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| <h1 id="fuzzing"><a class="header" href="#fuzzing">Fuzzing</a></h1> |
| <!-- date-check: Mar 2023 --> |
| <p>For the purposes of this guide, <em>fuzzing</em> is any testing methodology that |
| involves compiling a wide variety of programs in an attempt to uncover bugs in |
| rustc. Fuzzing is often used to find internal compiler errors (ICEs). Fuzzing |
| can be beneficial, because it can find bugs before users run into them and |
| provide small, self-contained programs that make the bug easier to track down. |
| However, some common mistakes can reduce the helpfulness of fuzzing and end up |
| making contributors’ lives harder. To maximize your positive impact on the Rust |
| project, please read this guide before reporting fuzzer-generated bugs!</p> |
| <h2 id="guidelines"><a class="header" href="#guidelines">Guidelines</a></h2> |
| <h3 id="in-a-nutshell"><a class="header" href="#in-a-nutshell">In a nutshell</a></h3> |
| <p><em>Please do:</em></p> |
| <ul> |
| <li>Ensure the bug is still present on the latest nightly rustc</li> |
| <li>Include a reasonably minimal, standalone example along with any bug report</li> |
| <li>Include all of the information requested in the bug report template</li> |
| <li>Search for existing reports with the same message and query stack</li> |
| <li>Format the test case with <code>rustfmt</code>, if it maintains the bug</li> |
| <li>Indicate that the bug was found by fuzzing</li> |
| </ul> |
| <p><em>Please don’t:</em></p> |
| <ul> |
| <li>Don’t report lots of bugs that use internal features, including but not |
| limited to <code>custom_mir</code>, <code>lang_items</code>, <code>no_core</code>, and <code>rustc_attrs</code>.</li> |
| <li>Don’t seed your fuzzer with inputs that are known to crash rustc (details |
| below).</li> |
| </ul> |
| <h3 id="discussion"><a class="header" href="#discussion">Discussion</a></h3> |
| <p>If you’re not sure whether or not an ICE is a duplicate of one that’s already |
| been reported, please go ahead and report it and link to issues you think might |
| be related. In general, ICEs on the same line but with different <em>query stacks</em> |
| are usually distinct bugs. For example, <a href="https://github.com/rust-lang/rust/issues/109020">#109020</a> and <a href="https://github.com/rust-lang/rust/issues/109129">#109129</a> |
| had similar error messages:</p> |
| <pre><code>error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <[closure@src/main.rs:36:25: 36:28] as std::ops::FnOnce<(Emplacable<()>,)>>::Output, maybe try to call `try_normalize_erasing_regions` instead |
| </code></pre> |
| <pre><code>error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <() as Project>::Assoc, maybe try to call `try_normalize_erasing_regions` instead |
| </code></pre> |
| <p>but different query stacks:</p> |
| <pre><code>query stack during panic: |
| #0 [fn_abi_of_instance] computing call ABI of `<[closure@src/main.rs:36:25: 36:28] as core::ops::function::FnOnce<(Emplacable<()>,)>>::call_once - shim(vtable)` |
| end of query stack |
| </code></pre> |
| <pre><code>query stack during panic: |
| #0 [check_mod_attrs] checking attributes in top-level module |
| #1 [analysis] running analysis passes on this crate |
| end of query stack |
| </code></pre> |
| <h2 id="building-a-corpus"><a class="header" href="#building-a-corpus">Building a corpus</a></h2> |
| <p>When building a corpus, be sure to avoid collecting tests that are already |
| known to crash rustc. A fuzzer that is seeded with such tests is more likely to |
| generate bugs with the same root cause, wasting everyone’s time. The simplest |
| way to avoid this is to loop over each file in the corpus, see if it causes an |
| ICE, and remove it if so.</p> |
| <p>To build a corpus, you may want to use:</p> |
| <ul> |
| <li>The rustc/rust-analyzer/clippy test suites (or even source code) — though avoid |
| tests that are already known to cause failures, which often begin with comments |
| like <code>//@ failure-status: 101</code> or <code>//@ known-bug: #NNN</code>.</li> |
| <li>The already-fixed ICEs in the archived <a href="https://github.com/rust-lang/glacier">Glacier</a> repository — though |
| avoid the unfixed ones in <code>ices/</code>!</li> |
| </ul> |
