Testing Your Own Implementation of GLIBC in NASM x64

Want to learn how to effectively test your custom `GLIBC` using `NASM` x64? This guide walks you through the process step by step to ensure a successful implementation.
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Testing Your Own Implementation of GLIBC in NASM x64: A Complete Guide

When developing a custom implementation of GLIBC in NASM x64, one of the most challenging aspects is the testing phase. You might find yourself battling with various complications that arise when trying to link and execute your shared library correctly. This post discusses how to navigate those hurdles efficiently and ensures that you can successfully test your implementation.

Understanding the Problem

You have developed your own putchar() function in assembly, compiling it into a shared object (.so) file and creating a test program in C. However, when you set a breakpoint in GDB, it continues to reference the original GLIBC function instead of your implementation. Your goal is to get your custom putchar() to be recognized and used during testing.

Here’s a breakdown of the typical issues:

The GDB using the existing GLIBC implementation of putchar().

Missing _start label leading to linkage errors.

Undefined references while trying to run your testing binary.

The Solution

Gratefully, with hints and suggestions from experienced community members, the solution can be straightforward if you follow the right approach. Here's how to structure your code correctly and set up your Makefile for successful testing.

Step 1: Modify Your Main Function

To test your custom assembly implementation effectively, you need to define an entry point for your application. Adding an _start label in assembly code serves this purpose. Here's how to do it:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Update Your Makefile

The configuration of your Makefile plays a crucial role in linking your shared library correctly and preventing interference from the default system libraries. Consider the following modifications:

Include Linker Options: Add options to specify where the linker should find your custom library.

Here’s the updated Makefile:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Additional Compiler Options

Use the following compiler options to enhance the debugging experience and ensure that you’re testing only your own library:

-ggdb3: Provides detailed debugging information for GDB.

-fno-builtin: Prevents GCC from replacing your functions with built-in equivalents.

-nostdlib: Avoids automatic inclusion of the standard libraries.

-fno-plt: Directly links to the Global Offset Table (GOT) without using Procedure Linkage Table (PLT).

Conclusion

By making these modifications to your assembly function and the Makefile, you can now effectively test your custom GLIBC implementation in NASM x64. No longer will GDB default to using GLIBC's built-in functions. Instead, you can confirm that your custom functions are working as intended.

Catch any remaining bugs, polish your code, and enjoy the process of creating a lightweight version of GLIBC tailored to your needs! Happy coding!