Exposing problems in json parsers of several programming languages.
Many JSON parsers (and many parsers in general) use recursion to parse nested structures. This is very convenient while programming the parser, but it has consequenses on what the parser can parse: indeed, the size of the call stack is usually limited to a value several orders of magnitude smaller than the available RAM, and this implies that a program with too many levels of recursion will fail.
However, the JSON specification doesn't contain any limit on how deeply nested JSON structures can be. This means that most JSON parsers fail on a valid input.
This repository contains tools to measure the limits of JSON parsers of different languages.
This repository contains a script called test_parser.sh that takes a JSON parser and uses binary search to find the smallest JSON structure it fails to parse and print its nesting level.
The json parser must be a program that reads JSON on its standard input, and exits with a status of 0 if it managed to parse it and any other status if an error occured.
test_parser.sh constructs json structures composed uniquely of nested arrays, and gives them to the program it tests. For instance, for a depth of 3, it builds the following json : [[[]]]. This allows to create a structure of only 2n bytes that has n nesting levels.
It uses binary search to find the smallest structure for which the programm fails.
On my machine (Ubuntu Linux 4.10.0-35-generic SMP x86_64 with 8Gb RAM, 8.4 MB maximum stack size), I found the following results, sorted from worst to best:
| language | json library | nesting level | file size | notes |
|---|---|---|---|---|
| ruby | json | 101 | 202 bytes | |
| rust | serde_json | 128 | 256 bytes | |
| php | json_decode |
512 | 1024 bytes | maximum depth is configurable |
| python3 | json | 994 | 2.0 KB | without sys.setrecursionlimit |
| C | jansson | 2049 | 4.0 KB | |
| java | Gson | 5670 | 11.3 KB | |
| javascript | JSON.parse |
5713 | 11.4 KB | |
| C++ | nlohmann::json | 13787 | 27.6 KB | segfault |
| ruby | Oj | ∞ | ∞ | |
| Haskell | Aeson | ∞ | ∞ | available RAM is the only limit |
I tried to test the most popular json library of each language. If you want to add a new language or a new library, feel free to open a pull request.
All the parameters were left to their defaut values. In particular, the result
for PHP is particular: json_decode accepts a depth parameter to configure
the maximum depth of the object to be parsed.