The qdev_unplug() function contains a g_assert(hotplug_ctrl) statement,
so QEMU crashes when the user tries to device_add + device_del a device
that does not have a corresponding hotplug controller. This could be
provoked for a couple of devices in the past (see commit 4c93950659487c7ad
or 84ebd3e8c7d4fe955 for example). So devices clearly need a hotplug
controller when they should be usable with device_add.
The code in qdev_device_add() already checks whether the bus has a proper
hotplug controller, but for devices that do not have a corresponding bus,
there is no appropriate check available yet. In that case we should check
whether the machine itself provides a suitable hotplug controller and
refuse to plug the device if none is available.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v2:
 - Do the check earlier in qdev_device_add
 - Use common new err_dev_del handler in case of failure

 hw/core/qdev.c         | 28 ++++++++++++++++++++--------
 include/hw/qdev-core.h |  1 +
 qdev-monitor.c         |  5 +++++
 3 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 606ab53..a953ec9 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -253,19 +253,31 @@ void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
     dev->alias_required_for_version = required_for_version;
 }
 
+HotplugHandler *qdev_get_machine_hotplug_handler(DeviceState *dev)
+{
+    MachineState *machine;
+    MachineClass *mc;
+    Object *m_obj = qdev_get_machine();
+
+    if (object_dynamic_cast(m_obj, TYPE_MACHINE)) {
+        machine = MACHINE(m_obj);
+        mc = MACHINE_GET_CLASS(machine);
+        if (mc->get_hotplug_handler) {
+            return mc->get_hotplug_handler(machine, dev);
+        }
+    }
+
+    return NULL;
+}
+
 HotplugHandler *qdev_get_hotplug_handler(DeviceState *dev)
 {
-    HotplugHandler *hotplug_ctrl = NULL;
+    HotplugHandler *hotplug_ctrl;
 
     if (dev->parent_bus && dev->parent_bus->hotplug_handler) {
         hotplug_ctrl = dev->parent_bus->hotplug_handler;
-    } else if (object_dynamic_cast(qdev_get_machine(), TYPE_MACHINE)) {
-        MachineState *machine = MACHINE(qdev_get_machine());
-        MachineClass *mc = MACHINE_GET_CLASS(machine);
-
-        if (mc->get_hotplug_handler) {
-            hotplug_ctrl = mc->get_hotplug_handler(machine, dev);
-        }
+    } else {
+        hotplug_ctrl = qdev_get_machine_hotplug_handler(dev);
     }
     return hotplug_ctrl;
 }
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 0891461..5aa536d 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -285,6 +285,7 @@ DeviceState *qdev_try_create(BusState *bus, const char *name);
 void qdev_init_nofail(DeviceState *dev);
 void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
                                  int required_for_version);
+HotplugHandler *qdev_get_machine_hotplug_handler(DeviceState *dev);
 HotplugHandler *qdev_get_hotplug_handler(DeviceState *dev);
 void qdev_unplug(DeviceState *dev, Error **errp);
 void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
diff --git a/qdev-monitor.c b/qdev-monitor.c
index cb2b109..f93455f 100644
--- a/qdev-monitor.c
+++ b/qdev-monitor.c
@@ -614,6 +614,11 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
 
     if (bus) {
         qdev_set_parent_bus(dev, bus);
+    } else if (qdev_hotplug && !qdev_get_machine_hotplug_handler(dev)) {
+        /* No bus, no machine hotplug handler --> device is not hotpluggable */
+        error_setg(&err, "Device '%s' can not be hotplugged on this machine",
+                   driver);
+        goto err_del_dev;
     }
 
     qdev_set_id(dev, qemu_opts_id(opts));
-- 
1.8.3.1

On Thu,  5 Oct 2017 14:32:18 +0200
Thomas Huth <thuth@redhat.com> wrote:

> The qdev_unplug() function contains a g_assert(hotplug_ctrl) statement,
> so QEMU crashes when the user tries to device_add + device_del a device
> that does not have a corresponding hotplug controller. This could be
> provoked for a couple of devices in the past (see commit 4c93950659487c7ad
> or 84ebd3e8c7d4fe955 for example). So devices clearly need a hotplug
> controller when they should be usable with device_add.
> The code in qdev_device_add() already checks whether the bus has a proper
> hotplug controller, but for devices that do not have a corresponding bus,
> there is no appropriate check available yet. In that case we should check
> whether the machine itself provides a suitable hotplug controller and
> refuse to plug the device if none is available.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>

> ---
>  v2:
>  - Do the check earlier in qdev_device_add
>  - Use common new err_dev_del handler in case of failure
> 
>  hw/core/qdev.c         | 28 ++++++++++++++++++++--------
>  include/hw/qdev-core.h |  1 +
>  qdev-monitor.c         |  5 +++++
>  3 files changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index 606ab53..a953ec9 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -253,19 +253,31 @@ void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
>      dev->alias_required_for_version = required_for_version;
>  }
>  
> +HotplugHandler *qdev_get_machine_hotplug_handler(DeviceState *dev)
> +{
> +    MachineState *machine;
> +    MachineClass *mc;
> +    Object *m_obj = qdev_get_machine();
> +
> +    if (object_dynamic_cast(m_obj, TYPE_MACHINE)) {
> +        machine = MACHINE(m_obj);
> +        mc = MACHINE_GET_CLASS(machine);
> +        if (mc->get_hotplug_handler) {
> +            return mc->get_hotplug_handler(machine, dev);
> +        }
> +    }
> +
> +    return NULL;
> +}
> +
>  HotplugHandler *qdev_get_hotplug_handler(DeviceState *dev)
>  {
> -    HotplugHandler *hotplug_ctrl = NULL;
> +    HotplugHandler *hotplug_ctrl;
>  
>      if (dev->parent_bus && dev->parent_bus->hotplug_handler) {
>          hotplug_ctrl = dev->parent_bus->hotplug_handler;
> -    } else if (object_dynamic_cast(qdev_get_machine(), TYPE_MACHINE)) {
> -        MachineState *machine = MACHINE(qdev_get_machine());
> -        MachineClass *mc = MACHINE_GET_CLASS(machine);
> -
> -        if (mc->get_hotplug_handler) {
> -            hotplug_ctrl = mc->get_hotplug_handler(machine, dev);
> -        }
> +    } else {
> +        hotplug_ctrl = qdev_get_machine_hotplug_handler(dev);
>      }
>      return hotplug_ctrl;
>  }
> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index 0891461..5aa536d 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -285,6 +285,7 @@ DeviceState *qdev_try_create(BusState *bus, const char *name);
>  void qdev_init_nofail(DeviceState *dev);
>  void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
>                                   int required_for_version);
> +HotplugHandler *qdev_get_machine_hotplug_handler(DeviceState *dev);
>  HotplugHandler *qdev_get_hotplug_handler(DeviceState *dev);
>  void qdev_unplug(DeviceState *dev, Error **errp);
>  void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
> diff --git a/qdev-monitor.c b/qdev-monitor.c
> index cb2b109..f93455f 100644
> --- a/qdev-monitor.c
> +++ b/qdev-monitor.c
> @@ -614,6 +614,11 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
>  
>      if (bus) {
>          qdev_set_parent_bus(dev, bus);
> +    } else if (qdev_hotplug && !qdev_get_machine_hotplug_handler(dev)) {
> +        /* No bus, no machine hotplug handler --> device is not hotpluggable */
> +        error_setg(&err, "Device '%s' can not be hotplugged on this machine",
> +                   driver);
> +        goto err_del_dev;
>      }
>  
>      qdev_set_id(dev, qemu_opts_id(opts));