# Sanitizers support

The rustc compiler contains support for following sanitizers:

* [AddressSanitizer][clang-asan] a faster memory error detector.
  Can detect out-of-bounds access to heap, stack, and globals, use after free, use
  after return, double free, invalid free, memory leaks.
* [ControlFlowIntegrity][clang-cfi] LLVM Control Flow Integrity (CFI) provides
  forward-edge control flow protection.
* [Hardware-assisted AddressSanitizer][clang-hwasan]  a tool similar to
  AddressSanitizer but based on partial hardware assistance.
* [KernelControlFlowIntegrity][clang-kcfi] LLVM Kernel Control Flow Integrity
  (KCFI) provides forward-edge control flow protection for operating systems kernels.
* [LeakSanitizer][clang-lsan] a run-time memory leak detector.
* [MemorySanitizer][clang-msan] a detector of uninitialized reads.
* [ThreadSanitizer][clang-tsan] a fast data race detector.

## How to use the sanitizers?

To enable a sanitizer compile with `-Z sanitizer=...` option, where value is one
of `address`, `cfi`, `hwaddress`, `kcfi`, `leak`, `memory` or `thread`.
For more details on how to use sanitizers,
please refer to the sanitizer flag in [The Unstable Book].

[The Unstable Book]: https://doc.rust-lang.org/unstable-book

## How are sanitizers implemented in rustc?

The implementation of sanitizers (except CFI) relies almost entirely on LLVM.
The rustc is an integration point for LLVM compile time instrumentation passes
and runtime libraries.
Highlight of the most important aspects of the implementation:

*  The sanitizer runtime libraries are part of the [compiler-rt] project, and
   [will be built][sanitizer-build] on [supported targets][sanitizer-targets]
   when enabled in `bootstrap.toml`:

   ```toml
   [build]
   sanitizers = true
   ```

   The runtimes are [placed into target libdir][sanitizer-copy].

*  During LLVM code generation, the functions intended for instrumentation are
   [marked][sanitizer-attribute] with appropriate LLVM attribute:
   `SanitizeAddress`, `SanitizeHWAddress`, `SanitizeMemory`, or `SanitizeThread`.
   By default, all functions are instrumented, but this
   behaviour can be changed with `#[sanitize(xyz = "on|off|<other>")]`.

*  The decision whether to perform instrumentation or not is possible only at a
   function granularity.
   In the cases were those decision differ between
   functions, it might be necessary to inhibit inlining, both at [MIR
   level][inline-mir] and [LLVM level][inline-llvm].

*  The LLVM IR generated by rustc is instrumented by [dedicated LLVM
   passes][sanitizer-pass], different for each sanitizer.
   Instrumentation passes are invoked after optimization passes.

*  When producing an executable, the sanitizer specific runtime library is
   [linked in][sanitizer-link].
   The libraries are searched for in the target libdir.
   First, the search is relative to the overridden system root, and subsequently,
   it is relative to the default system root.
   Fall-back to the default system root
   ensures that sanitizer runtimes remain available when using sysroot overrides
   constructed by cargo `-Z build-std` or xargo.

