Software systems powering OS kernels, basebands, bootloaders, firmware, IoT or automotive build the foundation of infrastructure that billions of people rely on every day. Testing these systems is crucial, especially as their complexity grows and they are often written in unsafe languages such as C/C++.

However, testing such complex systems poses significant challenges, e.g., custom hardware for which there is no emulator, or a non-trivial setup of testing and debugging on the target device. As a result, the commonly used testing techniques and tools are not always easily applicable.

An off-target (OT) testing is a promising technique which addresses these challenges: part of the code is extracted and adapted to run on a different hardware platform with better tool support, easier debugging and higher test throughput. Unfortunately, since the process of creating an OT program has been manual, the technique did not scale well and was mostly used in an ad hoc manner.

In this paper we present a novel complex systems testing approach called Auto Off-target (AoT). Based on the information extracted from the source code and from the build process, AoT can automatically generate OT programs in C. AoT goes beyond the code generation and provides mechanisms that help to recreate and discover the program state in the OT code. The generated OTs are self-contained and independent of the original build environment. As a result, pieces of complex or embedded software can be easily run, analyzed, debugged and tested on a standard x86_64 machine.

We evaluate AoT on tens of thousands of functions selected from OS kernels, a bootloader and a network stack. We demonstrate that majority of the generated OTs can be automatically tested with fuzzing and symbolic execution. We further used AoT in a bug finding campaign and discovered seven bugs in the Android redfin and oriole kernels powering Google Pixel 5 and 6 phones.