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GCC-based instrumentation for afl-fuzz

(See ../README.md for the general instruction manual.) (See ../llvm_mode/README.md for the LLVM-based instrumentation.)

!!! TODO items are: !!! => inline instrumentation has to work! !!!

1) Introduction

The code in this directory allows you to instrument programs for AFL using true compiler-level instrumentation, instead of the more crude assembly-level rewriting approach taken by afl-gcc and afl-clang. This has several interesting properties:

  • The compiler can make many optimizations that are hard to pull off when manually inserting assembly. As a result, some slow, CPU-bound programs will run up to around faster.

    The gains are less pronounced for fast binaries, where the speed is limited chiefly by the cost of creating new processes. In such cases, the gain will probably stay within 10%.

  • The instrumentation is CPU-independent. At least in principle, you should be able to rely on it to fuzz programs on non-x86 architectures (after building afl-fuzz with AFL_NOX86=1).

  • Because the feature relies on the internals of GCC, it is gcc-specific and will not work with LLVM (see ../llvm_mode for an alternative).

Once this implementation is shown to be sufficiently robust and portable, it will probably replace afl-gcc. For now, it can be built separately and co-exists with the original code.

The idea and much of the implementation comes from Laszlo Szekeres.

2) How to use

In order to leverage this mechanism, you need to have modern enough GCC (>= version 4.5.0) and the plugin headers installed on your system. That should be all you need. On Debian machines, these headers can be acquired by installing the gcc-<VERSION>-plugin-dev packages.

To build the instrumentation itself, type 'make'. This will generate binaries called afl-gcc-fast and afl-g++-fast in the parent directory. If the CC/CXX have been overridden, those compilers will be used from those wrappers without using AFL_CXX/AFL_CC settings. Once this is done, you can instrument third-party code in a way similar to the standard operating mode of AFL, e.g.:

CC=/path/to/afl/afl-gcc-fast ./configure [...options...] make

Be sure to also include CXX set to afl-g++-fast for C++ code.

The tool honors roughly the same environmental variables as afl-gcc (see env_variables.md. This includes AFL_INST_RATIO, AFL_USE_ASAN, AFL_HARDEN, and AFL_DONT_OPTIMIZE.

Note: if you want the GCC plugin to be installed on your system for all users, you need to build it before issuing 'make install' in the parent directory.

3) Gotchas, feedback, bugs

This is an early-stage mechanism, so field reports are welcome. You can send bug reports to [email protected].

4) Bonus feature #1: deferred initialization

AFL tries to optimize performance by executing the targeted binary just once, stopping it just before main(), and then cloning this "master" process to get a steady supply of targets to fuzz.

Although this approach eliminates much of the OS-, linker- and libc-level costs of executing the program, it does not always help with binaries that perform other time-consuming initialization steps - say, parsing a large config file before getting to the fuzzed data.

In such cases, it's beneficial to initialize the forkserver a bit later, once most of the initialization work is already done, but before the binary attempts to read the fuzzed input and parse it; in some cases, this can offer a 10x+ performance gain. You can implement delayed initialization in LLVM mode in a fairly simple way.

First, locate a suitable location in the code where the delayed cloning can take place. This needs to be done with extreme care to avoid breaking the binary. In particular, the program will probably malfunction if you select a location after:

  • The creation of any vital threads or child processes - since the forkserver can't clone them easily.

  • The initialization of timers via setitimer() or equivalent calls.

  • The creation of temporary files, network sockets, offset-sensitive file descriptors, and similar shared-state resources - but only provided that their state meaningfully influences the behavior of the program later on.

  • Any access to the fuzzed input, including reading the metadata about its size.

With the location selected, add this code in the appropriate spot:

#ifdef __AFL_HAVE_MANUAL_CONTROL
  __AFL_INIT();
#endif

You don't need the #ifdef guards, but they will make the program still work as usual when compiled with a tool other than afl-gcc-fast/afl-clang-fast.

Finally, recompile the program with afl-gcc-fast (afl-gcc or afl-clang will not generate a deferred-initialization binary) - and you should be all set!

5) Bonus feature #2: persistent mode

Some libraries provide APIs that are stateless, or whose state can be reset in between processing different input files. When such a reset is performed, a single long-lived process can be reused to try out multiple test cases, eliminating the need for repeated fork() calls and the associated OS overhead.

The basic structure of the program that does this would be:

  while (__AFL_LOOP(1000)) {

    /* Read input data. */
    /* Call library code to be fuzzed. */
    /* Reset state. */

  }

  /* Exit normally */

The numerical value specified within the loop controls the maximum number of iterations before AFL will restart the process from scratch. This minimizes the impact of memory leaks and similar glitches; 1000 is a good starting point.

A more detailed template is shown in ../examples/persistent_demo/. Similarly to the previous mode, the feature works only with afl-gcc-fast or afl-clang-fast; #ifdef guards can be used to suppress it when using other compilers.

Note that as with the previous mode, the feature is easy to misuse; if you do not reset the critical state fully, you may end up with false positives or waste a whole lot of CPU power doing nothing useful at all. Be particularly wary of memory leaks and the state of file descriptors.

When running in this mode, the execution paths will inherently vary a bit depending on whether the input loop is being entered for the first time or executed again. To avoid spurious warnings, the feature implies AFL_NO_VAR_CHECK and hides the "variable path" warnings in the UI.