1 files changed, 73 insertions, 0 deletions
diff --git a/gcc_plugin/README.instrument_file.md b/gcc_plugin/README.instrument_file.md
new file mode 100644
index 00000000..d0eaf6ff
--- /dev/null
+++ b/gcc_plugin/README.instrument_file.md
@@ -0,0 +1,73 @@
+========================================
+Using afl++ with partial instrumentation
+========================================
+
+  This file describes how you can selectively instrument only the source files
+  that are interesting to you using the gcc instrumentation provided by
+  afl++.
+
+  Plugin by hexcoder-.
+
+
+## 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 gcc
+plugin of AFLFuzz that allows you to specify on a source file level which files
+should be compiled with or without instrumentation.
+
+
+## 2) Building the gcc plugin
+
+The new code is part of the existing afl++ gcc plugin in the gcc_plugin/
+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-gcc-fast and afl-g++-fast respectively. The only required change is
+that you need to set the environment variable AFL_GCC_INSTRUMENT_FILE 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 instrument list (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 instrument list file containing:
+
+```
+feature_a/a1.cpp
+feature_a/a2.cpp
+```
+
+However if the instrument list 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 instrument list file is then set to AFL_GCC_INSTRUMENT_FILE when you compile
+your program. For each file that didn't match the instrument list, 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.