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fuzzing

Testing & Quality
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Implements, registers, and verifies fuzz tests in Chromium. Use when the user asks to add or write fuzzers in C++, or mentions fuzz testing or FUZZ_TEST.

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Source SKILL.md: https://github.com/chromium/chromium/blob/HEAD/agents/skills/fuzzing/SKILL.md

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Fuzzing (Chromium)

1. Setup

Ensure the output directory is configured:

gn gen out/fuzz --args='enable_fuzztest_fuzz=true is_debug=false is_asan=true \
is_component_build=false use_remoteexec=true'

2. Implement the FUZZ_TEST

Add to *_unittest.cc alongside existing tests:

#include "third_party/fuzztest/src/fuzztest/fuzztest.h"
#include "third_party/googletest/src/googletest/include/gtest/gtest.h"

// 1. Define the property function. Use a descriptive name that reflects
// the property being tested (e.g., "ParseFooDoesNotCrash" or "RoundTripIsLossless").
void MyPropertyFunctionDoesNotCrash(int i, const std::string& s) {
  // Call code under test. Focus on functions parsing untrusted input, complex
  // state machines, or data processing.
  bool result = MyComponent::DoSomething(i, s);
  // Add test assertions about invariants (e.g. "roundtrip equality", "valid
  // output structure"). Sanitizers like ASAN catch crashes.
  EXPECT_TRUE(result);
}

// 2. Register with FUZZ_TEST macro
FUZZ_TEST(MyComponentFuzzTest, MyPropertyFunctionDoesNotCrash)
  .WithDomains(
      fuzztest::InRange(0, 100),
      fuzztest::Arbitrary<std::string>()
  );

For complex types:

  • Construct from primitives: If the object has a parsing constructor (e.g. GURL(string)), accept the primitive and construct it inside your test function.
  • Define a local domain: Use fuzztest::Constructor or fuzztest::Map to build valid objects. 
    auto ArbitraryFoo() {
  return fuzztest::Constructor<Foo>(fuzztest::InRange(0, 10));
}

3. Register in BUILD.gn

You MUST register the test in the fuzztests list (in alphabetical order) of the executable test target.

Case A: File is in a test() target

test("my_component_unittests") {
  sources = [ "my_component_unittest.cc" ]

  # Format: SuiteName.TestName
  fuzztests = [
    "MyComponentFuzzTest.MyPropertyFunctionDoesNotCrash",
  ]

  # No dependency changes are needed here. The build system
  # automatically adds FuzzTest dependencies for targets
  # with a `fuzztests` list.

}

Case B: File is in a source_set(): Add //third_party/fuzztest:fuzztest to deps.

source_set("tests") {
  sources = [ "my_component_unittest.cc" ]
  deps = [
    "//third_party/fuzztest:fuzztest",
    # ...
  ]
}

Find the executable test() target that depends on this source_set(): gn refs out/fuzz //path/to:source_set --testonly=true --type=executable --all

Then, ensure it lists the fuzz test in its fuzztests variable (in alphabetical order).

4. Mandatory verification workflow

The task is incomplete until you successfully execute this sequence:

  1. Find the target and Build

Find the executable test() target that contains your test file:

gn refs out/fuzz //path/to/my_component_unittest.cc --type=executable --all

Build the identified target (e.g. unit_tests or browser_tests):

autoninja --quiet -C out/fuzz <target_name>

Note: If the build fails with an include not allowed by DEPS error, see the Common Issues & Fixes section below.

  1. Verify unit tests pass
./out/fuzz/<target_name> \
--gtest_filter="MyComponentFuzzTest.MyPropertyFunctionDoesNotCrash"
  1. Verify fuzzing mode doesn't crash
./out/fuzz/<target_name> \
--fuzz="MyComponentFuzzTest.MyPropertyFunctionDoesNotCrash" --fuzz_for=10s

Resources

  • Chromium Guide: testing/libfuzzer/getting_started.md
  • Macro Usage: third_party/fuzztest/src/doc/fuzz-test-macro.md
  • Domains: third_party/fuzztest/src/doc/domains-reference.md
  • Fixtures: third_party/fuzztest/src/doc/fixtures.md

Common Issues & Fixes

Build failure: include not allowed by DEPS

Symptom: Build fails with an error like: ERROR: include not allowed by DEPS: third_party/fuzztest/...

Fix: You must allow the fuzztest include in the nearest DEPS file (usually in the same directory as your test or a parent directory). Add +third_party/fuzztest to specific_include_rules for your test files:

specific_include_rules = {
  ".*_unittest\\.cc": [
    "+third_party/fuzztest",
  ],
}