Rumipsem - toy MIPS emulator written in Rust

Rumipsem aims to execute statically compiled ELF binaries for systems with MIPS32 CPUs. Architecture version is not exactly specified. However, majority of instructions are implemented for release 2, because the emulator is tested against processor MIPS 74Kc - CPU inside my OpenWrt router. Target platform is locked to Linux, because the emulator attempts to translate syscalls.

Build

The emulator is developed in stable release channel of Rust.

To build it, run in the project root:

cargo build --release

Result executable will be stored in target/release/rumipsem.

Usage

rumipsem [OPTIONS] -- ELF_BINARY [ARGS...]

The doubledash -- is not necessary, it just prevents the emulator from consuming arguments for the actual emulated program. Most notable option for the emulator is probably -v for verbosity. You can stack them as much as you like - -vvvv logs every single instruction emulated. For more options, run with --help.

Example

You can give it a try with BusyBox MIPS binary. During development, I was testing on version 1.28.1. So that should be able to run some basic things. Probably the most complex of functional tools is sh.

rumipsem [-vvvv] -- busybox-mips sh

Just be aware, that ioctl syscall does not work properly, so it thinks, its running on some ancient terminal. No prompt is displayed and tab completion does not work either. But it's able to execute normal programs from your system!

Current emulator status

Majority of instructions are implemented, but there are still some, that are not. Probably the most problematic missing feature is floats. Only load/stores are supported. Calculations with them are not.

Bigger problem for usability are missing syscall implementations. Some syscalls are even pretty much imposible to implement properly - ioctl takes an arbitrary data structure (or some scalar). But the data structures must be translated to match the native system in endiannity and field sizes. That means, every device driver would have to have a special translation code just for it. Other problem is mmap. It is possible to implement it, but it requires more advanced memory management than currently implemented.

Testing and coredumps

The emulator can execute coredumps. To work properly, the coredump must have been created just after loading the program into memory. Before the first instruction is executed. A small problem is, that entry point and stack pointer are not stored in the coredump. It is necessary to supply them manually to the emulator through command line options.

Coredumps are valuable, because the provide a way to test, if the emulator behaves the same way real hardware does. Using GDB on any MIPS device, you can run any executable, create coredump and then store every single instruction address and register value in a custom trace file. The emulator can than check itself for correctness. After every register read, the value can be compared to the trace file. And addresses of executed instructions are checked, so that the execution path does not diverge.