Auto merge of #1061 - fitzgen:add-csmith-instructions-to-contributing, r=pepyakin

Point to `csmith` instructions from CONTRIBUTING.md

r? @pepyakin

Author: bors-servo

CONTRIBUTING.md

@@ -19,6 +19,7 @@ out to us in a GitHub issue, or stop by
   - [Testing a Single Header's Bindings Generation and Compiling its Bindings](#testing-a-single-headers-bindings-generation-and-compiling-its-bindings)
   - [Authoring New Tests](#authoring-new-tests)
   - [Test Expectations and `libclang` Versions](#test-expectations-and-libclang-versions)
+  - [Fuzzing `bindgen` with `csmith`](#fuzzing-bindgen-with-csmith)
 - [Code Overview](#code-overview)
 - [Pull Requests and Code Reviews](#pull-requests-and-code-reviews)
 - [Generating Graphviz Dot Files](#generating-graphviz-dot-files)
@@ -193,6 +194,14 @@ Where `$VERSION` is one of:
 
 depending on which version of `libclang` you have installed.
 
+### Fuzzing `bindgen` with `csmith`
+
+We <3 finding hidden bugs and the people who help us find them! One way to help
+uncover hidden bugs is by running `csmith` to generate random headers to test
+`bindgen` against.
+
+See [./csmith-fuzzing/README.md](./csmith-fuzzing/README.md) for details.
+
 ## Code Overview
 
 `bindgen` takes C and C++ header files as input and generates corresponding Rust

csmith-fuzzing/README

@@ -0,0 +1,65 @@
+# Fuzzing `bindgen` with `csmith`
+
+[`csmith`][csmith] generates random C and C++ programs that can be used as test
+cases for compilers. When testing `bindgen` with `csmith`, we interpret the
+generated programs as header files, and emit Rust bindings to them. If `bindgen`
+panics, the emitted bindings won't compile with `rustc`, or the generated layout
+tests in the bindings fail, then we report an issue containing the test case!
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+
+- [Prerequisites](#prerequisites)
+- [Running the Fuzzer](#running-the-fuzzer)
+- [Reporting Issues](#reporting-issues)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+## Prerequisites
+
+Requires `python3`, `csmith` and `bindgen` to be in `$PATH`.
+
+Many OS package managers have a `csmith` package:
+
+```
+$ sudo apt install csmith
+$ brew install csmith
+$ # Etc...
+```
+
+## Running the Fuzzer
+
+Run `csmith` and test `bindgen` on the generated test cases with this command:
+
+```
+$ ./driver.py
+```
+
+The driver will keep running until it encounters an error in `bindgen`.
+
+Each invocation of `./driver.py` will use its own temporary directories, so
+running it in multiple terminals in parallel is supported.
+
+`csmith` is run with `--no-checksum --nomain --max-block-size 1
+--max-block-depth 1` which disables the `main` function, and makes function
+bodies as simple as possible as `bindgen` does not care about them, but they
+cannot be completely disabled in `csmith`. Run `csmith --help` to see what
+exactly those options do.
+
+## Reporting Issues
+
+Once the fuzz driver finds a test case that causes some kind of error in
+`bindgen` or its emitted bindings, it is helpful to
+[run C-Reduce on the test case][creducing] to remove the parts that are
+irrelevant to reproducing the error. This is ***very*** helpful for the folks
+who further investigate the issue and come up with a fix!
+
+Additionally, mention that you discovered the issue via `csmith` and we will add
+the `A-csmith` label. You can find all the issues discovered with `csmith`, and
+related to fuzzing with `csmith`, by looking up
+[all issues tagged with the `A-csmith` label][csmith-issues].
+
+[csmith]: https://github.com/csmith-project/csmith
+[creducing]: ../CONTRIBUTING.md#using-creduce-to-minimize-test-cases
+[csmith-issues]: https://github.com/rust-lang-nursery/rust-bindgen/issues?q=label%3AA-csmith

csmith-fuzzing/README.md

@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 import os, sys
 from subprocess import run, SubprocessError, DEVNULL, PIPE
 from tempfile import NamedTemporaryFile