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