6 ``pw_fuzzer`` provides developers with tools to write `libFuzzer`_ based
9 Fuzzing or fuzz testing is style of testing that stochastically generates inputs
10 to targeted interfaces in order to automatically find defects and/or
11 vulnerabilities. In other words, fuzzing is simply an automated way of testing
12 APIs with generated data.
14 A fuzzer is a program that is used to fuzz a interface. It typically has three
15 steps that it executes repeatedly:
17 #. Generate a new, context-free input. This is the *fuzzing engine*. For
18 ``pw_fuzzer``, this is `libFuzzer`_.
19 #. Use the input to exercise the targeted interface, or code being tested. This
20 is the *fuzz target function*. For ``pw_fuzzer``, these are the GN
21 ``sources`` and/or ``deps`` that define `LLVMFuzzerTestOneInput`_.
22 #. Monitor the code being tested for any abnormal conditions. This is the
23 *instrumentation*. For ``pw_fuzzer``, these are sanitizer runtimes from
24 LLVM's `compiler_rt`_.
28 ``pw_fuzzer`` is currently only supported on Linux and MacOS using clang.
30 .. image:: doc_resources/pw_fuzzer_coverage_guided.png
31 :alt: Coverage Guided Fuzzing with libFuzzer
37 To write a fuzzer, a developer needs to write a fuzz target function follwing
38 the `fuzz target function`__ guidelines given by libFuzzer:
42 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
43 DoSomethingInterestingWithMyAPI(data, size);
44 return 0; // Non-zero return values are reserved for future use.
47 .. __: LLVMFuzzerTestOneInput_
49 When writing you fuzz target function, you may want to consider:
51 - It is acceptable to return early if the input doesn't mean some constraints,
53 - If your fuzzer accepts data with a well-defined format, you can bootstrap
54 coverage by crafting examples and adding them to a `corpus`_.
55 - There are tools to `split a fuzzing input`_ into multiple fields if needed;
56 the `FuzzedDataProvider`_ is particularly easy to use.
57 - If your code acts on "transformed" inputs, such as encoded or compressed
58 inputs, you may want to try `structure aware fuzzing`.
59 - You can do `startup initialization`_ if you need to.
60 - If your code is non-deterministic or uses checksums, you may want to disable
61 those **only** when fuzzing by using LLVM's
62 `FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION`_
69 To build a fuzzer, do the following:
71 1. Add the GN target using ``pw_fuzzer`` GN template, and add it to your the
72 test group of the module:
76 # In $dir_my_module/BUILD.gn
77 import("$dir_pw_fuzzer/fuzzer.gni")
79 pw_fuzzer("my_fuzzer") {
80 sources = [ "my_fuzzer.cc" ]
84 pw_test_group("tests") {
86 ":existing_tests", ...
87 ":my_fuzzer", # <- Added!
91 2. Select your choice of sanitizers ("address" is also the current default).
92 See LLVM for `valid options`_.
96 $ gn gen out --args='pw_toolchain_SANITIZERS=["address"]'
98 3. Build normally, e.g. using ``pw watch``.
102 Running fuzzers locally
103 =======================
105 Based on the example above, the fuzzer output will be at
106 ``out/host/obj/my_module/my_fuzzer``. It can be invoked using the normal
107 `libFuzzer options`_ and `sanitizer runtime flags`_. For even more details, see
108 the libFuzzer section on `running a fuzzer`_.
110 For example, the following invocation disables "one definition rule" detection,
111 saves failing inputs to ``artifacts/``, treats any input that takes longer than
112 10 seconds as a failure, and stores the working corpus in ``corpus/``.
