json: Pass lexical errors and limit violations to callback

The callback to consume JSON values takes QObject *json, Error *err.
If both are null, the callback is supposed to make up an error by
itself.  This sucks.

qjson.c's consume_json() neglects to do so, which makes
qobject_from_json() null instead of failing.  I consider that a bug.

The culprit is json_message_process_token(): it passes two null
pointers when it runs into a lexical error or a limit violation.  Fix
it to pass a proper Error object then.  Update the callbacks:

* monitor.c's handle_qmp_command(): the code to make up an error is
  now dead, drop it.

* qga/main.c's process_event(): lumps the "both null" case together
  with the "not a JSON object" case.  The former is now gone.  The
  error message "Invalid JSON syntax" is misleading for the latter.
  Improve it to "Input must be a JSON object".

* qobject/qjson.c's consume_json(): no update; check-qjson
  demonstrates qobject_from_json() now sets an error on lexical
  errors, but still doesn't on some other errors.

* tests/libqtest.c's qmp_response(): the Error object is now reliable,
  so use it to improve the error message.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-40-armbru@redhat.com>

2 files changed, 17 insertions, 8 deletions

diff --git a/qobject/json-lexer.c b/qobject/json-lexer.c
index 96fe13621d..7c31c2c8ff 100644
--- a/qobject/json-lexer.c
+++ b/qobject/json-lexer.c
@@ -334,8 +334,7 @@ static void json_lexer_feed_char(JSONLexer *lexer, char ch, bool flush)
             /* XXX: To avoid having previous bad input leaving the parser in an
              * unresponsive state where we consume unpredictable amounts of
              * subsequent "good" input, percolate this error state up to the
-             * tokenizer/parser by forcing a NULL object to be emitted, then
-             * reset state.
+             * parser by emitting a JSON_ERROR token, then reset lexer state.
              *
              * Also note that this handling is required for reliable channel
              * negotiation between QMP and the guest agent, since chr(0xFF)
diff --git a/qobject/json-streamer.c b/qobject/json-streamer.c
index a373e0114a..e372ecc895 100644
--- a/qobject/json-streamer.c
+++ b/qobject/json-streamer.c
@@ -13,6 +13,7 @@
 
 #include "qemu/osdep.h"
 #include "qemu-common.h"
+#include "qapi/error.h"
 #include "qapi/qmp/json-lexer.h"
 #include "qapi/qmp/json-parser.h"
 #include "qapi/qmp/json-streamer.h"
@@ -57,6 +58,7 @@ void json_message_process_token(JSONLexer *lexer, GString *input,
         parser->bracket_count--;
         break;
     case JSON_ERROR:
+        error_setg(&err, "JSON parse error, stray '%s'", input->str);
         goto out_emit;
     default:
         break;
@@ -82,12 +84,20 @@ void json_message_process_token(JSONLexer *lexer, GString *input,
         goto out_emit;
     }
 
-    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.
-         */
+    /*
+     * Security consideration, we limit total memory allocated per object
+     * and the maximum recursion depth that a message can force.
+     */
+    if (parser->token_size > MAX_TOKEN_SIZE) {
+        error_setg(&err, "JSON token size limit exceeded");
+        goto out_emit;
+    }
+    if (g_queue_get_length(parser->tokens) > MAX_TOKEN_COUNT) {
+        error_setg(&err, "JSON token count limit exceeded");
+        goto out_emit;
+    }
+    if (parser->bracket_count + parser->brace_count > MAX_NESTING) {
+        error_setg(&err, "JSON nesting depth limit exceeded");
         goto out_emit;
     }