| <h2 id="extra-credit"><a class="header" href="#extra-credit">Extra credit</a></h2> |
| <p>Here are a few things you can do to help the Rust project after filing an ICE.</p> |
| <ul> |
| <li><a href="https://rust-lang.github.io/cargo-bisect-rustc/">Bisect</a> the bug to figure out when it was introduced. |
| If you find the regressing PR / commit, you can mark the issue with the label |
| <code>S-has-bisection</code>. If not, consider applying <code>E-needs-bisection</code> instead.</li> |
| <li>Fix “distractions”: problems with the test case that don’t contribute to |
| triggering the ICE, such as syntax errors or borrow-checking errors</li> |
| <li>Minimize the test case (see below). If successful, you can label the |
| issue with <code>S-has-mcve</code>. Otherwise, you can apply <code>E-needs-mcve</code>.</li> |
| <li>Add the minimal test case to the rust-lang/rust repo as a <a href="tests/compiletest.html#crash-tests">crash test</a>. |
| While you’re at it, consider including other “untracked” crashes in your PR. |
| Please don’t forget to mark all relevant issues with <code>S-bug-has-test</code> once |
| your PR is merged.</li> |
| </ul> |
| <p>See also <a href="https://forge.rust-lang.org/release/issue-triaging.html#applying-and-removing-labels">applying and removing labels</a>.</p> |
| <h2 id="minimization"><a class="header" href="#minimization">Minimization</a></h2> |
| <p>It is helpful to carefully <em>minimize</em> the fuzzer-generated input. When |
| minimizing, be careful to preserve the original error, and avoid introducing |
| distracting problems such as syntax, type-checking, or borrow-checking errors.</p> |
| <p>There are some tools that can help with minimization. If you’re not sure how |
| to avoid introducing syntax, type-, and borrow-checking errors while using |
| these tools, post both the complete and minimized test cases. Generally, |
| <em>syntax-aware</em> tools give the best results in the least amount of time. |
| <a href="https://github.com/langston-barrett/treereduce"><code>treereduce-rust</code></a> and <a href="https://github.com/renatahodovan/picireny">picireny</a> are syntax-aware. |
| <a href="https://github.com/googleprojectzero/halfempty"><code>halfempty</code></a> is not, but is generally a high-quality tool.</p> |
| <h2 id="effective-fuzzing"><a class="header" href="#effective-fuzzing">Effective fuzzing</a></h2> |
| <p>When fuzzing rustc, you may want to avoid generating machine code, since this |
| is mostly done by LLVM. Try <code>--emit=mir</code> instead.</p> |
| <p>A variety of compiler flags can uncover different issues. <code>-Zmir-opt-level=4</code> |
| will turn on MIR optimization passes that are not run by default, potentially |
| uncovering interesting bugs. <code>-Zvalidate-mir</code> can help uncover such bugs.</p> |
| <p>If you’re fuzzing a compiler you built, you may want to build it with <code>-C target-cpu=native</code> or even PGO/BOLT to squeeze out a few more executions per |
| second. Of course, it’s best to try multiple build configurations and see |
| what actually results in superior throughput.</p> |
| <p>You may want to build rustc from source with debug assertions to find |
| additional bugs, though this is a trade-off: it can slow down fuzzing by |
| requiring extra work for every execution. To enable debug assertions, add this |
| to <code>bootstrap.toml</code> when compiling rustc:</p> |
| <pre><code class="language-toml">[rust] |
| debug-assertions = true |
| </code></pre> |
| <p>ICEs that require debug assertions to reproduce should be tagged |
| <a href="https://github.com/rust-lang/rust/labels/requires-debug-assertions"><code>requires-debug-assertions</code></a>.</p> |
| <h2 id="existing-projects"><a class="header" href="#existing-projects">Existing projects</a></h2> |
| <ul> |
| <li><a href="https://github.com/dwrensha/fuzz-rustc">fuzz-rustc</a> demonstrates how to fuzz rustc with libfuzzer</li> |
| <li><a href="https://github.com/matthiaskrgr/icemaker/">icemaker</a> runs rustc and other tools on a large number of source |
| files with a variety of flags to catch ICEs</li> |
| <li><a href="https://github.com/langston-barrett/tree-splicer/">tree-splicer</a> generates new source files by combining existing |
| ones while maintaining correct syntax</li> |
| </ul> |
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