @@ static void virtio_balloon_to_target(void *opaque, ram_addr_t target,
     if (target) {
         dev->num_pages = (ram_size - target) >> VIRTIO_BALLOON_PFN_SHIFT;
         virtio_notify_config(&dev->vdev);
-    } else {
-        virtio_balloon_stat(opaque, cb, cb_data);
     }
 }
 
@@ -284,7 +281,7 @@ VirtIODevice *virtio_balloon_init(DeviceState *dev)
     s->svq = virtio_add_queue(&s->vdev, 128, virtio_balloon_receive_stats);
 
     reset_stats(s);
-    qemu_add_balloon_handler(virtio_balloon_to_target, s);
+    qemu_add_balloon_handler(virtio_balloon_to_target, virtio_balloon_stat, s);
 
     register_savevm(dev, "virtio-balloon", -1, 1,
                     virtio_balloon_save, virtio_balloon_load, s);