Understanding How to Return a Pointer in x86-64 Assembly: Debugging Common Pitfalls

Learn how to effectively return a pointer in x86-64 assembly, identify issues using GDB, and ensure correct data storage within structures.
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Understanding How to Return a Pointer in x86-64 Assembly: Debugging Common Pitfalls

When working with assembly language, especially in x86-64 architecture, one of the frequent tasks involves returning a pointer from a function. This scenario can arise in complex programs, especially those that handle data structures like arrays of device pointers. This guide will explore the common pitfalls of returning pointers in assembly, using a specific example to illustrate the important steps in debugging such issues effectively.

The Problem at Hand

In our case, we are attempting to return a pointer from the inventory function, which takes an array of device pointers to evaluate their performance based on variability. Our main concern arises after executing the program; although our logic seems sound, we encounter an unexpected outcome where the maximum variation is reported correctly, but the name associated with that maximum variation is empty. This issue raises several questions:

Why is the device's name not printing correctly?

Is there a problem with how we're handling storage for our character array within the Device structure?

The Root of the Issue

Upon rigorous analysis, the bug arises from a specific instruction in the assembly code that manipulates the structure's values. The problematic instruction found in calibrate.s is as follows:

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

However, avg is defined as a short in the Device structure, which means it needs to be stored as a 16-bit value. The assembly line should instead utilize:

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

This highlights that while we are computing averages, the storage operation is incorrectly attempting to store a 32-bit integer into a 16-bit short, leading to unexpected memory overwrites within the Device structure.

Debugging with GDB

To effectively troubleshoot the problem, we used GDB (GNU Debugger), which proved instrumental. Here’s a detailed breakdown of how we investigated:

Step 1: Set Breakpoints

We began by setting a breakpoint at the second printf call within the inventory function to check what string the %rdx register was pointing to at that moment.

Step 2: Examine Variables

Upon inspecting the value in $rdx, we found it pointed to an empty string, which indicated an issue. By checking the status of the things array in memory:

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

We observed that some names were unexpectedly modified to empty strings, suggesting irregular memory access.

Step 3: Utilize Watchpoints

To identify what operation was corrupting the data, we set a watchpoint on things[1].name[0] and re-ran the program. This allowed us to monitor changes to that specific piece of memory.

We noticed that the name unexpectedly changed from the expected character to a null value due to the mishandled storage in the earlier step of the calibrate function, confirming our suspicion about the movl instruction.

Conclusion

Returning a pointer from a function in assembly requires careful consideration of data types, storage mechanisms, and debugging techniques. The issue we encountered stemmed primarily from an improperly executed store operation, which led to corrupted data.

Key Takeaways

Always ensure that the data type matches the storage size in assembly language.

Utilize GDB effectively to set breakpoints, examine memory addresses, and use watchpoints to track variable changes.

Debugging assembly can be challenging, but understanding how registers and memory interact will lead to more effective resolutions of issues.

By following these strategies, developers can successfully navigate the complexities of assembly and ensure they are returning the correct pointers with the appro

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