hv-balloon: define dm_hot_add_with_region to avoid Coverity warning

Since the presence of a hot add memory region is optional in hot add
request message it wasn't part of this message declaration
(struct dm_hot_add).

Instead, the code allocated such enlarged message by simply adding the
necessary size for this extra field to the size of basic hot add message
struct.

However, Coverity considers accessing this extra member to be
an out-of-bounds access, even thought the memory is actually there.

Fix this by adding an extended variant of this message that explicitly has
an additional union dm_mem_page_range at its end.

CID: #1523903
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

hw/hyperv/hv-balloon.c

@@ -513,8 +513,8 @@ ret_idle:
 static void hv_balloon_hot_add_rb_wait(HvBalloon *balloon, StateDesc *stdesc)
 {
     VMBusChannel *chan = hv_balloon_get_channel(balloon);
-    struct dm_hot_add *ha;
-    size_t ha_size = sizeof(*ha) + sizeof(ha->range);
+    struct dm_hot_add_with_region *ha;
+    size_t ha_size = sizeof(*ha);
 
     assert(balloon->state == S_HOT_ADD_RB_WAIT);
 
@@ -530,8 +530,8 @@ static void hv_balloon_hot_add_posting(HvBalloon *balloon, StateDesc *stdesc)
     PageRange *hot_add_range = &balloon->hot_add_range;
     uint64_t *current_count = &balloon->ha_current_count;
     VMBusChannel *chan = hv_balloon_get_channel(balloon);
-    g_autofree struct dm_hot_add *ha = NULL;
-    size_t ha_size = sizeof(*ha) + sizeof(ha->range);
+    g_autofree struct dm_hot_add_with_region *ha = NULL;
+    size_t ha_size = sizeof(*ha);
     union dm_mem_page_range *ha_region;
     uint64_t align, chunk_max_size;
     ssize_t ret;
@@ -560,7 +560,7 @@ static void hv_balloon_hot_add_posting(HvBalloon *balloon, StateDesc *stdesc)
     *current_count = MIN(hot_add_range->count, chunk_max_size);
 
     ha = g_malloc0(ha_size);
-    ha_region = &(&ha->range)[1];
+    ha_region = &ha->region;
     ha->hdr.type = DM_MEM_HOT_ADD_REQUEST;
     ha->hdr.size = ha_size;
     ha->hdr.trans_id = balloon->trans_id;

include/hw/hyperv/dynmem-proto.h

@@ -328,7 +328,8 @@ struct dm_unballoon_response {
 /*
  * Hot add request message. Message sent from the host to the guest.
  *
- * mem_range: Memory range to hot add.
+ * range: Memory range to hot add.
+ * region: Explicit hot add memory region for guest to use. Optional.
  *
  */
 
@@ -337,6 +338,12 @@ struct dm_hot_add {
     union dm_mem_page_range range;
 } QEMU_PACKED;
 
+struct dm_hot_add_with_region {
+    struct dm_header hdr;
+    union dm_mem_page_range range;
+    union dm_mem_page_range region;
+} QEMU_PACKED;
+
 /*
  * Hot add response message.
  * This message is sent by the guest to report the status of a hot add request.