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74 lines
2.5 KiB
Markdown
74 lines
2.5 KiB
Markdown
========================================
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Using afl++ with partial instrumentation
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========================================
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This file describes how you can selectively instrument only the source files
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that are interesting to you using the gcc instrumentation provided by
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afl++.
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Plugin by hexcoder-.
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## 1) Description and purpose
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When building and testing complex programs where only a part of the program is
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the fuzzing target, it often helps to only instrument the necessary parts of
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the program, leaving the rest uninstrumented. This helps to focus the fuzzer
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on the important parts of the program, avoiding undesired noise and
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disturbance by uninteresting code being exercised.
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For this purpose, I have added a "partial instrumentation" support to the gcc
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plugin of AFLFuzz that allows you to specify on a source file level which files
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should be compiled with or without instrumentation.
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## 2) Building the gcc plugin
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The new code is part of the existing afl++ gcc plugin in the gcc_plugin/
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subdirectory. There is nothing specifically to do :)
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## 3) How to use the partial instrumentation mode
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In order to build with partial instrumentation, you need to build with
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afl-gcc-fast and afl-g++-fast respectively. The only required change is
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that you need to set the environment variable AFL_GCC_INSTRUMENT_FILE when calling
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the compiler.
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The environment variable must point to a file containing all the filenames
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that should be instrumented. For matching, the filename that is being compiled
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must end in the filename entry contained in this instrument list (to avoid breaking
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the matching when absolute paths are used during compilation).
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For example if your source tree looks like this:
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```
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project/
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project/feature_a/a1.cpp
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project/feature_a/a2.cpp
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project/feature_b/b1.cpp
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project/feature_b/b2.cpp
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```
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and you only want to test feature_a, then create a instrument list file containing:
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```
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feature_a/a1.cpp
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feature_a/a2.cpp
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```
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However if the instrument list file contains only this, it works as well:
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```
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a1.cpp
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a2.cpp
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```
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but it might lead to files being unwantedly instrumented if the same filename
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exists somewhere else in the project directories.
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The created instrument list file is then set to AFL_GCC_INSTRUMENT_FILE when you compile
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your program. For each file that didn't match the instrument list, the compiler will
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issue a warning at the end stating that no blocks were instrumented. If you
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didn't intend to instrument that file, then you can safely ignore that warning.
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