* Build the Fuzzer library as a static archive (or just a set of .o files).
Note that the Fuzzer contains the main() function.
Preferably do *not* use sanitizers while building the Fuzzer.
-* Build the library you are going to test with -fsanitize-coverage=[234]
+* Build the library you are going to test with
+ `-fsanitize-coverage={bb,edge}[,indirect-calls]`
and one of the sanitizers. We recommend to build the library in several
different modes (e.g. asan, msan, lsan, ubsan, etc) and even using different
optimizations options (e.g. -O0, -O1, -O2) to diversify testing.
* Build a test driver using the same options as the library.
The test driver is a C/C++ file containing interesting calls to the library
- inside a single function ``extern "C" void TestOneInput(const uint8_t *Data, size_t Size);``
+ inside a single function ``extern "C" void LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);``
* Link the Fuzzer, the library and the driver together into an executable
using the same sanitizer options as for the library.
* Collect the initial corpus of inputs for the
the sanitizer (asan, etc) it will be reported as usual and the reproducer
will be written to disk.
Each Fuzzer process is single-threaded (unless the library starts its own
- threads). You can run the Fuzzer on the same corpus in multiple processes.
+ threads). You can run the Fuzzer on the same corpus in multiple processes
in parallel. For run-time options run the Fuzzer binary with '-help=1'.
A simple function that does something interesting if it receives the input "HI!"::
cat << EOF >> test_fuzzer.cc
- extern "C" void TestOneInput(const unsigned char *data, unsigned long size) {
+ extern "C" void LLVMFuzzerTestOneInput(const unsigned char *data, unsigned long size) {
if (size > 0 && data[0] == 'H')
if (size > 1 && data[1] == 'I')
if (size > 2 && data[2] == '!')
# Build lib/Fuzzer files.
clang -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer
# Build test_fuzzer.cc with asan and link against lib/Fuzzer.
- clang++ -fsanitize=address -fsanitize-coverage=3 test_fuzzer.cc Fuzzer*.o
+ clang++ -fsanitize=address -fsanitize-coverage=edge test_fuzzer.cc Fuzzer*.o
# Run the fuzzer with no corpus.
./a.out
Here we show how to use lib/Fuzzer on something real, yet simple: pcre2_::
- COV_FLAGS=" -fsanitize-coverage=4 -mllvm -sanitizer-coverage-8bit-counters=1"
+ COV_FLAGS=" -fsanitize-coverage=edge,indirect-calls,8bit-counters"
# Get PCRE2
svn co svn://vcs.exim.org/pcre2/code/trunk pcre
# Get lib/Fuzzer. Assuming that you already have fresh clang in PATH.
cat << EOF > pcre_fuzzer.cc
#include <string.h>
#include "pcre2posix.h"
- extern "C" void TestOneInput(const unsigned char *data, size_t size) {
+ extern "C" void LLVMFuzzerTestOneInput(const unsigned char *data, size_t size) {
if (size < 1) return;
char *str = new char[size+1];
memcpy(str, data, size);
This time you were running the fuzzer with a non-empty input corpus (564 items).
As the first step, the fuzzer minimized the set to produce 344 interesting items (the ``INITED`` line)
+It is quite convenient to store test corpuses in git.
+As an example, here is a git repository with test inputs for the above PCRE2 fuzzer::
+
+ git clone https://github.com/kcc/fuzzing-with-sanitizers.git
+ ./pcre_fuzzer ./fuzzing-with-sanitizers/pcre2/C1/
+
You may run ``N`` independent fuzzer jobs in parallel on ``M`` CPUs::
N=100; M=4; ./pcre_fuzzer ./CORPUS -jobs=$N -workers=$M
-This is useful when you already have an exhaustive test corpus.
-If you've just started fuzzing with no good corpus running independent
-jobs will create a corpus with too many duplicates.
-One way to avoid this and still use all of your CPUs is to use the flag ``-exit_on_first=1``
-which will cause the fuzzer to exit on the first new synthesised input::
+By default (``-reload=1``) the fuzzer processes will periodically scan the CORPUS directory
+and reload any new tests. This way the test inputs found by one process will be picked up
+by all others.
