Gain insights into using GDB for debugging in Linux. Explore disassembly, memory investigation, and register updates. Learn about pointers, stack, and function parameters in x64 architecture.

gdb.tar.gz

C2-LA

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GNU Debugger (GDB) is used for debugging C/C++ programs in LINUX/UNIX environments. It is a good tool to investigate what is happening inside a program, and how the contents inside the memory are changed with the execution of the program.

The main focus of the course is the disassembly of the program, where we'll use simple operations in C. With the help of GDB, we'll examine the contents of the registers and memory. We'll also learn how they are changed while executing basic operations. We’ll then explore the use of pointers, stack, and function parameters, and analyze how different registers update their values. With the help of disassembly output, we’ll learn how a simple C program can be reconstructed. The disassembly output is important for debugging and core dump analysis. We’ll use assembly language for x64 architecture.

By the end of this course, you should be able to debug the programs and memory contents at assembly level when a program executes or probe into the problem when a program crashes.

Debugging, Disassembly & Reversing in Linux for x64 Architecture

# Pointers to add numbers

The following pseudocode is an instruction to add numbers using pointers:

```s
(rbx) + (rax) -> (rbx)
```

The pointers `(rax)`and `(rbx)` refer to the contents of memory cells whose addresses are stored in `%RAX` and `%RBX` CPU registers. 

# Add numbers in different environments

Now we’ll explore how we add numbers by using pointers in following languages:

## Add numbers in C/C++ using pointers
The expression `(rbx) + (rax) -> (rbx)` is equivalent to the following C/C++ language expression:
```c
*pb = *pb + *pa;
```
In this expression, the `*` operator is an instruction to retrieve memory contents pointed to by the pointers `pa` or `pb` (also called **pointer dereference**).

## Add numbers in assembly language using pointers
In assembly language, we use the instruction `ADD` for the `+` operator, but we cannot use multiple memory addresses in a one-step instruction:
```asm
addl (%rax), (%rbx) # invalid instruction
```

We can only use one memory reference per line, and we therefore need to employ another register as a temporary variable:
```s
(rax) -> register
(rbx) + register -> (rbx)
```

We write this sequence of instructions as:


Learn how to add numbers using pointers in gdb disassembly.

Adding Numbers Using Pointers

Introduction to the Course

Memory, Registers, and Simple Arithmetic

Code Optimization

Number Representations

Pointers

Bytes, Words, Double, and Quad Words

Pointers to Memory

Logical Instructions and RIP

Reconstructing a Program with Pointers

Memory and Stacks

Frame Pointer and Local Variables

Function Parameters

More Instructions

Function Pointer Parameters

Summary of Code Disassembly Patterns

x64 Debugging, Disassembly & Reversing Exam

Adding Numbers Using Pointers

Pointers to add numbers

Add numbers in different environments

Add numbers in C/C++ using pointers