[compiler-rt]: https://github.com/llvm/llvm-project/tree/main/compiler-rt
[sanitizer-build]: https://github.com/rust-lang/rust/blob/1ead4761e9e2f056385768614c23ffa7acb6a19e/src/bootstrap/src/core/build_steps/llvm.rs#L958-L1031
[sanitizer-targets]: https://github.com/rust-lang/rust/blob/1ead4761e9e2f056385768614c23ffa7acb6a19e/src/bootstrap/src/core/build_steps/llvm.rs#L1073-L1111
[sanitizer-copy]: https://github.com/rust-lang/rust/blob/1ead4761e9e2f056385768614c23ffa7acb6a19e/src/bootstrap/src/core/build_steps/compile.rs#L637-L676
[sanitizer-attribute]: https://github.com/rust-lang/rust/blob/1.55.0/compiler/rustc_codegen_llvm/src/attributes.rs#L42-L58
[inline-mir]: https://github.com/rust-lang/rust/blob/1.55.0/compiler/rustc_mir/src/transform/inline.rs#L314-L316
[inline-llvm]: https://github.com/rust-lang/llvm-project/blob/9330ec5a4c1df5fc1fa62f993ed6a04da68cb040/llvm/include/llvm/IR/Attributes.td#L225-L241
[sanitizer-pass]: https://github.com/rust-lang/rust/blob/1.55.0/compiler/rustc_codegen_llvm/src/back/write.rs#L660-L678
[sanitizer-link]: https://github.com/rust-lang/rust/blob/1.55.0/compiler/rustc_codegen_ssa/src/back/link.rs#L1053-L1089

## Testing sanitizers

Sanitizers are validated by code generation tests in
[`tests/codegen-llvm/sanitize*.rs`][test-cg] and end-to-end functional tests in
[`tests/ui/sanitizer/`][test-ui] directory.

Testing sanitizer functionality requires the sanitizer runtimes (built when
`sanitizer = true` in `bootstrap.toml`) and target providing support for particular a sanitizer.
When a sanitizer is unsupported on a given target, sanitizer tests will be ignored.
This behaviour is controlled by compiletest `needs-sanitizer-*` directives.

[test-cg]: https://github.com/rust-lang/rust/tree/HEAD/tests/codegen-llvm
[test-ui]: https://github.com/rust-lang/rust/tree/HEAD/tests/ui/sanitizer

## Enabling a sanitizer on a new target

To enable a sanitizer on a new target which is already supported by LLVM:

1. Include the sanitizer in the list of `supported_sanitizers` in [the target
   definition][target-definition].
   `rustc --target .. -Zsanitizer=..` should now recognize the sanitizer as supported.
2. [Build the runtime for the target and include it in the libdir.][sanitizer-targets]
3. [Teach compiletest that your target now supports the sanitizer.][compiletest-definition]
   Tests marked with `needs-sanitizer-*` should now run on the target.
4. Run tests `./x test --force-rerun tests/ui/sanitize/` to verify.
5. [--enable-sanitizers in the CI configuration][ci-configuration] to build and
   distribute the sanitizer runtime as part of the release process.

[target-definition]: https://github.com/rust-lang/rust/blob/1.55.0/compiler/rustc_target/src/spec/x86_64_unknown_linux_gnu.rs#L10-L11
[compiletest-definition]: https://github.com/rust-lang/rust/blob/1.55.0/src/tools/compiletest/src/util.rs#L87-L116
[ci-configuration]: https://github.com/rust-lang/rust/blob/1.55.0/src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile#L94

## Additional Information

* [Sanitizers project page](https://github.com/google/sanitizers/wiki/)
* [AddressSanitizer in Clang][clang-asan]
* [ControlFlowIntegrity in Clang][clang-cfi]
* [Hardware-assisted AddressSanitizer][clang-hwasan]
* [KernelControlFlowIntegrity in Clang][clang-kcfi]
* [LeakSanitizer in Clang][clang-lsan]
* [MemorySanitizer in Clang][clang-msan]
* [ThreadSanitizer in Clang][clang-tsan]

[clang-asan]: https://clang.llvm.org/docs/AddressSanitizer.html
[clang-cfi]: https://clang.llvm.org/docs/ControlFlowIntegrity.html
[clang-hwasan]: https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html
[clang-kcfi]: https://clang.llvm.org/docs/ControlFlowIntegrity.html#fsanitize-kcfi
[clang-lsan]: https://clang.llvm.org/docs/LeakSanitizer.html
[clang-msan]: https://clang.llvm.org/docs/MemorySanitizer.html
[clang-tsan]: https://clang.llvm.org/docs/ThreadSanitizer.html
