This originally is the work of an individual nicknamed laf-intel. His blog Circumventing Fuzzing Roadblocks with Compiler Transformations and GitLab repo laf-llvm-pass describe some code transformations that help AFL++ to enter conditional blocks, where conditions consist of comparisons of large values.
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
- simplify operators >= (and <=) into chains of > (<) and == comparisons
- change signed integer comparisons to a chain of sign-only comparison and unsigned integer comparisons
- split all unsigned integer comparisons with bit widths of 64, 32, or 16 bits to chains of 8 bits comparisons.
You can change the behavior 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 unique 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.
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. :-)