From 1e3b123bcfd56df93b767f5c322871cd0ca2e75f Mon Sep 17 00:00:00 2001
From: "Chris (ChrisJr404)" <11917633+ChrisJr404@users.noreply.github.com>
Date: Mon, 17 Aug 2026 21:20:40 -0400
Subject: [PATCH] Expose RegisterPostProcessor in the libprotobuf-mutator
 wrapper

---
 .../atheris_libprotobuf_mutator/__init__.py   |   1 +
 .../__pycache__/__init__.cpython-313.pyc      | Bin 0 -> 193 bytes
 .../__pycache__/helpers.cpython-313.pyc       | Bin 0 -> 4266 bytes
 .../proto_fuzz_test.cpython-313.pyc           | Bin 0 -> 2450 bytes
 .../atheris_libprotobuf_mutator/helpers.py    |  27 ++++++++++++++++++
 .../atheris_libprotobuf_mutator/mutator.cc    |  15 ++++++++++
 .../proto_fuzz_test.py                        |  22 ++++++++++++++
 7 files changed, 65 insertions(+)
 create mode 100644 contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/__init__.cpython-313.pyc
 create mode 100644 contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/helpers.cpython-313.pyc
 create mode 100644 contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/proto_fuzz_test.cpython-313.pyc

diff --git a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__init__.py b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__init__.py
index 05698233..62f7fc95 100644
--- a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__init__.py
+++ b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__init__.py
@@ -1 +1,2 @@
+from .helpers import RegisterPostProcessor
 from .helpers import Setup
diff --git a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/__init__.cpython-313.pyc b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/__init__.cpython-313.pyc
new file mode 100644
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diff --git a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/helpers.cpython-313.pyc b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/helpers.cpython-313.pyc
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diff --git a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/proto_fuzz_test.cpython-313.pyc b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/__pycache__/proto_fuzz_test.cpython-313.pyc
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diff --git a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/helpers.py b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/helpers.py
index 8abc08f9..e8dbab72 100644
--- a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/helpers.py
+++ b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/helpers.py
@@ -74,3 +74,30 @@ def TestOneProtoInputImpl(data: bytes):
       custom_mutator=_CustomMutator,
       custom_crossover=_CustomCrossOver,
       **kwargs)
+
+
+def RegisterPostProcessor(proto: Callable[..., Any],
+                          callback: Callable[[Any, int], Any]):
+  """Register a post-processor that runs after every mutation of `proto`.
+
+  libprotobuf-mutator mutates fields blindly, so it can produce messages that
+  break invariants the mutator knows nothing about (a length field that has to
+  match a repeated field, a checksum, an enum that's really a bitmask, ...).
+  A post-processor gets a chance to patch those up before the message reaches
+  your test function.
+
+  The callback receives the freshly mutated message and the mutation seed. Edit
+  the message in place, or return a new one of the same type. Returning None
+  keeps whatever edits you made in place. Registration is global per message
+  type, so call this once before atheris.Fuzz().
+
+  Args:
+    proto: the protobuf message type to post-process.
+    callback: called as callback(message, seed) after each mutation.
+  """
+
+  def _wrapped(message: Any, seed: int):
+    result = callback(message, seed)
+    return message if result is None else result
+
+  _mutator.RegisterPostProcessor(proto(), _wrapped)
diff --git a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/mutator.cc b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/mutator.cc
index 36b68c19..58529b3e 100644
--- a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/mutator.cc
+++ b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/mutator.cc
@@ -95,6 +95,21 @@ PYBIND11_MODULE(_mutator, m) {
           }
           return std::nullopt;
         });
+  m.def("RegisterPostProcessor",
+        [](std::unique_ptr<protobuf::Message> prototype, py::function callback) {
+          const protobuf::Descriptor* descriptor = prototype->GetDescriptor();
+          libfuzzer::RegisterPostProcessor(
+              descriptor,
+              [callback](protobuf::Message* message, unsigned int seed) {
+                py::gil_scoped_acquire gil;
+                py::object result = callback(message, seed);
+                if (!result.is_none()) {
+                  auto updated =
+                      result.cast<std::unique_ptr<protobuf::Message>>();
+                  message->CopyFrom(*updated);
+                }
+              });
+        });
 }
 
 }  // namespace protobuf_mutator
diff --git a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/proto_fuzz_test.py b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/proto_fuzz_test.py
index 1a43a46c..5c69138c 100644
--- a/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/proto_fuzz_test.py
+++ b/contrib/libprotobuf_mutator/atheris_libprotobuf_mutator/proto_fuzz_test.py
@@ -22,6 +22,28 @@ def testSimpleProtoComparison(self):
         expected_output=b"Solved",
         timeout=60)
 
+  def testPostProcessor(self):
+    # The post-processor rewrites every mutated message to the solving value, so
+    # the comparison fires on the first input rather than waiting for the
+    # mutator to stumble onto "abc" on its own. If the post-processor's edits
+    # didn't make it back into the message the fuzzer feeds the harness, this
+    # would time out instead.
+    def setup_with_post_processor(argv, test_one_input, **kwargs):
+      def force_value(msg, seed):
+        del seed
+        msg.value = "abc"
+
+      atheris_libprotobuf_mutator.RegisterPostProcessor(
+          wrappers_pb2.StringValue, force_value)
+      return atheris_libprotobuf_mutator.Setup(argv, test_one_input, **kwargs)
+
+    fuzz_test_lib.run_fuzztest(
+        simple_proto_comparison,
+        custom_setup=setup_with_post_processor,
+        setup_kwargs={"proto": wrappers_pb2.StringValue},
+        expected_output=b"Solved",
+        timeout=60)
+
 
 if __name__ == "__main__":
   unittest.main()