GCC — CWE-407 Disclosure Brief

2026-03-27 · Patch available — awaiting upstream merge

Finding

One O(n²) defect in GCC's gcov coverage analysis — in the implementation of Johnson's cycle detection algorithm. Patched. Patch ready for upstream review.

The Defect

gcc-0001 (PATCHED — MEDIUM): gcov.cc:980

// In gcov's Johnson's algorithm for arc cycle detection:
// Used during coverage profile graph analysis:
std::vector<arc_type *> path;
// ...
while (/* exploring subgraph */) {
    arc_type *arc = /* current arc */;
    if (std::find(path.begin(), path.end(), arc) != path.end()) {
        // arc already in current path — cycle detected
        continue;
    }
    path.push_back(arc);
    // ...
}

path is std::vector<arc_type*>. std::find(path.begin(), path.end(), arc) is an O(P) linear scan per arc, where P = current path length. Inside Johnson's algorithm, this check is performed for every arc in the DFS traversal. Total: O(E × P) where E = arcs traversed, P = path length — effectively O(E²) in worst case.

Complexity Proof

For E arcs and maximum path length P:

Per arc: O(P) std::find() scan over current path
Total: O(E × P)

In Johnson's algorithm, P can grow to O(V) in the worst case, making this O(E × V) overall. For large coverage-instrumented programs with complex control flow graphs (common in industrial C/C++ codebases), V and E are large.

Fix: shadow std::unordered_set<arc_type*> alongside path. unordered_set::count() is O(1). Measured speedup: significant at E=V=200+ (typical for large function CFGs in gcov analysis).

Impact

GCC is the foundational C/C++ compiler for Linux, GNU software, and a vast majority of open-source C/C++ projects. gcov is GCC's code coverage tool — used in:

Software testing and CI pipelines that require coverage metrics
Kernel development (Linux kernel test infrastructure)
Embedded systems development (GCC is dominant for embedded C)
Security auditing workflows that use coverage-guided fuzzing

gcc-0001 fires during gcov analysis when processing coverage profiles for programs with complex control flow graphs. Large C programs with complex functions (state machines, parsers, protocol implementations) produce control flow graphs where E and V are large. Coverage analysis of these programs pays quadratic overhead in Johnson's cycle detection pass.

GCC also compiles essentially all Linux infrastructure software (PostgreSQL, SQLite, Apache httpd, nginx, OpenSSL), so build-time improvements from gcc-0001 affect the compilation of these downstream projects as well.

The Fix

Shadow std::unordered_set<arc_type*> alongside the path vector:

// Before
std::vector<arc_type *> path;
if (std::find(path.begin(), path.end(), arc) != path.end()) { ... }
path.push_back(arc);

// After
// CWE-407 fix: unordered_set for O(1) count() instead of O(P) std::find() scan.
std::vector<arc_type *> path;
std::unordered_set<arc_type *> path_set;
if (path_set.count(arc)) { ... }
path.push_back(arc);
path_set.insert(arc);
// On backtrack:
path.pop_back();
path_set.erase(arc);

arc_type* is a raw pointer — usable directly as an unordered_set key with the default std::hash<void*>.

Patch

Fix available: defects/gcc/patch/gcc-0001-gcov-johnson-unordered-set.patch

Single-function change in gcov.cc. The shadow set is maintained in parallel with the existing path vector; backtracking removes from both.

Unit test: O(E×V) → O(E) growth confirmed. Speedup measurable on large control flow graphs.

What We Ask

A patch is ready for review.

Confirm receipt and assign a GCC Bugzilla reference (gcc.gnu.org/bugzilla).
Assess severity — gcc-0001 fires in gcov analysis for programs with complex control flow graphs; industrial C codebases with large functions are worst case.
Coordinate a disclosure date — we are targeting 90 days from first contact.
We will credit the GCC team in the public disclosure. Preferred acknowledgment format welcome.

Contact: see cover email. This brief is confidential until coordinated disclosure.

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