[YAMLIO] Add a generic YAML fuzzer harness

[scott.linder]

Summary

This is essentially a clone of the existing fuzzer added in https://reviews.llvm.org/D50839, but for the top-level parser, and only testing for sanitizer violations.

[dexonsmith]

LGTM.

[dexonsmith]

llvm/tools/llvm-yaml-parser-fuzzer/yaml-parser-fuzzer.cpp

16–17

On further thought: should this be iterating through all the documents?

[dexonsmith]

(Requesting changes to highlight my question.)

[dexonsmith]

I suggest changing this patch to include the isValidYaml function from https://reviews.llvm.org/D84050 and call that, and then that revision can be rebased on top of this one. If you do that, this LGTM.

[scott.linder]

In D91573#2403817, @dexonsmith wrote:

I suggest changing this patch to include the isValidYaml function from https://reviews.llvm.org/D84050 and call that, and then that revision can be rebased on top of this one. If you do that, this LGTM.

This is what I ended up doing, thank you! I also adopted the vector<uint8_t> style here and copied back the matching comment from the next patch. Let me know if you notice any mistakes.

llvm/tools/llvm-yaml-parser-fuzzer/CMakeLists.txt

+set(LLVM_LINK_COMPONENTS
+  Support
+  FuzzMutate
+)
+
+add_llvm_fuzzer(llvm-yaml-parser-fuzzer
+  yaml-parser-fuzzer.cpp
+  DUMMY_MAIN DummyYAMLParserFuzzer.cpp
+  )

llvm/tools/llvm-yaml-parser-fuzzer/DummyYAMLParserFuzzer.cpp

+//===--- DummyYAMLParserFuzzer.cpp ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Implementation of main so we can build and test without linking libFuzzer.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/FuzzMutate/FuzzerCLI.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+int main(int argc, char *argv[]) {
+  return llvm::runFuzzerOnInputs(argc, argv, LLVMFuzzerTestOneInput);
+}

llvm/tools/llvm-yaml-parser-fuzzer/yaml-parser-fuzzer.cpp

+//===-- yaml-parser-fuzzer.cpp - Fuzzer for YAML parser -------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/YAMLParser.h"
+
+using namespace llvm;
+
+static bool isValidYaml(const uint8_t *Data, size_t Size) {
+  SourceMgr SM;
+  yaml::Stream Stream(StringRef(reinterpret_cast<const char *>(Data), Size),
+                      SM);
+  return Stream.validate();
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  std::vector<uint8_t> Input(Data, Data + Size);
+
+  // Ensure we don't crash on byte strings where the only null character is
+  // one-past-the-end of the actual input to the parser.
+  Input.erase(std::remove(Input.begin(), Input.end(), 0), Input.end());
+  Input.push_back(0);
+  Input.shrink_to_fit();
+  isValidYaml(Input.data(), Input.size() - 1);
+
+  return 0;
+}