hw/core/qdev-clock: add a reference on aliased clocks

When aliasing a clock with the qdev_alias_clock() function, a new link
property is created on the device aliasing the clock. The link points
to the aliased clock and use the OBJ_PROP_LINK_STRONG flag. This
property is read only since it does not provide a check callback for
modifications.

The object_property_add_link() documentation stats that with
OBJ_PROP_LINK_STRONG properties, the linked object reference count get
decremented when the property is deleted. But it is _not_ incremented on
creation (object_property_add_link() does not actually know the link).

This commit increments the reference count on the aliased clock to
ensure the aliased clock stays alive during the property lifetime, and
to avoid a double-free memory error when the property gets deleted.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Luc Michel <luc@lmichel.fr>
Message-Id: <20201020091024.320381-1-luc@lmichel.fr>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/core/qdev-clock.c

@@ -61,6 +61,14 @@ static NamedClockList *qdev_init_clocklist(DeviceState *dev, const char *name,
                                  object_get_typename(OBJECT(clk)),
                                  (Object **) &ncl->clock,
                                  NULL, OBJ_PROP_LINK_STRONG);
+        /*
+         * Since the link property has the OBJ_PROP_LINK_STRONG flag, the clk
+         * object reference count gets decremented on property deletion.
+         * However object_property_add_link does not increment it since it
+         * doesn't know the linked object. Increment it here to ensure the
+         * aliased clock stays alive during this device life-time.
+         */
+        object_ref(OBJECT(clk));
     }
 
     ncl->clock = clk;