diff options
Diffstat (limited to 'llvm_mode/README.neverzero.md')
| -rw-r--r-- | llvm_mode/README.neverzero.md | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/llvm_mode/README.neverzero.md b/llvm_mode/README.neverzero.md new file mode 100644 index 00000000..5fcf7b47 --- /dev/null +++ b/llvm_mode/README.neverzero.md @@ -0,0 +1,24 @@ +# NeverZero counters for LLVM instrumentation + +## Usage + +In larger, complex or reiterative programs the map that collects the edge pairs +can easily fill up and wrap. +This is not that much of an issue - unless by chance it wraps just to a 0 +when the program execution ends. +In this case afl-fuzz is not able to see that the pair has been accessed and +will ignore it. + +NeverZero prevents this behaviour. If a counter wraps, it jumps over the 0 +directly to a 1. This improves path discovery (by a very little amount) +at a very little cost (one instruction per edge). + +This is implemented in afl-gcc, however for llvm_mode this is optional if +the llvm version is below 9 - as there is a perfomance bug that is only fixed +in version 9 and onwards. + +If you want to enable this for llvm < 9 then set + +``` +export AFL_LLVM_NOT_ZERO=1 +``` |