Getting Started with libFuzzer in Chrome

Prerequisites: libFuzzer in Chrome is supported with GN on Linux only.

This document will walk you through:

  • setting up your build enviroment.
  • creating your first fuzzer.
  • running the fuzzer and verifying its vitals.

Configure Build

Use use_libfuzzer GN argument together with sanitizer to generate build files:

# With address sanitizer
gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true enable_nacl=false' --check

Supported sanitizer configurations are:

GN ArgumentDescription
is_asan=trueenables Address Sanitizer to catch problems like buffer overruns.
is_msan=trueenables Memory Sanitizer to catch problems like uninitialed reads.
is_ubsan_security=trueenables Undefined Behavior Sanitizer to catch[1] undefined behavior like integer overflow.

Write Fuzzer Function

Create a new .cc file and define a LLVMFuzzerTestOneInput function:

extern "C" int LLVMFuzzerTestOneInput(const unsigned char *data, size_t size) {
  // put your fuzzing code here and use data+size as input.
  return 0;
} is a simple example of real-world fuzzer.

Define GN Target

Define fuzzer_test GN target:

fuzzer_test("my_fuzzer") {
  sources = [ "" ]
  deps = [ ... ]

Build and Run Fuzzer Locally

Build with ninja as usual and run:

ninja -C out/libfuzzer url_parse_fuzzer

Your fuzzer should produce output like this:

INFO: Seed: 1787335005
INFO: -max_len is not provided, using 64
INFO: PreferSmall: 1
#0      READ   units: 1 exec/s: 0
#1      INITED cov: 2361 bits: 95 indir: 29 units: 1 exec/s: 0
#2      NEW    cov: 2710 bits: 359 indir: 36 units: 2 exec/s: 0 L: 64 MS: 0 
#3      NEW    cov: 2715 bits: 371 indir: 37 units: 3 exec/s: 0 L: 64 MS: 1 ShuffleBytes-
#5      NEW    cov: 2728 bits: 375 indir: 38 units: 4 exec/s: 0 L: 63 MS: 3 ShuffleBytes-ShuffleBytes-EraseByte-
#6      NEW    cov: 2729 bits: 384 indir: 38 units: 5 exec/s: 0 L: 10 MS: 4 ShuffleBytes-ShuffleBytes-EraseByte-CrossOver-
#7      NEW    cov: 2733 bits: 424 indir: 39 units: 6 exec/s: 0 L: 63 MS: 1 ShuffleBytes-
#8      NEW    cov: 2733 bits: 426 indir: 39 units: 7 exec/s: 0 L: 63 MS: 2 ShuffleBytes-ChangeByte-
#11     NEW    cov: 2733 bits: 447 indir: 39 units: 8 exec/s: 0 L: 33 MS: 5 ShuffleBytes-ChangeByte-ChangeASCIIInt-ChangeBit-CrossOver-
#12     NEW    cov: 2733 bits: 451 indir: 39 units: 9 exec/s: 0 L: 62 MS: 1 CrossOver-
#16     NEW    cov: 2733 bits: 454 indir: 39 units: 10 exec/s: 0 L: 61 MS: 5 CrossOver-ChangeBit-ChangeBit-EraseByte-ChangeBit-
#18     NEW    cov: 2733 bits: 458 indir: 39 units: 11 exec/s: 0 L: 24 MS: 2 CrossOver-CrossOver-

The ... NEW ... line appears when libFuzzer finds new and interesting input. The efficient fuzzer should be able to finds lots of them rather quickly.

The ... pulse ... line will appear periodically to show the current status.

Default value for maximum testcase length

By default, when run manually, libFuzzer uses -max_len=64 or takes the length of the biggest testcase in corpus if corpus is not empty. ClusterFuzz takes random value in range from 1 to 10000 for each fuzzing session and passes that value to libFuzzers. If corpus contains testcases of size greater than max_len, libFuzzer will use only first max_len bytes of such testcases.

You can specify custom max_len value to be used by ClusterFuzz. For more information check out Maximum Testcase Length section of the Efficient Fuzzer Guide.

Submitting Fuzzer to ClusterFuzz

ClusterFuzz builds and executes all fuzzer_test targets in the source tree. The only thing you should do is to submit a fuzzer into Chrome.

Next Steps


[1] By default UBSan doesn't crash once undefined behavior has been detected. To make it crash the following additional option should be provided:

UBSAN_OPTIONS=halt_on_error=1 ./fuzzer <corpus_directory_or_single_testcase_path>

Other useful options (used by ClusterFuzz) are:

UBSAN_OPTIONS=symbolize=1:halt_on_error=1:print_stacktrace=1 ./fuzzer <corpus_directory_or_single_testcase_path>