Rump File Systems: Kernel Code Reborn

When kernel functionality is desired in userspace, the common approach is to reimplement it for userspace interfaces. We show that use of existing kernel file systems in userspace programs is possible without modifying the kernel file system code base. Two different operating modes are explored: 1) a transparent mode, in which the file system is mounted in the typical fashion by using the kernel code as a userspace server, and 2) a standalone mode, in which applications can use a kernel file system as a library. The first mode provides isolation from the trusted computing base and a secure way for mounting untrusted file systems on a monolithic kernel. The second mode is useful for file system utilities and applications, such as populating an image or viewing the contents without requiring host operating system kernel support. Additional uses for both modes include debugging, development and testing. The design and implementation of the Runnable Userspace Meta Program file system (rump fs) framework for NetBSD is presented. Using rump, ten diskbased file systems, a memory file system, a network file system and a userspace framework file system have been tested to be functional. File system performance for an estimated typical workload is found to be ±5% of kernel performance. The prototype of a similar framework for Linux was also implemented and portability was verified: Linux file systems work on NetBSD and NetBSD file systems work on Linux. Finally, the implementation is shown to be maintainable by examining the 1.5 year period it has been a part of NetBSD.