wasmtime-0002: AdapterOptions ancestors Vec O(n) scan per trampoline

Severity: MEDIUM CWE: CWE-407 (Algorithmic Complexity — linear membership test in compilation path) Speedup: >10x at D=50 nesting depth (deeply nested components) Target: wasmtime (bytecodealliance/wasmtime) Files:

crates/environ/src/fact.rs:130 — ancestors: Vec<RuntimeComponentInstanceIndex>
crates/environ/src/fact/trampoline.rs:121 — adapter.lower.ancestors.contains(&adapter.lift.instance)
crates/environ/src/fact/trampoline.rs:122 — adapter.lift.ancestors.contains(&adapter.lower.instance)
crates/environ/src/component/translate/adapt.rs:171 — same field in DFG

Description

When generating component model adapter trampolines, wasmtime checks for illegal re-entrancy by testing whether one adapter's instance appears in the ancestor chain of the other adapter:

if adapter.lift.instance == adapter.lower.instance
    || adapter.lower.ancestors.contains(&adapter.lift.instance)
    || adapter.lift.ancestors.contains(&adapter.lower.instance)

Both ancestors fields are Vec<RuntimeComponentInstanceIndex>, populated as the full chain of instantiating component instances (depth-first order).

For a component tree of nesting depth D, the ancestor chain has length D. The contains() call performs a linear scan through all D ancestors.

This runs once per adapter trampoline during compilation. With A adapters in a deeply nested component (D levels, A adapter functions), total work is O(A * D) in the worst case.

In practice Wasm component ecosystems are developing rapidly — large component graphs with many adapters and deep nesting are expected in production (e.g. WASI Preview 2 composites, wasm-compose pipelines).

Root Cause

The ancestor list is built as a Vec at inline.rs:1583-1588 from a frame stack. Since element identity (not ordering) is what matters for the re-entrancy check, this should be a HashSet or a sorted Vec with binary search.

Patch

See patch/wasmtime-0002.patch

Complexity Before

ancestors.contains() with D nesting depth: O(D) per check A adapters × 2 checks each: O(A * D)

Complexity After

With IndexSet<RuntimeComponentInstanceIndex> (or HashSet): O(1) per contains check → O(A) total

Reproduction

cd defects/wasmtime/unit && javac -d . *.java && java -ea unit.AncestorsLinearScanTest

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