Comprehensive security review: Memory allocator passes production readiness audit

[Copilot]

Verified all previously identified issues resolved in commit 8551006. Performed extensive line-by-line security and correctness analysis of the complete allocator implementation.

Issues Confirmed Fixed

• Fragment initialization (line 240): freed field now properly initialized to 0 before calling free(), preventing reads of uninitialized memory
• Heap corruption detection (lines 60-74): Guard added to _extend_address() validates pointer ordering, detects heap corruption with abort in DEBUG builds
• Type consistency: All SIZE_MAX → UINTPTR_MAX conversions complete
• Logic inversions: _extend_address() and _defragment_address() correct after !_is_free() fixes
• Error handling: fprintf(stderr, ...) replaces undefined eprintf

Comprehensive Analysis Results

Memory Safety ✓

• All pointer arithmetic bounds-checked
• Integer overflow guards at critical points (lines 162, 180, 306)
• Fragment creation within validated heap space
• Alignment consistently enforced via _align() at allocation entry points

Defragmentation Logic ✓

• Extend/coalesce operations correctly handle free blocks
• Sentinel block (size=0, freed=0) properly excluded from allocation
• _heap_last tracking maintained across all operations

Double-Free Protection ✓

• Counter-based detection with saturation at UINT8_MAX
• DEBUG builds abort on detection, release builds return early

Known Limitations (by design)

• Not thread-safe (acceptable for single-threaded embedded kernel)
• free() after alloc_free() undefined (user error, not allocator bug)
• Byte-wise copy in realloc (correct but could use memcpy for performance)
• TODO remains: re-fragment excess space in realloc shrink case

Verdict

Production-ready for intended use case. No critical bugs found. Code demonstrates solid understanding of memory management with thoughtful optimizations and comprehensive safety checks.

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