rust-ub-risk-audit

Rust UB Risk Audit - make unsafe assumptions explicit

Use this skill to review Rust code for undefined behavior risk at the points where the compiler cannot enforce the full contract: unsafe code, FFI, raw pointers, aliasing, layout assumptions, lifetime extension, concurrency, and dependency boundaries. The goal is not to prove the program safe. The goal is to produce a precise inventory, identify broken or undocumented safety contracts, and leave the project with fixes and verification steps that match the actual risk.

Critical Constraints

• Treat every unsafe use as a contract with evidence, not as a style issue.
• Do not clear a risk because the code "looks normal"; tie the verdict to an invariant, a caller guarantee, a test result, or a narrower refactor.
• Keep static reasoning and dynamic tool results separate. Miri, sanitizers, fuzzing, and tests can expose defects, but passing them does not prove that every aliasing, lifetime, or FFI contract is valid.
• Prefer reducing the unsafe surface before adding comments. A smaller unsafe block with checked inputs is stronger than a paragraph explaining why a wide unsafe region should be okay.
• Report uncertainty as an audit gap. Unknown ABI ownership, missing header contracts, untested feature flags, or unreviewed native libraries are findings when they can change safety.

When To Use

Use this skill for:

• Reviewing unsafe Rust blocks, unsafe functions, unsafe traits, or unsafe impls.
• Auditing FFI boundaries, exported C ABI functions, callbacks, and native library wrappers.
• Investigating raw pointer dereferences, pointer casts, transmute, packed fields, unions, MaybeUninit, ManuallyDrop, or layout-sensitive code.
• Checking Send, Sync, Unpin, pinning, atomics, locks, and interior mutability where unsoundness can become a data race or invalid reference.
• Reviewing dependency boundaries where a crate imports unsafe abstractions, generated bindings, native code, or feature-dependent behavior.

Do not use this skill as a general Rust lint pass. If no unsafe, FFI, layout, or concurrency boundary exists, say that and limit the report to the evidence that established the low-risk scope.

Inputs To Gather

Inspect only the repository and artifacts relevant to the requested audit:

• Rust source, build scripts, generated bindings, examples, and tests.
• Cargo.toml, Cargo.lock, workspace configuration, feature flags, and target cfg gates.
• FFI headers, bindgen configuration, native build scripts, and wrapper docs.
• Existing safety comments, design notes, issue history, and prior crash reports when they are in scope for the task.
• CI, sanitizer, Miri, fuzzing, Loom, or stress-test outputs if already present.

If generated code is committed, audit the committed surface and identify the generator and source contract. If generated code is not committed, audit the generation command and the checked-in input that drives it.

Quick Start

1. Define the audit boundary: crate, workspace, module, feature set, platform, and whether dependencies or native libraries are in scope.
2. Build an unsafe inventory with file paths, symbols, and risk categories.
3. For each unsafe boundary, write the safety contract in plain language: preconditions, ownership, aliasing, lifetimes, layout, threading, panic or unwind behavior, and caller obligations.
4. Compare the contract to the code that establishes it. Mark missing checks, invalid assumptions, and undocumented caller requirements.
5. Run targeted verification commands where available. Record skipped commands with the reason.
6. Deliver a risk-ranked report with evidence, fixes, and residual gaps.

Inventory Commands

Use commands like these from the repository root, adjusting for workspace shape:

rg -n "\bunsafe\b|extern \"|#\[no_mangle\]|#\[export_name|repr\(|transmute|from_raw|into_raw|MaybeUninit|ManuallyDrop|UnsafeCell|NonNull|Send|Sync" .
cargo metadata --format-version 1 --no-deps
cargo tree -e features

If the repository has multiple crates or generated output, record which paths were included and which were excluded. Do not let a broad rg result replace manual review; it is only an index.

Audit Procedure

1. Scope The Boundary

State the exact audit target before judging findings:

• Workspace member or crate name.
• Cargo features and target triples that change unsafe code paths.
• Public API, internal API, FFI boundary, or dependency boundary.
• Whether native libraries, generated bindings, proc macros, or build scripts are in scope.
• Verification commands that can run in the current environment.

If scope is ambiguous, choose the smallest defensible boundary and name what remains unaudited.

2. Build The Unsafe Inventory

Capture each relevant item:

• unsafe fn, unsafe block, unsafe trait, unsafe impl, and extern block.
• Raw pointer creation, cast, arithmetic, dereference, or conversion into a reference.
• Ownership transfer through Box::from_raw, Vec::from_raw_parts, CString::from_raw, handles, descriptors, and custom allocators.
• Layout-sensitive code using repr(C), repr(transparent), repr(packed), unions, discriminants, transmute, or byte casting.
• Initialization and drop-sensitive code using MaybeUninit, ManuallyDrop, mem::zeroed, ptr::read, ptr::write, or drop_in_place.
• Concurrency-sensitive code using UnsafeCell, atomics, lock-free structures, callback threads, unsafe Send, or unsafe Sync.

For each item, assign one or more categories: FFI, aliasing, lifetime, layout, initialization, drop, concurrency, dependency, or public API contract.

3. Write The Safety Contract

Every unsafe boundary needs an explicit contract. Record:

• What the caller must guarantee before entering the boundary.
• What the boundary validates itself before executing unsafe operations.
• Which pointer ranges are valid, aligned, initialized, and non-null.
• Whether unique or shared access exists, and what prevents incompatible aliases.
• How long references, buffers, callbacks, and handles remain valid.
• Which thread may call it and which thread may drop returned values.
• What happens on panic, error, cancellation, partial initialization, and drop.
• Which representation, endianness, and ABI assumptions must hold.

Missing contracts are findings when callers cannot infer the requirements from types alone.