117 $ ASAN_OPTIONS=detect_odr_violation=0 \
118 out/host_clang_fuzz/obj/pw_fuzzer/bin/toy_fuzzer \
119 -artifact_prefix=artifacts/ \
122 INFO: Seed: 305325345
123 INFO: Loaded 1 modules (46 inline 8-bit counters): 46 [0x38dfc0, 0x38dfee),
124 INFO: Loaded 1 PC tables (46 PCs): 46 [0x23aaf0,0x23add0),
125 INFO: 0 files found in corpus
126 INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
127 INFO: A corpus is not provided, starting from an empty corpus
128 #2 INITED cov: 2 ft: 3 corp: 1/1b exec/s: 0 rss: 27Mb
129 #4 NEW cov: 3 ft: 4 corp: 2/3b lim: 4 exec/s: 0 rss: 27Mb L: 2/2 MS: 2 ShuffleBytes-InsertByte-
130 #11 NEW cov: 7 ft: 8 corp: 3/7b lim: 4 exec/s: 0 rss: 27Mb L: 4/4 MS: 2 EraseBytes-CrossOver-
131 #27 REDUCE cov: 7 ft: 8 corp: 3/6b lim: 4 exec/s: 0 rss: 27Mb L: 3/3 MS: 1 EraseBytes-
132 #29 REDUCE cov: 7 ft: 8 corp: 3/5b lim: 4 exec/s: 0 rss: 27Mb L: 2/2 MS: 2 ChangeBit-EraseBytes-
133 #445 REDUCE cov: 9 ft: 10 corp: 4/13b lim: 8 exec/s: 0 rss: 27Mb L: 8/8 MS: 1 InsertRepeatedBytes-
134 #12104 NEW cov: 11 ft: 12 corp: 5/24b lim: 122 exec/s: 0 rss: 28Mb L: 11/11 MS: 4 CMP-InsertByte-ShuffleBytes-ChangeByte- DE: "\xff\xff"-
135 #12321 NEW cov: 12 ft: 13 corp: 6/31b lim: 122 exec/s: 0 rss: 28Mb L: 7/11 MS: 2 CopyPart-EraseBytes-
136 #12459 REDUCE cov: 12 ft: 13 corp: 6/28b lim: 122 exec/s: 0 rss: 28Mb L: 8/8 MS: 3 CMP-InsertByte-EraseBytes- DE: "\x00\x00"-
137 #12826 REDUCE cov: 12 ft: 13 corp: 6/26b lim: 122 exec/s: 0 rss: 28Mb L: 5/8 MS: 2 ShuffleBytes-EraseBytes-
138 #14824 REDUCE cov: 12 ft: 13 corp: 6/25b lim: 135 exec/s: 0 rss: 28Mb L: 4/8 MS: 3 PersAutoDict-ShuffleBytes-EraseBytes- DE: "\x00\x00"-
139 #15106 REDUCE cov: 12 ft: 13 corp: 6/24b lim: 135 exec/s: 0 rss: 28Mb L: 3/8 MS: 2 ChangeByte-EraseBytes-
141 #197809 REDUCE cov: 35 ft: 36 corp: 22/129b lim: 1800 exec/s: 0 rss: 79Mb L: 9/9 MS: 1 InsertByte-
142 #216250 REDUCE cov: 35 ft: 36 corp: 22/128b lim: 1980 exec/s: 0 rss: 87Mb L: 8/8 MS: 1 EraseBytes-
143 #242761 REDUCE cov: 35 ft: 36 corp: 22/127b lim: 2237 exec/s: 0 rss: 101Mb L: 7/8 MS: 1 EraseBytes-
144 ==126148== ERROR: libFuzzer: deadly signal
145 #0 0x35b981 in __sanitizer_print_stack_trace ../recipe_cleanup/clangFu99hg/llvm_build_dir/tools/clang/stage2-bins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/compiler-rt/lib/asan/asan_stack.cpp:86:3
146 #1 0x2bcdb5 in fuzzer::PrintStackTrace() (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2bcdb5)
147 #2 0x2a2ac9 in fuzzer::Fuzzer::CrashCallback() (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a2ac9)
148 #3 0x7f866684151f (/lib/x86_64-linux-gnu/libpthread.so.0+0x1351f)
149 #4 0x3831df in (anonymous namespace)::toy_example(char const*, char const*) /home/aarongreen/src/pigweed/out/host/../../pw_fuzzer/examples/toy_fuzzer.cc:49:15
150 #5 0x3831df in LLVMFuzzerTestOneInput /home/aarongreen/src/pigweed/out/host/../../pw_fuzzer/examples/toy_fuzzer.cc:80:3
151 #6 0x2a4025 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a4025)
152 #7 0x2a3774 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a3774)
153 #8 0x2a5769 in fuzzer::Fuzzer::MutateAndTestOne() (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a5769)
154 #9 0x2a6185 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, fuzzer::fuzzer_allocator<fuzzer::SizedFile> >&) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x2a6185)
155 #10 0x294c8a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x294c8a)
156 #11 0x2bd422 in main ../recipe_cleanup/clangFu99hg/llvm_build_dir/tools/clang/stage2-bins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/compiler-rt/lib/fuzzer/FuzzerMain.cpp:19:10
157 #12 0x7f8666684bba in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x26bba)
158 #13 0x26ae19 in _start (/home/aarongreen/src/pigweed/out/host/obj/pw_fuzzer/toy_fuzzer+0x26ae19)
