Interior unsafe is an essential design paradigm advocated by the Rust community in system software development. However, there is little official guidance or few best practices regarding how to encapsulate unsafe code and achieve interior unsafe. To address this issue, this paper studies how interior unsafe is achieved in practice, aiming to identify best practices to guide Rust code design concerning unsafe code encapsulation. Specifically, we propose a novel unsafety isolation graph to model the essential usage and encapsulation of unsafe code. Based on the graph, we further propose four major isolation types and nine structural patterns to split a graph into several small self-contained subgraphs. These subgraphs can serve as useful audit units for examining the soundness of unsafe code encapsulation. We applied our approach to four real-world Rust projects. The experimental results demonstrate that our method is effective in characterizing their encapsulation code. Additionally, we identified three encapsulation patterns and two common issues in these projects that could complicate soundness verification or incur unsoundness issues.

WebAssembly (Wasm) is a bytecode format originally serving as a compilation target for Web applications. It has recently been used increasingly on the server side, e.g., providing a safer, faster, and more portable alternative to Linux containers. With the popularity of server-side Wasm applications, it is essential to study performance issues (i.e., abnormal latency) in Wasm runtimes, as they may cause a significant impact on server-side applications. However, there is still a lack of attention to performance issues in server-side Wasm runtimes. In this paper, we design a novel differential testing approach WarpDiff to identify performance issues in server-side Wasm runtimes. The key insight is that in normal cases, the execution time of the same test case on different Wasm runtimes should follow an oracle ratio. We identify abnormal cases where the execution time ratio significantly deviates from the oracle ratio and subsequently locate the Wasm runtimes that cause the performance issues. We apply WarpDiff to test five popular server-side Wasm runtimes using 123 test cases from the LLVM test suite and demonstrate the top 10 abnormal cases we identified. We further conduct an in-depth analysis of these abnormal cases and summarize seven performance issues, all of which have been confirmed by the developers. We hope our work can inspire future investigation on improving Wasm runtime implementation and thus promoting the development of server-side Wasm applications.