4. Check Pointer Aliasing And Lifetimes

Look for these failure modes:

• A mutable reference exists while another live reference or raw pointer can observe the same memory in an incompatible way.
• A raw pointer is converted to a reference without proving alignment, initialization, non-nullness, and valid lifetime.
• Slices are built from pointer and length pairs without checking allocation provenance, length overflow, or element initialization.
• References are stored beyond the lifetime of the source object, callback, temporary, stack frame, or foreign allocation.
• Pinned data can move after a self-reference, intrusive link, or external pointer observes its address.
• Drop order invalidates memory while another value still points into it.

Prefer fixes that move checks before unsafe operations and keep references short-lived.

5. Check FFI Boundaries

For each imported or exported function, review:

• ABI string, symbol name, calling convention, target cfg, and link attributes.
• Ownership transfer for pointers, strings, slices, handles, and buffers.
• Nullability, alignment, length units, terminators, and encoding.
• Allocation and deallocation pairing across language or library boundaries.
• Error reporting, errno-like state, sentinel values, and partial writes.
• Callback lifetime, reentrancy, thread affinity, and user data pointers.
• Panic and unwind behavior across the boundary.
• Header or upstream contract drift, especially when bindings are generated.

Use wrapper types to encode ownership and lifetime when possible. Keep raw FFI types at the edge and convert into checked Rust types before wider use.

6. Check Layout And Initialization Assumptions

Review each layout-dependent operation:

• repr(C) and repr(transparent) match the external contract they claim.
• repr(packed) fields are not borrowed in a way that creates unaligned references.
• Transmute and byte-casting preserve validity, alignment, size, and inhabited value constraints.
• Zeroed memory is valid for the type being created.
• MaybeUninit paths initialize every field before assume-init and correctly drop only initialized fields on error.
• Unions and discriminants are guarded by a tag or external invariant.
• Endianness and pointer-width assumptions are explicit when data crosses process, file, or network boundaries.

When a layout assumption exists only in a comment, prefer a compile-time check, type wrapper, or test that fails when the assumption changes.

7. Check Concurrency And Interior Mutability

Focus on places where Rust's normal sharing rules are bypassed:

• Unsafe Send and unsafe Sync impls have a type-level invariant that holds for all fields and all feature combinations.
• UnsafeCell access is synchronized or constrained so incompatible access cannot happen concurrently.
• Atomics use orderings that match the data dependency being protected.
• Lock-free structures handle reclamation, ABA risk, and destructor timing.
• Foreign callbacks do not race with Rust drops, shutdown, or reconfiguration.
• Thread-local state and global mutable state are initialized and torn down in a defined order.

Flag "works under this scheduler" arguments unless the code has a deterministic ownership, synchronization, or state-machine reason.

8. Check Dependency Boundaries

Dependencies can import safety contracts that the local crate must uphold:

• Crates that expose unsafe APIs, FFI bindings, native build steps, or feature gates that change representation or threading behavior.
• Wrapper crates whose safety depends on an upstream C library version.
• Byte-casting, serialization, memory-map, SIMD, allocator, async runtime, or lock-free crates used near unsafe boundaries.
• Public APIs that pass local types into dependency unsafe functions.

Record whether the local crate validates the dependency's preconditions or delegates them to callers. If dependency behavior is version-sensitive, cite the checked version from Cargo.lock or mark the version unknown.

9. Run Targeted Verification

Choose checks that fit the risk surface and repository support:

cargo test --workspace --all-features
cargo test --workspace --no-default-features
cargo miri test
RUSTFLAGS="-Z sanitizer=address" cargo +nightly test --target x86_64-unknown-linux-gnu

Only run nightly, Miri, sanitizer, fuzzing, or model-checking commands when the toolchain and target make sense. If a command cannot run, include the attempted command and the blocker in the report.

Finding Severity

Use severity to communicate fix order:

• Critical: a reachable path can create invalid references, data races, invalid drops, cross-ABI unwind, or memory corruption with plausible inputs.
• High: the safety contract is likely violated for a supported API, feature, or platform, even if a concrete crash was not reproduced.
• Medium: a contract is under-specified or insufficiently checked, and misuse is plausible from local callers or documented public APIs.
• Low: hardening, documentation, or tests would reduce future risk, but current code has a credible invariant.
• Info: inventory, tool output, or scope notes without an immediate defect.

Do not downgrade a finding because a tool did not reproduce it. Downgrade only when the code or type system establishes the missing invariant.

Output Specification

Return a concise audit report:

1. Scope and environment: crate, features, platform assumptions, commands run.
2. Unsafe inventory: table or bullets grouped by file and category.
3. Findings: severity, location, risk, evidence, recommended fix, and verification step.
4. Clean areas: boundaries reviewed with no issue found and why.
5. Gaps: skipped paths, missing contracts, unavailable tools, or dependencies not audited.

For each finding, include enough local evidence for a maintainer to reproduce the reasoning without rereading the whole audit.

Review Checklist

Before finishing, verify:

• Every unsafe block, unsafe function, unsafe trait, unsafe impl, and extern boundary in scope appears in the inventory.
• Pointer dereferences have alignment, initialization, provenance, bounds, and lifetime evidence.
• Aliasing and lifetime assumptions are enforced by types, checks, or a narrow unsafe contract.
• FFI ownership, nullability, allocation pairing, callbacks, and unwind behavior are explicit.
• Layout assumptions have a representation guarantee or a failing check.
• Concurrency invariants cover unsafe Send, unsafe Sync, atomics, shared mutable state, and foreign callbacks.
• Dependency safety contracts are either locally validated or deliberately delegated with documented caller obligations.
• Verification commands and blockers are recorded accurately.