Understanding the __attribute__((__bounded__(__string__,2,3))) in OpenBSD's bcrypt Implementation

Discover how the `__bounded__` attribute works in OpenBSD and its significance for memory safety in C programming. Learn how this can aid in porting code across platforms.
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Understanding the __attribute__((__bounded__(__string__,2,3))) in OpenBSD's bcrypt Implementation

When delving into OpenBSD's bcrypt code, you might encounter the __attribute__((__bounded__(__string__,2,3))) declaration. This could trigger confusion, especially if you're getting warnings about unknown attributes. If you're looking for clarity on what this attribute means and how to implement a similar attribute on another platform, you've come to the right place!

What is the __bounded__ Attribute?

The __bounded__ attribute is a feature available in some versions of GCC used within OpenBSD. It is designed to enhance the safety and reliability of C function implementations by enforcing checks on argument memory sizes. Here’s a breakdown of how it works:

Purpose of __bounded__

Memory Safety: The __bounded__ attribute helps prevent buffer overflows by specifying lengths for pointers passed to functions.

Type Augmentation: It allows the compiler to better understand the relationship between function parameters, especially when one argument represents the size of another.

Breakdown of the Syntax

Function Declaration: The attribute is attached to a function declaration to indicate that one of its parameters bounds the size of another.

Example: In the provided code snippet, void SHA256Update(SHA2_CTX *, const void *, size_t) __attribute__((__bounded__(__string__,2,3)));, we can break it down as follows:

__bounded__: Marks the function to enable the bounds checking.

__string__: Indicates that the argument is treated as a string for additional checks.

2 and 3: Indicate which parameters are being used for the size check. Here, the second parameter is the pointer to the string, and the third is its length.

Practical Implications

The attributes are particularly beneficial for developers looking to write robust C code. By specifying that the second parameter can only point to memory that is at most the size defined by the third parameter, you can avoid common errors like accessing out-of-bounds memory.

Things to Note

Platform Specific: It’s vital to note that __bounded__ and its function rely specifically on OpenBSD's fork of GCC, which means they may not be available in other compilers. Familiarity with the appropriate attributes in other platforms is essential for effective code portability.

Historical Context: There was a short-lived GNU C extension similar to __bounded__ that existed for a brief period from around 2000 to 2003. However, it was poorly documented and is less relevant today.

Conclusion

Understanding the __attribute__((__bounded__(__string__,2,3))) can greatly enhance your C programming skills, especially in handling memory safely. As you work to port OpenBSD code to other platforms, investigating similar attributes or constructs available in those environments will ensure that you maintain the same levels of safety and reliability. If you find yourself consistently working with security-critical applications, becoming adept in such attributes will be a crucial asset in your developer toolkit.

By mastering these concepts, you can bolster your coding practices and protect your projects from common pitfalls associated with memory handling. Happy coding!

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