* 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,8bit-counters]`
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
fuzzer (a directory with test inputs, one file per input).
The better your inputs are the faster you will find something interesting.
Also try to keep your inputs small, otherwise the Fuzzer will run too slow.
+ By default, the Fuzzer limits the size of every input by 64 bytes
+ (use ``-max_len=N`` to override).
* Run the fuzzer with the test corpus. As new interesting test cases are
discovered they will be added to the corpus. If a bug is discovered by
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
- in parallel. For run-time options run the Fuzzer binary with '-help=1'.
+ in parallel.
The Fuzzer is similar in concept to AFL_,
but the Fuzzer itself does not (and should not) depend on any
part of LLVM and can be used for other projects w/o requiring the rest of LLVM.
+Flags
+=====
+The most important flags are::
+
+ seed 0 Random seed. If 0, seed is generated.
+ runs -1 Number of individual test runs (-1 for infinite runs).
+ max_len 64 Maximal length of the test input.
+ cross_over 1 If 1, cross over inputs.
+ mutate_depth 5 Apply this number of consecutive mutations to each input.
+ timeout -1 Timeout in seconds (if positive). If one unit runs more than this number of seconds the process will abort.
+ help 0 Print help.
+ save_minimized_corpus 0 If 1, the minimized corpus is saved into the first input directory
+ jobs 0 Number of jobs to run. If jobs >= 1 we spawn this number of jobs in separate worker processes with stdout/stderr redirected to fuzz-JOB.log.
+ workers 0 Number of simultaneous worker processes to run the jobs. If zero, "min(jobs,NumberOfCpuCores()/2)" is used.
+ tokens 0 Use the file with tokens (one token per line) to fuzz a token based input language.
+ apply_tokens 0 Read the given input file, substitute bytes with tokens and write the result to stdout.
+ sync_command 0 Execute an external command "<sync_command> <test_corpus>" to synchronize the test corpus.
+ sync_timeout 600 Minimal timeout between syncs.
+
+For the full list of flags run the fuzzer binary with ``-help=1``.
+
Usage examples
==============
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 $COV_FLAGS")
# Get and build LibFuzzer
assert (SSL_CTX_use_PrivateKey_file(sctx, "server.key", SSL_FILETYPE_PEM));
return 0;
}
- extern "C" void TestOneInput(unsigned char *Data, size_t Size) {
+ extern "C" void LLVMFuzzerTestOneInput(unsigned char *Data, size_t Size) {
static int unused = Init();
SSL *server = SSL_new(sctx);
BIO *sinbio = BIO_new(BIO_s_mem());
How good is my fuzzer?
----------------------
-Once you implement your target function ``TestOneInput`` and fuzz it to death,
+Once you implement your target function ``LLVMFuzzerTestOneInput`` and fuzz it to death,
you will want to know whether the function or the corpus can be improved further.
One easy to use metric is, of course, code coverage.
You can get the coverage for your corpus like this::
Then you can subtract the set of covered PCs from the set of all instrumented PCs in the binary,
see SanitizerCoverage_ for details.
+User-supplied mutators
+----------------------
+
+LibFuzzer allows to use custom (user-supplied) mutators,
+see FuzzerInterface.h_
+
Fuzzing components of LLVM
==========================
Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=23057
+Buildbot
+--------
+
+We have a buildbot that runs the above fuzzers for LLVM components
+24/7/365 at http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer .
+
+Pre-fuzzed test inputs in git
+-----------------------------
+
+The buildbot occumulates large test corpuses over time.
+The corpuses are stored in git on github and can be used like this::
+
+ git clone https://github.com/kcc/fuzzing-with-sanitizers.git
+ bin/clang-format-fuzzer fuzzing-with-sanitizers/llvm/clang-format/C1
+ bin/clang-fuzzer fuzzing-with-sanitizers/llvm/clang/C1/
+ bin/clang-fuzzer fuzzing-with-sanitizers/llvm/clang/TOK1 -tokens=$LLVM/llvm/lib/Fuzzer/cxx_fuzzer_tokens.txt
+
+
FAQ
=========================
.. _SanitizerCoverage: http://clang.llvm.org/docs/SanitizerCoverage.html
.. _Heartbleed: http://en.wikipedia.org/wiki/Heartbleed
+
+.. _FuzzerInterface.h: https://github.com/llvm-mirror/llvm/blob/master/lib/Fuzzer/FuzzerInterface.h
projects / oota-llvm.git / blobdiff