laf-intel instrumentation

Introduction

This originally is the work of an individual nicknamed laf-intel. His blog [Circumventing Fuzzing Roadblocks with Compiler Transformations] (https://lafintel.wordpress.com/) and gitlab repo [laf-llvm-pass] (https://gitlab.com/laf-intel/laf-llvm-pass/) describe some code transformations that help AFL++ to enter conditional blocks, where conditions consist of comparisons of large values.

Usage

By default these passes will not run when you compile programs using afl-clang-fast. Hence, you can use AFL as usual. To enable the passes you must set environment variables before you compile the target project.

The following options exist:

export AFL_LLVM_LAF_SPLIT_SWITCHES=1

Enables the split-switches pass.

export AFL_LLVM_LAF_TRANSFORM_COMPARES=1

Enables the transform-compares pass (strcmp, memcmp, strncmp, strcasecmp, strncasecmp).

export AFL_LLVM_LAF_SPLIT_COMPARES=1

Enables the split-compares pass. By default it will

1. simplify operators >= (and <=) into chains of > (<) and == comparisons
2. change signed integer comparisons to a chain of sign-only comparison and unsigned integer comparisons
3. split all unsigned integer comparisons with bit widths of 64, 32 or 16 bits to chains of 8 bits comparisons.

You can change the behaviour of the last step by setting export AFL_LLVM_LAF_SPLIT_COMPARES_BITW=<bit_width>, where bit_width may be 64, 32 or 16. For example, a bit_width of 16 would split larger comparisons down to 16 bit comparisons.

A new experimental feature is splitting floating point comparisons into a series of sign, exponent and mantissa comparisons followed by splitting each of them into 8 bit comparisons when necessary. It is activated with the AFL_LLVM_LAF_SPLIT_FLOATS setting. Please note that full IEEE 754 functionality is not preserved, that is values of nan and infinity will probably behave differently.

Note that setting this automatically activates AFL_LLVM_LAF_SPLIT_COMPARES

You can also set AFL_LLVM_LAF_ALL and have all of the above enabled :-)