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>

1 files changed, 5 insertions, 3 deletions

diff --git a/balloon.h b/balloon.h
index 06a8a46..a6c31d5 100644
--- a/balloon.h
+++ b/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);