160 NOTE: libFuzzer has rudimentary signal handlers.
161 Combine libFuzzer with AddressSanitizer or similar for better crash reports.
162 SUMMARY: libFuzzer: deadly signal
163 MS: 1 CrossOver-; base unit: 9f479f7a6e3a21363397a25da3168218ba182a16
164 0x68,0x65,0x6c,0x6c,0x6f,0x0,0x77,0x6f,0x72,0x6c,0x64,0x0,0x0,0x0,
165 hello\x00world\x00\x00\x00
166 artifact_prefix='artifacts'; Test unit written to artifacts/crash-6e4fdc7ffd04131ea15dd243a0890b1b606f4831
167 Base64: aGVsbG8Ad29ybGQAAAA=
169 Running fuzzers on OSS-Fuzz
170 ===========================
172 Pigweed is integrated with `OSS-Fuzz`_, a continuous fuzzing infrastructure for
173 open source software. Fuzzers listed in in ``pw_test_groups`` will automatically
174 start being run within a day or so of appearing in the git repository.
176 Bugs produced by OSS-Fuzz can be found in its `Monorail instance`_. These bugs
179 * A detailed report, including a symbolized backtrace.
180 * A revision range indicating when the bug has been detected.
181 * A minimized testcase, which is a fuzzer input that can be used to reproduce
186 #. Build_ the fuzzers as described above.
187 #. Download the minimized testcase.
188 #. Run_ the fuzzer with the testcase as an argument.
190 For example, if the testcase is saved as "~/Downloads/testcase"
191 and the fuzzer is the same as in the examples above, you could run:
195 $ ./out/host/obj/pw_fuzzer/toy_fuzzer ~/Downloads/testcase
197 If you need to recreate the OSS-Fuzz environment locally, you can use its
198 documentation on `reproducing`_ issues.
200 In particular, you can recreate the OSS-Fuzz environment using:
204 $ python infra/helper.py pull_images
205 $ python infra/helper.py build_image pigweed
206 $ python infra/helper.py build_fuzzers --sanitizer <address/undefined> pigweed
208 With that environment, you can run the reproduce bugs using:
212 python infra/helper.py reproduce pigweed <pw_module>_<fuzzer_name> ~/Downloads/testcase
214 You can even verify fixes in your local source checkout:
218 $ python infra/helper.py build_fuzzers --sanitizer <address/undefined> pigweed $PW_ROOT
219 $ python infra/helper.py reproduce pigweed <pw_module>_<fuzzer_name> ~/Downloads/testcase
221 .. _compiler_rt: https://compiler-rt.llvm.org/
222 .. _corpus: https://llvm.org/docs/LibFuzzer.html#corpus
223 .. _FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION: https://llvm.org/docs/LibFuzzer.html#fuzzer-friendly-build-mode
224 .. _FuzzedDataProvider: https://github.com/llvm/llvm-project/blob/master/compiler-rt/include/fuzzer/FuzzedDataProvider.h
225 .. _libFuzzer: https://llvm.org/docs/LibFuzzer.html
226 .. _libFuzzer options: https://llvm.org/docs/LibFuzzer.html#options
227 .. _LLVMFuzzerTestOneInput: https://llvm.org/docs/LibFuzzer.html#fuzz-target
228 .. _monorail instance: https://bugs.chromium.org/p/oss-fuzz
229 .. _oss-fuzz: https://github.com/google/oss-fuzz
230 .. _reproducing: https://google.github.io/oss-fuzz/advanced-topics/reproducing/
231 .. _running a fuzzer: https://llvm.org/docs/LibFuzzer.html#running
232 .. _sanitizer runtime flags: https://github.com/google/sanitizers/wiki/SanitizerCommonFlags
233 .. _split a fuzzing input: https://github.com/google/fuzzing/blob/master/docs/split-inputs.md
234 .. _startup initialization: https://llvm.org/docs/LibFuzzer.html#startup-initialization
235 .. _structure aware fuzzing: https://github.com/google/fuzzing/blob/master/docs/structure-aware_fuzzing.md
236 .. _valid options: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html