Add README.md for aflpp_extractor

Author: P1umer

utils/aflpp_extractor/README.md

@@ -0,0 +1,83 @@
+# AFL++ Plugin: Data-Generator Extractor
+## Overview
+This plugin allows AFL++ to use LibFuzzer's data generators for fuzzing. The plugin extracts generators from LibFuzzer and integrates them into AFL++.
+
+## Example Building
+
+1. Build the AFL++ compiler.
+This will create the patched `afl-cc`.
+
+2. Build the plugin:
+```
+cd utils/aflpp_extractor
+make
+```
+
+This will compile and create the files:
+- `libAFLExtractor.a`: The static library containing the AFL++ Extractor plugin.
+- `extractor_test_fuzzer`: The library that demonstrates how to extract generator using this plugin.
+- `reproducer`: The program that reads an input file, processes it with the extracted generator, and writes the result to an output file.
+
+## For Your Use
+Assume you have a generation-based libfuzzer `fuzzer_harness.c`:
+```
+size_t generate_magic_data(const uint8_t *data, size_t size){
+
+    /* 1. Consume the random data */
+    /* 2. Construct random data into more complex data to   */
+    /*    satisfy a certain structure, syntax or semantics  */
+}
+
+/* libFuzzer's required entry point. */
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  /* Generate magic data */
+  uint8_t * magic_data = generate_magic_data(data, size);
+
+  /* Call function to be fuzzed using the magic_data */
+  target_function(modified_data);
+
+  /* Reset state. e.g. free something */
+}
+```
+And this is the clang compile command to build for libfuzzer: 
+```
+clang -o fuzz -fsanitize=fuzzer fuzzer_harness.cc
+```
+OK, if you find the data generator to be quite ingenious and you want to use it in AFL++, set it up as follows:
+
+#### 1) Include the header before `LLVMFuzzerTestOneInput`:
+```
+#include "fuzzer_ext.h"
+```
+or just add this:
+```
+typedef struct mutate_helper {
+    uint8_t *buf;
+    size_t len;
+} mutate_helper_t;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+    extern void mutate_helper_buffer_copy(unsigned char *buf, size_t buf_size);
+#ifdef __cplusplus
+}
+#endif 
+```
+
+#### 2) Call `mutate_helper_buffer_copy` after the `generate_magic_data`:
+```
+mutate_helper_buffer_copy(modified_data, size + 2);
+```
+
+#### 3) Remove the `target_function` call.
+```
+# target_function(modified_data);
+```
+
+#### 4) Recompile this libfuzzer, replacing clang with `afl-clang-fast`, with `AFL_PATH` needed.
+```
+afl-clang-fast -o fuzz -fsanitize=fuzzer fuzzer_harness.cc
+```
+The afl-clang-fast will automatically produce a shared library with `libAFLExtractor.a` linked to it, which will be used by `afl-fuzz` later on.
+