Merge #17942: doc: Improve fuzzing docs for macOS users

b6c3e84 doc: Improve fuzzing docs for macOS users (Fabian Jahr)

Pull request description:

  Adds several helpful hints for macOS users trying to get fuzzers to run locally using AFL or libFuzzer. These are partly based on this comment bitcoin/bitcoin#17657 (comment) and discussions in the review club for #17860. See: https://bitcoincore.reviews/17860.html

  Based on the doc in the current state I could not compile fuzzers for AFL or libFuzzer. Using these hints, I can
  - compile and run fuzzers with AFL
  - compile but **not** run fuzzers with libFuzzer

  Fuzzers compiled with libFuzzers may be running but don't produce any output. Looking for others to test this to see if it is an issue with my local system. Especially interesting if you have been running libFuzzer fuzzers successfully on macOS before.

  Edit: Closes #17914

ACKs for top commit:
  MarcoFalke:
    ACK b6c3e84
  Sjors:
    ACK b6c3e84
  fanquake:
    ACK b6c3e84 - I think this has been nitpicked enough, and importantly the commands look better now.

Tree-SHA512: fdbacbcf10e9353a4ac3d22edf88663e33185ad2f244b986ff74c513de05f9fa62c4d8b17985d2f9288834c124b352cf52280627b5ff095735b411b12482e2ec

Author: MarcoFalke

doc/fuzzing.md

@@ -7,11 +7,8 @@ describe how to use it with AFL and libFuzzer.
 
 ## Preparing fuzzing
 
-AFL needs an input directory with examples, and an output directory where it
-will place examples that it found. These can be anywhere in the file system,
-we'll define environment variables to make it easy to reference them.
-
-libFuzzer will use the input directory as output directory.
+The fuzzer needs some inputs to work on, but the inputs or seeds can be used
+interchangeably between libFuzzer and AFL.
 
 Extract the example seeds (or other starting inputs) into the inputs
 directory before starting fuzzing.
@@ -21,13 +18,19 @@ git clone https://github.com/bitcoin-core/qa-assets
 export DIR_FUZZ_IN=$PWD/qa-assets/fuzz_seed_corpus
 ```
 
-Only for AFL:
+AFL needs an input directory with examples, and an output directory where it
+will place examples that it found. These can be anywhere in the file system,
+we'll define environment variables to make it easy to reference them.
+
+So, only for AFL you need to configure the outputs path:
 
 ```
 mkdir outputs
 export AFLOUT=$PWD/outputs
 ```
 
+libFuzzer will use the input directory as output directory.
+
 ## AFL
 
 ### Building AFL
@@ -41,16 +44,25 @@ make
 export AFLPATH=$PWD
 ```
 
+For macOS you may need to ignore x86 compilation checks when running `make`:
+`AFL_NO_X86=1 make`.
+
 ### Instrumentation
 
 To build Bitcoin Core using AFL instrumentation (this assumes that the
 `AFLPATH` was set as above):
 ```
 ./configure --disable-ccache --disable-shared --enable-tests --enable-fuzz CC=${AFLPATH}/afl-gcc CXX=${AFLPATH}/afl-g++
 export AFL_HARDEN=1
-cd src/
 make
 ```
+
+If you are using clang you will need to substitute `afl-gcc` with `afl-clang`
+and `afl-g++` with `afl-clang++`, so the first line above becomes:
+```
+./configure --disable-ccache --disable-shared --enable-tests --enable-fuzz CC=${AFLPATH}/afl-clang CXX=${AFLPATH}/afl-clang++
+```
+
 We disable ccache because we don't want to pollute the ccache with instrumented
 objects, and similarly don't want to use non-instrumented cached objects linked
 in.
@@ -60,25 +72,32 @@ The fuzzing can be sped up significantly (~200x) by using `afl-clang-fast` and
 compiling using `afl-clang-fast`/`afl-clang-fast++` the resulting
 binary will be instrumented in such a way that the AFL
 features "persistent mode" and "deferred forkserver" can be used. See
-https://github.com/mcarpenter/afl/tree/master/llvm_mode for details.
+https://github.com/google/AFL/tree/master/llvm_mode for details.
 
 ### Fuzzing
 
 To start the actual fuzzing use:
 
 ```
-export FUZZ_TARGET=fuzz_target_foo  # Pick a fuzz_target
+export FUZZ_TARGET=bech32  # Pick a fuzz_target
 mkdir ${AFLOUT}/${FUZZ_TARGET}
-$AFLPATH/afl-fuzz -i ${DIR_FUZZ_IN}/${FUZZ_TARGET} -o ${AFLOUT}/${FUZZ_TARGET} -m52 -- test/fuzz/${FUZZ_TARGET}
+$AFLPATH/afl-fuzz -i ${DIR_FUZZ_IN}/${FUZZ_TARGET} -o ${AFLOUT}/${FUZZ_TARGET} -m52 -- src/test/fuzz/${FUZZ_TARGET}
 ```
 
 You may have to change a few kernel parameters to test optimally - `afl-fuzz`
 will print an error and suggestion if so.
 
+On macOS you may need to set `AFL_NO_FORKSRV=1` to get the target to run.
+```
+export FUZZ_TARGET=bech32  # Pick a fuzz_target
+mkdir ${AFLOUT}/${FUZZ_TARGET}
+AFL_NO_FORKSRV=1 $AFLPATH/afl-fuzz -i ${DIR_FUZZ_IN}/${FUZZ_TARGET} -o ${AFLOUT}/${FUZZ_TARGET} -m52 -- src/test/fuzz/${FUZZ_TARGET}
+```
+
 ## libFuzzer
 
-A recent version of `clang`, the address/undefined sanitizers (ASan/UBSan) and libFuzzer is needed (all
-found in the `compiler-rt` runtime libraries package).
+A recent version of `clang`, the address/undefined sanitizers (ASan/UBSan) and
+libFuzzer is needed (all found in the `compiler-rt` runtime libraries package).
 
 To build all fuzz targets with libFuzzer, run
 
@@ -87,11 +106,33 @@ To build all fuzz targets with libFuzzer, run
 make
 ```
 
-The fuzzer needs some inputs to work on, but the inputs or seeds can be used
-interchangeably between libFuzzer and AFL.
-
 See https://llvm.org/docs/LibFuzzer.html#running on how to run the libFuzzer
 instrumented executable.
 
-Alternatively run the script in `./test/fuzz/test_runner.py` and provide it
-with the `${DIR_FUZZ_IN}` created earlier.
+Alternatively, you can run the script through the fuzzing test harness (only
+libFuzzer supported so far). You need to pass it the inputs directory and
+the specific test target you want to run.
+
+```
+./test/fuzz/test_runner.py ${DIR_FUZZ_IN} bech32
+```
+
+### macOS hints for libFuzzer
+
+The default clang/llvm version supplied by Apple on macOS does not include
+fuzzing libraries, so macOS users will need to install a full version, for
+example using `brew install llvm`.
+
+Should you run into problems with the address sanitizer, it is possible you
+may need to run `./configure` with `--disable-asm` to avoid errors
+with certain assembly code from Bitcoin Core's code. See [developer notes on sanitizers](https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#sanitizers)
+for more information.
+
+You may also need to take care of giving the correct path for clang and
+clang++, like `CC=/path/to/clang CXX=/path/to/clang++` if the non-systems
+clang does not come first in your path.
+
+Full configure that was tested on macOS Catalina with `brew` installed `llvm`:
+```
+./configure --disable-ccache --enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=/usr/local/opt/llvm/bin/clang CXX=/usr/local/opt/llvm/bin/clang++ --disable-asm
+```