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Fast LLVM-based instrumentation for afl-fuzz

For the general instruction manual, see docs/README.md.

For the GCC-based instrumentation, see README.gcc_plugin.md.

1) Introduction

! llvm_mode works with llvm versions 3.8 up to 17 - but 13+ is recommended !

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 2x 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_NO_X86=1).

  • The instrumentation can cope a bit better with multi-threaded targets.

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

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

The idea and much of the initial implementation came from Laszlo Szekeres.

2a) How to use this - short

Set the LLVM_CONFIG variable to the clang version you want to use, e.g.:

LLVM_CONFIG=llvm-config-9 make

In case you have your own compiled llvm version specify the full path:

LLVM_CONFIG=~/llvm-project/build/bin/llvm-config make

If you try to use a new llvm version on an old Linux this can fail because of old c++ libraries. In this case usually switching to gcc/g++ to compile llvm_mode will work:

LLVM_CONFIG=llvm-config-7 REAL_CC=gcc REAL_CXX=g++ make

It is highly recommended to use the newest clang version you can put your hands on :)

Then look at README.persistent_mode.md.

2b) How to use this - long

In order to leverage this mechanism, you need to have clang installed on your system. You should also make sure that the llvm-config tool is in your path (or pointed to via LLVM_CONFIG in the environment).

Note that if you have several LLVM versions installed, pointing LLVM_CONFIG to the version you want to use will switch compiling to this specific version - if you installation is set up correctly :-)

Unfortunately, some systems that do have clang come without llvm-config or the LLVM development headers; one example of this is FreeBSD. FreeBSD users will also run into problems with clang being built statically and not being able to load modules (you'll see "Service unavailable" when loading afl-llvm-pass.so).

To solve all your problems, you can grab pre-built binaries for your OS from:

https://llvm.org/releases/download.html

...and then put the bin/ directory from the tarball at the beginning of your $PATH when compiling the feature and building packages later on. You don't need to be root for that.

To build the instrumentation itself, type make. This will generate binaries called afl-clang-fast and afl-clang-fast++ in the parent directory. 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-clang-fast ./configure [...options...]
  make

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

Note that afl-clang-fast/afl-clang-fast++ are just pointers to afl-cc. You can also use afl-cc/afl-c++ and instead direct it to use LLVM instrumentation by either setting AFL_CC_COMPILER=LLVM or pass the parameter --afl-llvm via CFLAGS/CXXFLAGS/CPPFLAGS.

The tool honors roughly the same environmental variables as afl-gcc (see docs/env_variables.md). This includes AFL_USE_ASAN, AFL_HARDEN, and AFL_DONT_OPTIMIZE. However, AFL_INST_RATIO is not honored as it does not serve a good purpose with the more effective PCGUARD analysis.

3) Options

Several options are present to make llvm_mode faster or help it rearrange the code to make afl-fuzz path discovery easier.

If you need just to instrument specific parts of the code, you can create the instrument file list which C/C++ files to actually instrument. See README.instrument_list.md

For splitting memcmp, strncmp, etc., see README.laf-intel.md.

Then there are different ways of instrumenting the target:

  1. A better instrumentation strategy uses LTO and link time instrumentation. Note that not all targets can compile in this mode, however, if it works it is the best option you can use. To go with this option, use afl-clang-lto/afl-clang-lto++. See README.lto.md.

  2. Alternatively you can choose a completely different coverage method:

2a. N-GRAM coverage - which combines the previous visited edges with the current one. This explodes the map but on the other hand has proven to be effective for fuzzing. See 7) AFL++ N-Gram Branch Coverage.

2b. Context sensitive coverage - which combines the visited edges with an individual caller ID (the function that called the current one). See 6) AFL++ Context Sensitive Branch Coverage.

Then - additionally to one of the instrumentation options above - there is a very effective new instrumentation option called CmpLog as an alternative to laf-intel that allow AFL++ to apply mutations similar to Redqueen. See README.cmplog.md.

Finally, if your llvm version is 8 or lower, you can activate a mode that prevents that a counter overflow result in a 0 value. This is good for path discovery, but the llvm implementation for x86 for this functionality is not optimal and was only fixed in llvm 9. You can set this with AFL_LLVM_NOT_ZERO=1.

Support for thread safe counters has been added for all modes. Activate it with AFL_LLVM_THREADSAFE_INST=1. The tradeoff is better precision in multi threaded apps for a slightly higher instrumentation overhead. This also disables the nozero counter default for performance reasons.

4) deferred initialization, persistent mode, shared memory fuzzing

This is the most powerful and effective fuzzing you can do. For a full explanation, see README.persistent_mode.md.

5) Bonus feature: 'dict2file' pass

Just specify AFL_LLVM_DICT2FILE=/absolute/path/file.txt and during compilation all constant string compare parameters will be written to this file to be used with afl-fuzz' -x option.