- N=100; M=4; ./pcre_fuzzer ./CORPUS -jobs=$N -workers=$M -exit_on_first=1
+If ``-workers=$M`` is not supplied, ``min($N,NumberOfCpuCore/2)`` will be used.
Heartbleed
----------
wget https://www.openssl.org/source/openssl-1.0.1f.tar.gz
tar xf openssl-1.0.1f.tar.gz
- COV_FLAGS="-fsanitize-coverage=4" # -mllvm -sanitizer-coverage-8bit-counters=1"
+ COV_FLAGS="-fsanitize-coverage=edge,indirect-calls" # -fsanitize-coverage=8bit-counters
(cd openssl-1.0.1f/ && ./config &&
make -j 32 CC="clang -g -fsanitize=address $COV_FLAGS")
# Get and build LibFuzzer
assert (SSL_CTX_use_PrivateKey_file(sctx, "server.key", SSL_FILETYPE_PEM));
return 0;
}
- extern "C" void TestOneInput(unsigned char *Data, size_t Size) {
+ extern "C" void LLVMFuzzerTestOneInput(unsigned char *Data, size_t Size) {
static int unused = Init();
SSL *server = SSL_new(sctx);
BIO *sinbio = BIO_new(BIO_s_mem());
but before passing this input to the target library it will replace every byte ``b`` with the ``b``-th token.
If there are less than ``b`` tokens, a space will be added instead.
+AFL compatibility
+-----------------
+LibFuzzer can be used in parallel with AFL_ on the same test corpus.
+Both fuzzers expect the test corpus to reside in a directory, one file per input.
+You can run both fuzzers on the same corpus in parallel::
+
+ ./afl-fuzz -i testcase_dir -o findings_dir /path/to/program -r @@
+ ./llvm-fuzz testcase_dir findings_dir # Will write new tests to testcase_dir
+
+Periodically restart both fuzzers so that they can use each other's findings.
+
+How good is my fuzzer?
+----------------------
+
+Once you implement your target function ``LLVMFuzzerTestOneInput`` and fuzz it to death,
+you will want to know whether the function or the corpus can be improved further.
+One easy to use metric is, of course, code coverage.
+You can get the coverage for your corpus like this::
+
+ ASAN_OPTIONS=coverage_pcs=1 ./fuzzer CORPUS_DIR -runs=0
+
+This will run all the tests in the CORPUS_DIR but will not generate any new tests
+and dump covered PCs to disk before exiting.
+Then you can subtract the set of covered PCs from the set of all instrumented PCs in the binary,
+see SanitizerCoverage_ for details.
Fuzzing components of LLVM
==========================
Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=23057
+Buildbot
+--------
+
+We have a buildbot that runs the above fuzzers for LLVM components
+24/7/365 at http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer .
+
+Pre-fuzzed test inputs in git
+-----------------------------
+
+The buildbot occumulates large test corpuses over time.
+The corpuses are stored in git on github and can be used like this::
+
+ git clone https://github.com/kcc/fuzzing-with-sanitizers.git
+ bin/clang-format-fuzzer fuzzing-with-sanitizers/llvm/clang-format/C1
+ bin/clang-fuzzer fuzzing-with-sanitizers/llvm/clang/C1/
+ bin/clang-fuzzer fuzzing-with-sanitizers/llvm/clang/TOK1 -tokens=$LLVM/llvm/lib/Fuzzer/cxx_fuzzer_tokens.txt
+
+
FAQ
=========================
.. _AFL: http://lcamtuf.coredump.cx/afl/
-.. _SanitizerCoverage: https://code.google.com/p/address-sanitizer/wiki/AsanCoverage
+.. _SanitizerCoverage: http://clang.llvm.org/docs/SanitizerCoverage.html
.. _Heartbleed: http://en.wikipedia.org/wiki/Heartbleed
projects / oota-llvm.git / blobdiff