checker: ignore fields marked unused

While comparing qemu-1.0 json output with qemu-2.1, a few fields got
marked unused.  These need to be skipped over, and not flagged as
mismatches.

For handling unused fields, the exact number of bytes need to be skipped
over as the size of the unused field.

Currently, only the term "unused" is matched.  When more field names
turn up, this will have to be updated based on the whitelist matching
method to match more such terms.

Signed-off-by: Amit Shah <amit.shah@redhat.com>

scripts/vmstate-static-checker.py

@@ -144,6 +144,7 @@ def check_fields(src_fields, dest_fields, desc, sec):
 
     advance_src = True
     advance_dest = True
+    unused_count = 0
 
     while True:
         if advance_src:
@@ -156,9 +157,10 @@ def check_fields(src_fields, dest_fields, desc, sec):
                 s_iter = s_iter_list.pop()
                 continue
         else:
-            # We want to avoid advancing just once -- when entering a
-            # dest substruct, or when exiting one.
-            advance_src = True
+            if unused_count == 0:
+                # We want to avoid advancing just once -- when entering a
+                # dest substruct, or when exiting one.
+                advance_src = True
 
         if advance_dest:
             try:
@@ -177,7 +179,37 @@ def check_fields(src_fields, dest_fields, desc, sec):
                 advance_src = False
                 continue
         else:
-            advance_dest = True
+            if unused_count == 0:
+                advance_dest = True
+
+        if unused_count > 0:
+            if advance_dest == False:
+                unused_count = unused_count - s_item["size"]
+                if unused_count == 0:
+                    advance_dest = True
+                    continue
+                if unused_count < 0:
+                    print "Section \"" + sec + "\",",
+                    print "Description \"" + desc + "\":",
+                    print "unused size mismatch near \"",
+                    print s_item["field"] + "\""
+                    bump_taint()
+                    break
+                continue
+
+            if advance_src == False:
+                unused_count = unused_count - d_item["size"]
+                if unused_count == 0:
+                    advance_src = True
+                    continue
+                if unused_count < 0:
+                    print "Section \"" + sec + "\",",
+                    print "Description \"" + desc + "\":",
+                    print "unused size mismatch near \"",
+                    print d_item["field"] + "\""
+                    bump_taint()
+                    break
+                continue
 
         if not check_fields_match(desc, s_item["field"], d_item["field"]):
             # Some fields were put in substructs, keeping the
@@ -208,6 +240,20 @@ def check_fields(src_fields, dest_fields, desc, sec):
                 advance_dest = False
                 continue
 
+            if s_item["field"] == "unused" or d_item["field"] == "unused":
+                if s_item["size"] == d_item["size"]:
+                    continue
+
+                if d_item["field"] == "unused":
+                    advance_dest = False
+                    unused_count = d_item["size"] - s_item["size"]
+                    continue
+
+                if s_item["field"] == "unused":
+                    advance_src = False
+                    unused_count = s_item["size"] - d_item["size"]
+                    continue
+
             print "Section \"" + sec + "\",",
             print "Description \"" + desc + "\":",
             print "expected field \"" + s_item["field"] + "\",",