BUSTA: Teaching Small Language Models to Reason About Binary Execution

Large Language Models (LLM) are increasingly applied to Reverse Engineering (RE) workflows. RE, however, differs from common LLM coding tasks as it requires models to reason about low-level program behavior. Furthermore, the deployment of tooling in resource-constrained sandboxed RE environments limits the ability to use frontier-scale solutions. We examine whether a targeted LLM training approach can transfer distinct RE skills to open models via inverse basic-block execution: given an x86-64 basic block and a desired output register state, the model predicts the input register values that produce it. Fine-tuning Qwen3-1.7B and Nemotron-3-Nano with Group Relative Policy Optimization (GRPO) raises knowable-register prediction accuracy from 73.2% to 85.3% and from 63.3% to 95.2%, respectively. Frontier models still score highest (up to 99.2%), but our fine- tuned Nemotron-3-Nano comes within four points at a fraction of the parameter count. We release both fine-tuned models and the training dataset.