json/tests/fuzzing.md

# Fuzz testing

Each parser of the library (JSON, BJData, BON8, BSON, CBOR, MessagePack, and UBJSON) can be fuzz tested. Currently,
[libFuzzer](https://llvm.org/docs/LibFuzzer.html) and [afl++](https://github.com/AFLplusplus/AFLplusplus) are supported.

Additionally, `parse_json_view_fuzzer` (`tests/src/fuzzer-parse_json_view.cpp`) cross-checks `json_document`/`json_view`
(the zero-copy, read-only view declared in `json_view.hpp`) against `basic_json` on the same JSON text: it asserts that
`json_document::accept` agrees with `json::accept`, that an accepted input materializes to the same value `json::parse`
produces, and that a rejected input makes both parsers throw with an identical `what()`. It checks this for a
`std::string` input (borrowed, with a NUL after the last byte) and for an exact-size `std::vector<std::uint8_t>`
(borrowed, with nothing after the last byte), and for the `ignore_comments` and `ignore_trailing_commas` options, which
are taken from the low bits of the first input byte (the byte stays part of the text). It takes plain JSON text, so it
reuses the `corpus_json` corpus rather than a format of its own.

`json_view_image_fuzzer` (`tests/src/fuzzer-json_view_image.cpp`) tests the images of `json_document` (`save()` and
`load()`). It uses each input twice: as an image, which `load()` must either reject with `parse_error.116` or read
safely (with `image_check::full`, the document must also serialize to the JSON it reads as), and as a JSON text, whose
image must load and serialize to the same text. A corpus of images can be made from JSON files with a small program
that calls `json_document::parse(text).save()`; plain JSON files work as well.

## What the fuzzers check

Each fuzzer driver (`tests/src/fuzzer-parse_*.cpp`) parses its input twice: once with `allow_exceptions = false` and
once with exceptions. Both calls must agree. Where parsing with exceptions fails, the call without exceptions must
return a discarded value (or throw the same kind of non-parse error), and it must never throw a `parse_error`. Where
parsing succeeds, both calls must return the same value. The drivers then serialize the value, parse the result back,
and check that nothing was lost.

Finally, each driver reads the same input through the other input adapters: a `std::istringstream`, a non-contiguous
iterator range (`std::list<std::uint8_t>`), and, on POSIX systems, a `FILE*` from `fmemopen`. The pointer range takes the
fast paths for contiguous input (bulk string scanning, multi-byte loads, lazy position tracking), while the other
adapters read byte by byte. Every adapter must give the same value, or the same exception with the same id, message,
and byte position. The JSON driver checks this for `parse` and `accept`, each with and without `ignore_comments`. The
shared code is `check_input_adapters()` in `tests/src/fuzzer_common.hpp`.

The drivers check all of this with `assert`, so they refuse to build with `NDEBUG`.

## Corpus creation

For most effective fuzzing, a [corpus](https://llvm.org/docs/LibFuzzer.html#corpus) should be provided. A corpus is a
directory with some simple input files that cover several features of the parser and is hence a good starting point
for mutations.

```shell
TEST_DATA_VERSION=3.2.0
wget https://github.com/nlohmann/json_test_data/archive/refs/tags/v$TEST_DATA_VERSION.zip
unzip v$TEST_DATA_VERSION.zip
rm v$TEST_DATA_VERSION.zip
for FORMAT in json bjdata bon8 bson cbor msgpack ubjson
do
  rm -fr corpus_$FORMAT
  mkdir corpus_$FORMAT
  find json_test_data-$TEST_DATA_VERSION -size -5k -name "*.$FORMAT" -exec cp "{}" "corpus_$FORMAT" \;
done
rm -fr json_test_data-$TEST_DATA_VERSION
```

The generated corpus can be used with both libFuzzer and afl++. The remainder of this documentation assumes the corpus
directories have been created in the `tests` directory.

## libFuzzer

To use libFuzzer, you need to pass `-fsanitize=fuzzer` as `FUZZER_ENGINE`. In the `tests` directory, call

```shell
make fuzzers FUZZER_ENGINE="-fsanitize=fuzzer"
```

This creates a fuzz tester binary for each parser that supports these
[command line options](https://llvm.org/docs/LibFuzzer.html#options).

