qjson: Limit number of tokens in addition to total size

Commit 29c75ddd "json-streamer: limit the maximum recursion depth and
maximum token count" attempts to guard against excessive heap usage by
limiting total token size (it says "token count", but that's a lie).

Total token size is a rather imprecise predictor of heap usage: many
small tokens use more space than few large tokens with the same input
size, because there's a constant per-token overhead: 37 bytes on my
system.

Tighten this up: limit the token count to 2Mi.  Chosen to roughly
match the 64MiB total token size limit.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1448486613-17634-13-git-send-email-armbru@redhat.com>

qobject/json-streamer.c

@@ -16,6 +16,7 @@
 #include "qapi/qmp/json-streamer.h"
 
 #define MAX_TOKEN_SIZE (64ULL << 20)
+#define MAX_TOKEN_COUNT (2ULL << 20)
 #define MAX_NESTING (1ULL << 10)
 
 static void json_message_free_tokens(JSONMessageParser *parser)
@@ -68,6 +69,7 @@ static void json_message_process_token(JSONLexer *lexer, GString *input,
          parser->bracket_count == 0)) {
         goto out_emit;
     } else if (parser->token_size > MAX_TOKEN_SIZE ||
+               g_queue_get_length(parser->tokens) > MAX_TOKEN_COUNT ||
                parser->bracket_count + parser->brace_count > MAX_NESTING) {
         /* Security consideration, we limit total memory allocated per object
          * and the maximum recursion depth that a message can force.