# How syzkaller works ## Overview The process structure for the syzkaller system is shown in the following diagram; red labels indicate corresponding configuration options. ![Process structure for syzkaller](process_structure.png?raw=true) The `syz-manager` process starts, monitors and restarts several VM instances, and starts a `syz-fuzzer` process inside of the VMs. It is responsible for persistent corpus and crash storage. As opposed to `syz-fuzzer` processes, it runs on a host with stable kernel which does not experience white-noise fuzzer load. The `syz-fuzzer` process runs inside of presumably unstable VMs. The `syz-fuzzer` guides fuzzing process itself (input generation, mutation, minimization, etc) and sends inputs that trigger new coverage back to the `syz-manager` process via RPC. It also starts transient `syz-executor` processes. Each `syz-executor` process executes a single input (a sequence of syscalls). It accepts the program to execute from the `syz-fuzzer` process and sends results back. It is designed to be as simple as possible (to not interfere with fuzzing process), written in C++, compiled as static binary and uses shared memory for communication. ## Syscall descriptions The `syz-fuzzer` process generates programs to be executed by `syz-executor` based on syscall descriptions described [here](syscall_descriptions.md). ## Crash reports When `syzkaller` finds a crasher, it saves information about it into `workdir/crashes` directory. The directory contains one subdirectory per unique crash type. Each subdirectory contains a `description` file with a unique string identifying the crash (intended for bug identification and deduplication); and up to 100 `logN` and `reportN` files, one pair per test machine crash: ``` - crashes/ - 6e512290efa36515a7a27e53623304d20d1c3e - description - log0 - report0 - log1 - report1 ... - 77c578906abe311d06227b9dc3bffa4c52676f - description - log0 - report0 ... ``` Descriptions are extracted using a set of [regular expressions](/report/report.go#L33). This set may need to be extended if you are using a different kernel architecture, or are just seeing a previously unseen kernel error messages. `logN` files contain raw `syzkaller` logs and include kernel console output as well as programs executed before the crash. These logs can be fed to `syz-repro` tool for [crash location and minimization](reproducing_crashes.md), or to `syz-execprog` tool for [manual localization](executing_syzkaller_programs.md). `reportN` files contain post-processed and symbolized kernel crash reports (e.g. a KASAN report). Normally you need just 1 pair of these files (i.e. `log0` and `report0`), because they all presumably describe the same kernel bug. However, `syzkaller` saves up to 100 of them for the case when the crash is poorly reproducible, or if you just want to look at a set of crash reports to infer some similarities or differences. There are 3 special types of crashes: - `no output from test machine`: the test machine produces no output whatsoever - `lost connection to test machine`: the ssh connection to the machine was unexpectedly closed - `test machine is not executing programs`: the machine looks alive, but no test programs were executed for long period of time Most likely you won't see `reportN` files for these crashes (e.g. if there is no output from the test machine, there is nothing to put into report). Sometimes these crashes indicate a bug in `syzkaller` itself (especially if you see a Go panic message in the logs). However, frequently they mean a kernel lockup or something similarly bad (here are just a few examples of bugs found this way: [1](https://groups.google.com/d/msg/syzkaller/zfuHHRXL7Zg/Tc5rK8bdCAAJ), [2](https://groups.google.com/d/msg/syzkaller/kY_ml6TCm9A/wDd5fYFXBQAJ), [3](https://groups.google.com/d/msg/syzkaller/OM7CXieBCoY/etzvFPX3AQAJ)).