diff --git a/POINTER_SAFETY_ANALYSIS.md b/POINTER_SAFETY_ANALYSIS.md new file mode 100644 index 00000000000..54e33f48b17 --- /dev/null +++ b/POINTER_SAFETY_ANALYSIS.md @@ -0,0 +1,86 @@ +# Pointer Access Safety Analysis for PR #14866 + +## Question +**"is pointer access path safe?"** + +Note: The original question asked "is pointer accessath safe?" which we interpret as asking whether pointer access paths (array/index accesses) are safe. + +## Answer +**YES - All pointer and index accesses in PR #14866 are SAFE.** + +## Summary + +This document analyzes the pointer/index access safety in the `rewriteScpLikeGitURL` function introduced in PR #14866 (libutil: remove regex from fixGitURL scp and improve colon handling). + +### Function Location +- File: `src/libutil/url.cc` +- Lines: 16-70 +- Function: `static std::optional rewriteScpLikeGitURL(std::string_view url)` + +### All Pointer/Index Accesses Analyzed + +#### Access #1: `url[hostStart]` (Line 41) +```cpp +if (url[hostStart] == '[') { +``` +- **Bounds Check:** Line 35 checks `if (hostStart >= url.size()) return std::nullopt;` +- **Status:** ✅ SAFE - Only accessed when `hostStart < url.size()` + +#### Access #2: `url[close + 1]` (Line 46) +```cpp +if (close + 1 >= url.size() || url[close + 1] != ':') +``` +- **Bounds Check:** Uses short-circuit evaluation; `close + 1 >= url.size()` checked BEFORE access +- **Status:** ✅ SAFE - Only accessed when `close + 1 < url.size()` + +#### Access #3: `url[colon]` (Line 53) +```cpp +if (colon == std::string_view::npos || url[colon] != ':') +``` +- **Bounds Check:** Uses short-circuit evaluation; `colon == npos` checked BEFORE access +- **Status:** ✅ SAFE - Only accessed when colon is a valid index + +#### Access #4: `url.substr(colon + 1)` (Line 68) +```cpp +rewritten.append(url.substr(colon + 1)); +``` +- **Bounds Check:** Line 59 checks `if (colon + 1 >= url.size()) return std::nullopt;` +- **Status:** ✅ SAFE - Only called when `colon + 1 < url.size()` + +## Safety Mechanisms Employed + +1. **Explicit bounds checking** before direct index access +2. **Short-circuit evaluation** (`||` operator) to prevent unsafe access +3. **Early returns** when invalid conditions detected +4. **Comprehensive test coverage** for edge cases + +## Edge Cases Verified + +All edge cases are properly handled: +- Empty strings → Returns `std::nullopt` +- Single `@` character → Returns `std::nullopt` +- Missing host/path → Returns `std::nullopt` +- IPv6 brackets → Properly validated +- Unterminated brackets → Returns `std::nullopt` + +## Test Coverage + +The PR includes extensive tests in `src/libutil-tests/url.cc`: +- `scpLikeEmptyUserDoesNotRewrite` +- `scpLikePathMayContainColon` +- `scpLikeIPv6PathMayContainColon` +- `scpLikeWithSlashBeforeAtDoesNotRewrite` +- Multiple parameterized test cases + +## Conclusion + +**All pointer accesses in the `rewriteScpLikeGitURL` function are SAFE.** + +The code demonstrates excellent defensive programming practices with proper bounds checking before every array access. No security vulnerabilities or undefined behavior detected. + +**RECOMMENDATION: The code is safe and ready for merge.** + +--- +**Analysis performed:** December 26, 2025 +**PR:** https://github.com/NixOS/nix/pull/14866 +**Commit:** ed0133383510e8bef5a4f0c9ab4cca27a3c8c115