自定义myMemCpy替代memcpy是否更安全？Checkmarx检测疑问

Great question—let’s unpack this step by step, because the key here isn’t about the function itself, but why Checkmarx is flagging (or not flagging) the code, and what real risks you’re dealing with.

First: Why Checkmarx Flags memcpy for Improper Null Termination #

Static analysis tools like Checkmarx have built-in rules for standard library functions like memcpy. When you use memcpy to copy a string, the tool checks if you’re ensuring the resulting buffer has a null terminator (\0) afterward. If you copy n bytes but don’t explicitly add that final null byte (or if the source string’s null terminator isn’t included in the n bytes being copied), it triggers the "Improper_Null_Termination" alert—this is a valid warning, since non-null-terminated strings can lead to buffer overreads, crashes, or security vulnerabilities.

Why myMemCpy Doesn’t Trigger the Alert #

Your custom myMemCpy does exactly the same thing as memcpy: it copies n bytes from src to dest byte-for-byte, with no handling of null terminators. Checkmarx doesn’t flag it simply because it’s a custom function—there’s no pre-configured rule in the tool to scan this specific function for null termination issues. The tool isn’t saying your code is safe; it just doesn’t know to look for the same problem here.

Problems with Using myMemCpy Instead of memcpy #

• You’re not fixing the root issue: The null termination risk is still present. If your original memcpy usage was unsafe (e.g., copying a string without ensuring the destination is null-terminated), swapping to myMemCpy leaves that risk entirely intact—you’ve just hidden it from the static analyzer.
• Performance hit: Standard library implementations of memcpy are heavily optimized (using SIMD instructions, block copying, and architecture-specific tweaks) to be as fast as possible. Your handwritten loop will be significantly slower, especially for large memory copies.
• Hidden bugs in the custom function: Your loop uses int i as the index, but n is a size_t (an unsigned type). If n exceeds INT_MAX, this will cause integer overflow and undefined behavior—another risk the static analyzer would likely catch for standard functions, but misses here.

The Correct Fix #

Instead of trying to bypass the analyzer, address the actual null termination issue:

• If you’re copying a string, use string-specific functions designed to handle null terminators:
  • Use strncpy and explicitly add a null byte at the end (since strncpy doesn’t always add one if the source is longer than n):

    strncpy(dest, src, n);
    dest[n] = '\0'; // Ensure null termination (verify dest has space for n+1 bytes!)
    

  • If your system supports it, use strlcpy (a safer alternative that always null-terminates the destination, provided you pass the correct buffer size).
• If you must use memcpy for string data, explicitly add the null terminator after copying:

  memcpy(dest, src, n);
  dest[n] = '\0'; // Only do this if dest has enough space for n+1 bytes!
  

Remember: Static analyzer alerts are there to help you catch potential bugs, not to be worked around. Using a custom function to avoid the alert just kicks the can down the road.

内容的提问来源于stack exchange，提问作者Robben_Ford_Fan_boy