[fuzzer] Update README.md

tests/fuzz/README.md

@@ -2,33 +2,87 @@
 
 Each fuzzing target can be built with multiple engines.
 
+## fuzz.py
+
+`fuzz.py` is a helper script for building and running fuzzers.
+Run `./fuzz.py -h` for the commands and run `./fuzz.py COMMAND -h` for
+command specific help.
+
+### Generating Data
+
+`fuzz.py` provides a utility to generate seed data for each fuzzer.
+
+```
+make -C ../tests decodecorpus
+./fuzz.py gen TARGET
+```
+
+By default it outputs 100 samples, each at most 8KB into `corpora/TARGET-seed`,
+but that can be configured with the `--number`, `--max-size-log` and `--seed`
+flags.
+
+### Build
+It respects the usual build environment variables `CC`, `CFLAGS`, etc.
+The environment variables can be overridden with the corresponding flags
+`--cc`, `--cflags`, etc.
+The specific fuzzing engine is selected with `LIB_FUZZING_ENGINE` or
+`--lib-fuzzing-engine`, the default is `libregression.a`.
+It has flags that can easily set up sanitizers `--enable-{a,ub,m}san`, and
+coverage instrumentation `--enable-coverage`.
+It sets sane defaults which can be overriden with flags `--debug`,
+`--enable-ubsan-pointer-overlow`, etc.
+Run `./fuzz.py build -h` for help.
+
+### Running Fuzzers
+
+`./fuzz.py` can run `libfuzzer`, `afl`, and `regression` tests.
+See the help of the relevant command for options.
+Flags not parsed by `fuzz.py` are passed to the fuzzing engine.
+The command used to run the fuzzer is printed for debugging.
+
 ## LibFuzzer
 
-You can install `libFuzzer` with `make libFuzzer`. Then you can make each target
+```
-with `make target LDFLAGS=-L. CC=clang CXX=clang++`.
+# Build libfuzzer if necessary
+make libFuzzer
+# Build the fuzz targets
+./fuzz.py build all --enable-coverage --enable-asan --enable-ubsan --lib-fuzzing-engine Fuzzer/libFuzzer.a --cc clang --cxx clang++
+# OR equivalently
+CC=clang CXX=clang++ LIB_FUZZING_ENGINE=Fuzzer/libFuzzer.a ./fuzz.py build all --enable-coverage --enable-asan --enable-ubsan
+# Run the fuzzer
+./fuzz.py libfuzzer TARGET -max_len=8192 -jobs=4
+```
+
+where `TARGET` could be `simple_decompress`, `stream_round_trip`, etc.
+
+### MSAN
+
+Fuzzing with `libFuzzer` and `MSAN` will require building a C++ standard library
+and libFuzzer with MSAN.
+`fuzz.py` respects the environment variables / flags `MSAN_EXTRA_CPPFLAGS`,
+`MSAN_EXTRA_CFLAGS`, `MSAN_EXTRA_CXXFLAGS`, `MSAN_EXTRA_LDFLAGS` to easily pass
+the extra parameters only for MSAN.
 
 ## AFL
 
-The regression driver also serves as a binary for `afl-fuzz`. You can make each
+The default `LIB_FUZZING_ENGINE` is `libregression.a`, which produces a binary
-target with one of these commands:
+that AFL can use.
 
 ```
-make target-regression CC=afl-clang CXX=afl-clang++
+# Build the fuzz targets
-AFL_MSAN=1 make target-regression-msan CC=afl-clang CXX=afl-clang++
+CC=afl-clang CXX=afl-clang++ ./fuzz.py build all --enable-asan --enable-ubsan
-AFL_ASAN=1 make target-regression-uasan CC=afl-clang CXX=afl-clang++
+# Run the fuzzer without a memory limit because of ASAN
+./fuzz.py afl TARGET -m none
 ```
 
-Then run as `./target @@`.
-
 ## Regression Testing
 
-Each fuzz target has a corpus checked into the repo under `fuzz/corpora/`.
+The regression rest supports the `all` target to run all the fuzzers in one
-You can run regression tests on the corpora to ensure that inputs which
+command.
-previously exposed bugs still pass. You can make these targets to run the
-regression tests with different sanitizers.
 
 ```
-make regression-test
+CC=clang CXX=clang++ ./fuzz.py build all --enable-asan --enable-ubsan
-make regression-test-msan
+./fuzz.py regression all
-make regression-test-uasan
+CC=clang CXX=clang++ ./fuzz.py build all --enable-msan
+./fuzz.py regression all
 ```