[Qemu-devel] [PATCH v2] qdev_try_create(): Assert that devices we put onto the system bus are SysBusDevices

[Peter Maydell <peter.maydell@linaro.org>]

If qdev_try_create() is passed NULL for the bus, it will automatically
put the newly created device onto the default system bus. However
if the device is not actually a SysBusDevice then this will result
in later crashes (for instance when running the monitor "info qtree"
command) because code reasonably assumes that all devices on the system
bus are system bus devices.

Generally the mistake is that the calling code should create the
object with object_new(TYPE_FOO) rather than qdev_create(NULL, TYPE_FOO);
see commit 6749695eaaf346c1 for an example of fixing this bug.

Assert in qdev_try_create() if the device isn't suitable to put on
the system bus, so that this mistake results in failure earlier
and more reliably.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
v1->v2: use an explicit g_assert() rather than relying on the one
hidden inside a SYS_BUS_DEVICE(dev) cast macro, as suggested by Paolo.

Andreas: the SD card patch which had to go in before this patch is
already in master, so you can just take this patch via your QOM tree
without it causing any awkward ordering issues.

 hw/core/qdev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index db41aa1..15b6713 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -35,6 +35,7 @@
 #include "qemu/error-report.h"
 #include "hw/hotplug.h"
 #include "hw/boards.h"
+#include "hw/sysbus.h"
 #include "qapi-event.h"
 
 int qdev_hotplug = 0;
@@ -161,6 +162,12 @@ DeviceState *qdev_try_create(BusState *bus, const char *type)
     }
 
     if (!bus) {
+        /* Assert that the device really is a SysBusDevice before
+         * we put it onto the sysbus. (Non-sysbus devices which aren't
+         * being put onto a bus should be created with object_new(TYPE_FOO),
+         * not qdev_create(NULL, TYPE_FOO).)
+         */
+        g_assert(object_dynamic_cast(OBJECT(dev), TYPE_SYS_BUS_DEVICE) != NULL);
         bus = sysbus_get_default();
     }
 
-- 
1.9.1