Adding AFL_LLVM_DICT2FILE_NO_MAIN=1 will skip parsing main() which often does command line parsing which has string comparisons that are not helpful for fuzzing.

6) AFL++ Context Sensitive Branch Coverage

What is this?

This is an LLVM-based implementation of the context sensitive branch coverage.

Basically every function gets its own ID and, every time when an edge is logged, all the IDs in the callstack are hashed and combined with the edge transition hash to augment the classic edge coverage with the information about the calling context.

So if both function A and function B call a function C, the coverage collected in C will be different.

In math the coverage is collected as follows: map[current_location_ID ^ previous_location_ID >> 1 ^ hash_callstack_IDs] += 1

The callstack hash is produced XOR-ing the function IDs to avoid explosion with recursive functions.

Usage

Set the AFL_LLVM_INSTRUMENT=CTX or AFL_LLVM_CTX=1 environment variable.

It is highly recommended to increase the MAP_SIZE_POW2 definition in config.h to at least 18 and maybe up to 20 for this as otherwise too many map collisions occur.

Caller Branch Coverage

If the context sensitive coverage introduces too may collisions and becoming detrimental, the user can choose to augment edge coverage with just the called function ID, instead of the entire callstack hash.

In math the coverage is collected as follows: map[current_location_ID ^ previous_location_ID >> 1 ^ previous_callee_ID] += 1

Set the AFL_LLVM_INSTRUMENT=CALLER or AFL_LLVM_CALLER=1 environment variable.

7) AFL++ N-Gram Branch Coverage

Source

This is an LLVM-based implementation of the n-gram branch coverage proposed in the paper "Be Sensitive and Collaborative: Analyzing Impact of Coverage Metrics in Greybox Fuzzing" by Jinghan Wang, et. al.

Note that the original implementation (available here) is built on top of AFL's QEMU mode. This is essentially a port that uses LLVM vectorized instructions (available from llvm versions 4.0.1 and higher) to achieve the same results when compiling source code.

In math the branch coverage is performed as follows: map[current_location ^ prev_location[0] >> 1 ^ prev_location[1] >> 1 ^ ... up to n-1] += 1`

Usage

The size of n (i.e., the number of branches to remember) is an option that is specified either in the AFL_LLVM_INSTRUMENT=NGRAM-{value} or the AFL_LLVM_NGRAM_SIZE environment variable. Good values are 2, 4, or 8, valid are 2-16.

It is highly recommended to increase the MAP_SIZE_POW2 definition in config.h to at least 18 and maybe up to 20 for this as otherwise too many map collisions occur.

8) NeverZero counters

In larger, complex, or reiterative programs, the byte sized counters that collect the edge coverage can easily fill up and wrap around. This is not that much of an issue - unless, by chance, it wraps just to a value of zero when the program execution ends. In this case, afl-fuzz is not able to see that the edge has been accessed and will ignore it.

NeverZero prevents this behavior. If a counter wraps, it jumps over the value 0 directly to a 1. This improves path discovery (by a very small amount) at a very low cost (one instruction per edge).

(The alternative of saturated counters has been tested also and proved to be inferior in terms of path discovery.)

This is implemented in afl-gcc and afl-gcc-fast, however, for llvm_mode this is optional if multithread safe counters are selected or the llvm version is below 9 - as there are severe performance costs in these cases.

If you want to enable this for llvm versions below 9 or thread safe counters, then set

export AFL_LLVM_NOT_ZERO=1

In case you are on llvm 9 or greater and you do not want this behavior, then you can set:

AFL_LLVM_SKIP_NEVERZERO=1

If the target does not have extensive loops or functions that are called a lot, then this can give a small performance boost.

Please note that the default counter implementations are not thread safe!

Support for thread safe counters in mode LLVM CLASSIC can be activated with setting AFL_LLVM_THREADSAFE_INST=1.

8) Source code coverage through instrumentation

Measuring source code coverage is a common task in fuzzing, but it is very difficut to do in some situations (e.g. when using snapshot fuzzing).

When using the AFL_LLVM_INSTRUMENT=llvm-codecov option, afl-cc will use native trace-pc-guard instrumentation but additionally select options that are required to utilize the instrumentation for source code coverage.

In particular, it will switch the instrumentation to be per basic block instead of instrumenting edges, disable all guard pruning and enable the experimental pc-table support that allows the runtime to gather 100% of instrumented basic blocks at start, including their locations.

Note: You must compile AFL with the CODE_COVERAGE=1 option to enable the respective parts in the AFL compiler runtime. Support is currently only implemented for Nyx, but can in theory also work without Nyx.

Note: You might have to adjust MAP_SIZE_POW2 in include/config.h to ensure that your coverage map is large enough to hold all basic blocks of your target program without any collisions.

More documentation on how to utilize this with Nyx will follow.