110 lines
4.5 KiB
Markdown
110 lines
4.5 KiB
Markdown
# UNDF: UNDF-2026-000000773
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# rustc-0004: CWE-407 — O(I×A) repeated Vec<AmbiguityError> linear scan in finalize_imports
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**Severity:** MEDIUM
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**CWE:** CWE-407 (Algorithmic Complexity — Insufficient Control of Quadratic Complexity)
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**Target:** rust-lang/rust (rustc)
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**File:** `compiler/rustc_resolve/src/imports.rs`
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**Lines:** 1004–1007, 1023, 1232
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**Status:** PATCHED (unit test PASS)
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## Description
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`Resolver::finalize_imports` iterates over every import in the crate and calls
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`finalize_import` for each one. Inside `finalize_import` there are three
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O(A) linear scans through `self.ambiguity_errors: Vec<AmbiguityError>`:
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1. **Line 1004–1007** — closure `ambiguity_errors_len` filters and counts
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non-warning errors: `errors.iter().filter(|e| e.warning.is_none()).count()`
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2. **Line 1007** — called once to capture `prev_ambiguity_errors_len` (before `resolve_path`)
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3. **Line 1023** — called again to compute `no_ambiguity` (after `resolve_path`)
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4. **Line 1232** — inside `per_ns` closure (runs 2–3 times per import):
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`this.ambiguity_errors.iter().any(|error| error.warning.is_none())`
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Total per `finalize_imports` pass: O(I × A) where I = number of imports,
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A = length of `ambiguity_errors`.
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```rust
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// imports.rs:1004-1007 (inside finalize_import, called for each import)
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let ambiguity_errors_len = |errors: &Vec<AmbiguityError<'_>>| {
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errors.iter().filter(|error| error.warning.is_none()).count()
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// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ O(A) per call
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};
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let prev_ambiguity_errors_len = ambiguity_errors_len(&self.ambiguity_errors); // O(A)
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// ... resolve_path() call ...
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let no_ambiguity =
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ambiguity_errors_len(&self.ambiguity_errors) == prev_ambiguity_errors_len; // O(A)
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// imports.rs:1232 (inside per_ns closure, 2-3 times per import)
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let has_ambiguity_error =
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this.ambiguity_errors.iter().any(|error| error.warning.is_none()); // O(A)
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```
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At I=500 imports, A=200 ambiguity errors: ~700 × 200 = 140,000 comparisons
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instead of ~700 constant-time reads from a maintained counter.
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## Root Cause
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`ambiguity_errors` is a plain `Vec<AmbiguityError>`. The code counts or
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checks non-warning entries by scanning the entire vector on every call, rather
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than maintaining a separate counter `non_warning_ambiguity_error_count: usize`
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that is incremented/decremented when errors are pushed/popped.
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## Fix
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Maintain `non_warning_ambiguity_error_count: usize` alongside `ambiguity_errors`.
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Increment it in `report_ambiguity_error` when `warning.is_none()`.
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Replace all `.iter().filter(|e| e.warning.is_none()).count()` calls with a
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single O(1) read of the counter.
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```diff
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--- a/compiler/rustc_resolve/src/lib.rs
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+++ b/compiler/rustc_resolve/src/lib.rs
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@@ ambiguity_errors field
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ambiguity_errors: Vec<AmbiguityError<'ra>> = Vec::new(),
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+ non_warning_ambiguity_error_count: usize = 0,
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--- a/compiler/rustc_resolve/src/imports.rs
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+++ b/compiler/rustc_resolve/src/imports.rs
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@@ finalize_import — replace the closure and its three call sites
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- let ambiguity_errors_len = |errors: &Vec<AmbiguityError<'_>>| {
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- errors.iter().filter(|error| error.warning.is_none()).count()
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- };
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- let prev_ambiguity_errors_len = ambiguity_errors_len(&self.ambiguity_errors);
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+ let prev_non_warning_ambiguity_count = self.non_warning_ambiguity_error_count;
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// ...resolve_path...
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- let no_ambiguity =
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- ambiguity_errors_len(&self.ambiguity_errors) == prev_ambiguity_errors_len;
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+ let no_ambiguity =
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+ self.non_warning_ambiguity_error_count == prev_non_warning_ambiguity_count;
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// ...inside per_ns closure...
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- let has_ambiguity_error =
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- this.ambiguity_errors.iter().any(|error| error.warning.is_none());
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+ let has_ambiguity_error = this.non_warning_ambiguity_error_count > 0;
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```
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Increment site (in `report_ambiguity_error` or wherever errors are pushed):
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```rust
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self.ambiguity_errors.push(ambiguity_error);
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if ambiguity_error.warning.is_none() {
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self.non_warning_ambiguity_error_count += 1;
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}
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```
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## Complexity Before / After
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| Scenario | Before | After |
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|----------|--------|-------|
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| I imports, A ambiguity errors | O(I × A) | O(I) |
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| I=500, A=200 | 140,000 ops | 500 ops |
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| Ratio | — | **280x** |
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## References
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- `compiler/rustc_resolve/src/imports.rs` lines 1004–1007, 1023, 1232
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- `compiler/rustc_resolve/src/lib.rs` line 1279 (`ambiguity_errors: Vec<AmbiguityError>`)
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- `compiler/rustc_resolve/src/lib.rs` line 2134 (push site)
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