Project 6: Safe Memory Allocator Wrapper

A secure memory allocation wrapper that prevents integer overflow in size calculations, zeroes memory on free (preventing info leaks), detects double-frees, and tracks allocations for debugging.

Quick Reference

Attribute Value
Primary Language C
Alternative Languages C++, Rust
Difficulty Level 2: Intermediate
Time Estimate 1-2 weeks
Knowledge Area Secure Coding / Memory Safety
Tooling Valgrind, AddressSanitizer
Prerequisites Project 2 (integer overflow), basic dynamic memory

What You Will Build

A secure memory allocation wrapper that prevents integer overflow in size calculations, zeroes memory on free (preventing info leaks), detects double-frees, and tracks allocations for debugging.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • Safe size calculation → maps to integer overflow before malloc
  • Preventing use-after-free → maps to poison freed memory
  • Detecting double-free → maps to allocation tracking
  • Memory zeroing on free → maps to preventing info leaks

Key Concepts

  • calloc vs malloc: CERT C MEM04-C
  • Clearing Sensitive Data: CERT C MEM03-C
  • Allocation Tracking: Valgrind internals

Real-World Outcome

$ ./test_safe_alloc

Testing safe_malloc...
  ✓ Normal allocation: 1024 bytes allocated
  ✓ Zero size: Returns NULL
  ✓ Overflow protection: safe_calloc(SIZE_MAX, 2) returns NULL

Testing safe_free...
  ✓ Memory zeroed on free (sensitive data cleared)
  ✓ Double-free detected: FATAL: double free at 0x12340000
  ✓ Use-after-free detected: Memory poisoned with 0xDEADBEEF

Testing safe_realloc...
  ✓ Normal realloc: 1024 -> 2048 bytes
  ✓ Shrink realloc: Zeroes extra bytes
  ✓ Overflow protection: safe_realloc(ptr, SIZE_MAX) returns NULL

Memory stats:
  Allocations: 15
  Frees: 14
  Current allocated: 1024 bytes
  LEAK DETECTED: 1 allocation not freed!

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: LEARN_SECURE_C_AND_EXPLOIT_AWARENESS.md
  • “Effective C, 2nd Edition” by Robert C. Seacord