In case your default compiler is not a Clang compiler that includes libFuzzer (Clang 6.0 or later), you need to set the
`CXX` variable accordingly. Note the compiler provided by Xcode (AppleClang) does not contain libFuzzer. Please install
Clang via Homebrew calling `brew install llvm` and add `CXX=$(brew --prefix llvm)/bin/clang` to the `make` call:

```shell
make fuzzers FUZZER_ENGINE="-fsanitize=fuzzer" CXX=$(brew --prefix llvm)/bin/clang
```

Then pass the corpus directory as command-line argument (assuming it is located in `tests`):

```shell
./parse_cbor_fuzzer corpus_cbor
```

The fuzzer should be able to run indefinitely without crashing. In case of a crash, the tested input is dumped into
a file starting with `crash-`.

To also detect memory leaks, build with AddressSanitizer (`FUZZER_ENGINE="-fsanitize=fuzzer,address"`): libFuzzer then
runs LeakSanitizer by default (`-detect_leaks=1`). LeakSanitizer is not available with Apple Clang on macOS.

## afl++

To use afl++, you need to pass `-fsanitize=fuzzer` as `FUZZER_ENGINE`. It will be replaced by a `libAFLDriver.a` to
re-use the same code written for libFuzzer with afl++. Furthermore, set `afl-clang-fast++` as compiler.

```shell
CXX=afl-clang-fast++ make fuzzers FUZZER_ENGINE="-fsanitize=fuzzer" 
```

Then the fuzzer is called like this in the `tests` directory:

```shell
afl-fuzz -i corpus_cbor -o out  -- ./parse_cbor_fuzzer 
```

The fuzzer should be able to run indefinitely without crashing. In case of a crash, the tested input is written to the
directory `out`.

## OSS-Fuzz

The library is further fuzz-tested 24/7 by Google's [OSS-Fuzz project](https://github.com/google/oss-fuzz). It uses
the same `fuzzers` target as above and also relies on the `FUZZER_ENGINE` variable. See the used
[build script](https://github.com/google/oss-fuzz/blob/master/projects/json/build.sh) for more information. Its default
`address` sanitizer includes LeakSanitizer, so OSS-Fuzz and the CIFuzz workflow (`.github/workflows/cifuzz.yml`) report
memory leaks, too.

In case the build at OSS-Fuzz fails, an issue will be created automatically.

### Handling OSS-Fuzz reports

OSS-Fuzz files the crashes it finds in its own [issue tracker](https://issues.oss-fuzz.com), not on GitHub. So that
each report can be traced to the change that fixed it, and each fix to the report it answers, fixes follow these
conventions:

- **Reference the OSS-Fuzz issue in the pull request**, next to any GitHub issue it closes, as `OSS-Fuzz: <id>` (for
  example, `OSS-Fuzz: 563659413`), and in the commit message. The ID alone does not disclose the crash. If the report
  was triaged into a GitHub issue, link the OSS-Fuzz issue there too.
- **Turn the reproducer into a unit test.** Download the testcase from the OSS-Fuzz report, reduce it if possible, and
  add it as a regression test to the unit test of the affected format (e.g., `tests/src/unit-bjdata.cpp`), with a
  comment naming the OSS-Fuzz issue. This way the input is checked by every CI run rather than only by OSS-Fuzz, and
  it stays covered even if OSS-Fuzz later closes the report as not reproducible.
- **Keep the fuzzer drivers and the unit tests in sync.** The round-trip checks of the BJData, BON8, BSON, CBOR,
  MessagePack and UBJSON drivers are also run on a fixed corpus in the unit tests (see
  `tests/src/round_trip_corpus.hpp` and the "round-trip invariants" test cases), so a regression shows up in CI
  first. When a driver's checks change, change the unit tests with them.
- **Record in the report whether the bug shipped.** OSS-Fuzz asks whether a crash was a short-lived regression or
  affects a released version; answer it when the fix is merged, as it decides whether the fix needs a release note or
  a security advisory (see the [security policy](../.github/SECURITY.md)).

After the fix is merged, OSS-Fuzz re-runs the reproducer on its next build and marks the report as verified and
closed. If it does not, the fix is incomplete.