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Using afl++ with partial instrumentation

This file describes how you can selectively instrument only the source files that are interesting to you using the LLVM instrumentation provided by afl++

Originally developed by Christian Holler (:decoder) choller@mozilla.com.

1) Description and purpose

When building and testing complex programs where only a part of the program is the fuzzing target, it often helps to only instrument the necessary parts of the program, leaving the rest uninstrumented. This helps to focus the fuzzer on the important parts of the program, avoiding undesired noise and disturbance by uninteresting code being exercised.

For this purpose, I have added a "partial instrumentation" support to the LLVM mode of AFLFuzz that allows you to specify on a source file level which files should be compiled with or without instrumentation.

2) Building the LLVM module

The new code is part of the existing afl++ LLVM module in the llvm_mode/ subdirectory. There is nothing specifically to do :)

3) How to use the partial instrumentation mode

In order to build with partial instrumentation, you need to build with afl-clang-fast and afl-clang-fast++ respectively. The only required change is that you need to set the environment variable AFL_LLVM_WHITELIST when calling the compiler.

The environment variable must point to a file containing all the filenames that should be instrumented. For matching, the filename that is being compiled must end in the filename entry contained in this whitelist (to avoid breaking the matching when absolute paths are used during compilation).

For example if your source tree looks like this:

project/
project/feature_a/a1.cpp
project/feature_a/a2.cpp
project/feature_b/b1.cpp
project/feature_b/b2.cpp

and you only want to test feature_a, then create a whitelist file containing:

feature_a/a1.cpp
feature_a/a2.cpp

However if the whitelist file contains only this, it works as well:

a1.cpp
a2.cpp

but it might lead to files being unwantedly instrumented if the same filename exists somewhere else in the project directories.

The created whitelist file is then set to AFL_LLVM_WHITELIST when you compile your program. For each file that didn't match the whitelist, the compiler will issue a warning at the end stating that no blocks were instrumented. If you didn't intend to instrument that file, then you can safely ignore that warning.

For old LLVM versions this feature might require to be compiled with debug information (-g), however at least from llvm version 6.0 onwards this is not required anymore (and might hurt performance and crash detection, so better not use -g).