Beyond Edge Coverage: Per-Task Data-Flow Extraction at Kernel Function Boundaries via LLVM

Abstract: Coverage-guided kernel fuzzers such as syzkaller rely on edge coverage (trace-pc) as their sole feedback signal. This context-blind approach cannot distinguish execution paths that differ only in argument values. for example, two invocations of copy_from_user() with different size parameters hit identical basic blocks yet have vastly different security implications. We present BOUNDARY FLOW, an LLVM-based instrumentation framework that extends Linux KCOV with data-flow extraction of function arguments and return values. A compiler pass (-fsanitize-coverage=dataflow-args, dataflow-ret) emits lightweight callbacks capturing a structured tuple <PC, arg_idx, arg_size, ptr, offsets[]> at function entry and <PC, ret_size, ptr, offsets[]> at return. Composite types are automatically decomposed via DWARF DICompositeType metadata with zero source annotation. A separate kernel device(/sys/kernel/debug/kcov_dataflow) provides lock-free per-task ring buffers with no inter ference to existing KCOV or syzkaller infrastructure. We demonstrate dual utility: fuzzers gain state-aware feedback for mutation guidance into value-dependent state transitions, and security analysts obtain deterministic argument records for root-cause analysis without printk or kprobe overhead. A post-compilation pipeline (rustc, opt, llc) enables Rust kernel module instrumentation without modifying rustc, the only runtime method for capturing Rust function arguments given that drgn/vmcore fails under-O2 DWARF elision. Evaluated on five vulnerability classes (OOB, UAF, double-free, 10 deep chain propagation, Rust FFI, Rust for Linux Modules) with <3% overhead on instrumented paths.