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Fuzz-testing Bitcoin Core
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==========================
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A special test harness `test_bitcoin_fuzzy` is provided to provide an easy
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entry point for fuzzers and the like. In this document we'll describe how to
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use it with AFL.
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Building AFL
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-------------
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It is recommended to always use the latest version of afl:
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```
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wget http://lcamtuf.coredump.cx/afl/releases/afl-latest.tgz
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tar -zxvf afl-latest.tgz
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cd afl-<version>
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make
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export AFLPATH=$PWD
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```
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Instrumentation
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----------------
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To build Bitcoin Core using AFL instrumentation (this assumes that the
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`AFLPATH` was set as above):
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```
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./configure --disable-ccache --disable-shared --enable-tests CC=${AFLPATH}/afl-gcc CXX=${AFLPATH}/afl-g++
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export AFL_HARDEN=1
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cd src/
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make test/test_bitcoin_fuzzy
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```
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We disable ccache because we don't want to pollute the ccache with instrumented
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objects, and similarly don't want to use non-instrumented cached objects linked
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in.
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The fuzzing can be sped up significantly (~200x) by using `afl-clang-fast` and
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`afl-clang-fast++` in place of `afl-gcc` and `afl-g++` when compiling. When
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compiling using `afl-clang-fast`/`afl-clang-fast++` the resulting
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`test_bitcoin_fuzzy` binary will be instrumented in such a way that the AFL
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features "persistent mode" and "deferred forkserver" can be used. See
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https://github.com/mcarpenter/afl/tree/master/llvm_mode for details.
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Preparing fuzzing
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------------------
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AFL needs an input directory with examples, and an output directory where it
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will place examples that it found. These can be anywhere in the file system,
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we'll define environment variables to make it easy to reference them.
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```
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mkdir inputs
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AFLIN=$PWD/inputs
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mkdir outputs
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AFLOUT=$PWD/outputs
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```
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Example inputs are available from:
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- https://download.visucore.com/bitcoin/bitcoin_fuzzy_in.tar.xz
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- http://strateman.ninja/fuzzing.tar.xz
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Extract these (or other starting inputs) into the `inputs` directory before starting fuzzing.
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Fuzzing
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--------
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To start the actual fuzzing use:
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```
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$AFLPATH/afl-fuzz -i ${AFLIN} -o ${AFLOUT} -m52 -- test/test_bitcoin_fuzzy
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```
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You may have to change a few kernel parameters to test optimally - `afl-fuzz`
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will print an error and suggestion if so.
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