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Check the fuzzers' UBJSON/BJData round-trip invariants in the unit tests (#5569)

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* Check the fuzzers' UBJSON/BJData round-trip invariants in the unit tests

The strongest correctness checks for the UBJSON and BJData writers lived
only in the OSS-Fuzz drivers: anything from_ubjson()/from_bjdata()
returns must serialize with every option combination, parse back, and
re-serialize stably. Those checks only run at OSS-Fuzz, so regressions
surfaced days later as external reports - the same BJData assert pair
was reported five times over three years, and #5494's harness change
was followed by OSS-Fuzz 563659413 within a day.

Add "UBJSON round-trip invariants" and "BJData round-trip invariants"
test cases that run the drivers' checks on a fixed, deterministic corpus
(tests/src/round_trip_corpus.hpp): integer and float boundaries,
non-finite numbers, strings, binary values, optimized containers, deep
nesting, the JData annotated-array matrix, and seeded random containers.
They also check two properties the drivers do not: the first round trip
preserves the value, and re-serializing reproduces the exact bytes. For
BJData both exclude values containing a binary value, which is read back
as an array of integers unless it was written as a Draft 3 optimized
binary array; this carve-out is now documented in bjdata.md. Run against
the headers before #5542, the BJData test fails, including on the shape
from OSS-Fuzz 563659413.

Also document how OSS-Fuzz reports are handled (reference them as
"OSS-Fuzz: <id>", turn the reproducer into a unit test, keep drivers and
unit tests in sync) in tests/fuzzing.md, and link it from the PR
template and the quality assurance page.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Add the OSS-Fuzz reproducers for 474400817 and 474480402 as unit tests

Following the convention added to tests/fuzzing.md, the reproducers of
the two BJData fuzzer asserts tracked since January are now unit tests:

- 474400817 (assert(false)): an empty object _ArraySize_ was written as
  the ND-array header length, which from_bjdata() could not read back.
  Fixed by #5455.

- 474480402 (to_bjdata(j2, false, false) == vec2): a one-byte Draft 3
  binary array is written in Draft 2 mode as a uint8 array and then
  re-serialized with the int8 marker. This is the documented exception to
  byte stability, not a library bug; OSS-Fuzz closed it after #5494
  relaxed the harness to value stability. The test pins the exact bytes
  so the exception stays deliberate.

The 563659413 reproducer is already a unit test (#5542). A comment also
ties the existing UBJSON excessive-count test to the timeout OSS-Fuzz
reported for that shape (testcase 6347769435193344).

OSS-Fuzz: 474400817
OSS-Fuzz: 474480402

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix GCC -Weffc++ and -Wuseless-cast warnings in the round-trip corpus

Initialize the atoms in the member initialization list, and drop the cast of
the generator's result, which already is std::size_t on 64-bit Linux.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-25 08:29:02 +02:00
committed by GitHub
parent 80bf54a5a2
commit cc472af13f
9 changed files with 408 additions and 1 deletions
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@@ -79,3 +79,26 @@ the same `fuzzers` target as above and also relies on the `FUZZER_ENGINE` variab
[build script](https://github.com/google/oss-fuzz/blob/master/projects/json/build.sh) for more information.
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 UBJSON and BJData 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.