Cross-checking Semantic Correctness: The Case of Finding File System Bugs
Taesoo Kim
Abstract:
Today, systems software is too complex to be bug-free. To find bugs in systems software, developers often rely on code checkers, like Linux's Sparse. However, the capability of existing tools used in commodity, large-scale systems is limited to finding only shallow bugs that tend to be introduced by simple programmer mistakes, and so do not require a deep understanding of code to find them. Unfortunately, the majority of bugs as well as those that are difficult to find are semantic ones, which violate high-level rules or invariants (e.g., missing a permission check). Thus, it is difficult for code checkers lacking the understanding of a programmer's true intention to reason about semantic correctness.
To solve this problem, we present Juxta, a tool that automatically infers high-level semantics directly from source code. The key idea in Juxta is to compare and contrast multiple existing implementations that obey latent yet implicit high-level semantics. For example, the implementation of open() at the file system layer expects to handle an out-of-space error from the disk in all file systems. We applied Juxta to 54 file systems in the stock Linux kernel (680K LoC), found 118 previously unknown semantic bugs (one bug per 5.8K LoC), and provided corresponding patches to 39 different file systems, including mature, popular ones like ext4, btrfs, XFS, and NFS. These semantic bugs are not easy to locate, as all the ones found by Juxta have existed for over 6.2 years on average. Not only do our empirical results look promising, but the design of Juxta is generic enough to be extended easily beyond file systems to any software that has multiple implementations, like Web browsers or protocols at the same layer of a network stack.
Bio:
Taesoo Kim is an Assistant Professor in the School Computer Science at Georgia Tech. He is interested in building a system that has underline principles for why it should be secure. Those principles include the design of a system, analysis of its implementation, and clear separation of trusted components. He holds a B.S. from KAIST (2009), a S.M. (2011) and a Ph.D. (2014) from MIT in CS.