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Embedded Systems Reverse Engineering

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Week 1: Introduction and Overview of Embedded Reverse Engineering: Ethics, Scoping, and Basic Concepts

🎯 What You'll Learn This Week

By the end of this week, you will be able to:

Understand what a microcontroller is and how it works
Know the basic registers of the ARM Cortex-M33 processor
Understand memory layout (Flash vs RAM) and why it matters
Understand how the stack works in embedded systems
Set up and connect GDB to your Pico 2 for debugging
Use Ghidra for static analysis of your binary
Read basic ARM assembly instructions and understand what they do

📚 Part 1: Understanding the Basics

What is a Microcontroller?

Think of a microcontroller as a tiny computer on a single chip. Just like your laptop has a processor, memory, and storage, a microcontroller has all of these packed into one small chip. The RP2350 is the microcontroller chip that powers the Raspberry Pi Pico 2.

What is the ARM Cortex-M33?

The RP2350 has two "brains" inside it - we call these cores. One brain uses ARM Cortex-M33 instructions, and the other can use RISC-V instructions. In this course, we'll focus on the ARM Cortex-M33 core because it's more commonly used in the industry.

What is Reverse Engineering?

Reverse engineering is like being a detective for code. Instead of writing code and compiling it, we take compiled code (the 1s and 0s that the computer actually runs) and figure out what it does. This is useful for:

Understanding how things work
Finding bugs or security issues
Learning how software interacts with hardware

📚 Part 2: Understanding Processor Registers

What is a Register?

A register is like a tiny, super-fast storage box inside the processor. The processor uses registers to hold numbers while it's doing calculations. Think of them like the short-term memory your brain uses when doing math in your head.

The ARM Cortex-M33 Registers

The ARM Cortex-M33 has several important registers:

Register	Also Called	Purpose
`r0` - `r12`	General Purpose	Store numbers, pass data between functions
`r13`	SP (Stack Pointer)	Keeps track of where we are in the stack
`r14`	LR (Link Register)	Remembers where to go back after a function
`r15`	PC (Program Counter)	Points to the next instruction to run

General Purpose Registers (`r0` - `r12`)

These 13 registers are your "scratch paper." When the processor needs to add two numbers, subtract, or do any calculation, it uses these registers to hold the values.

Example: If you want to add 5 + 3:

Put 5 in r0
Put 3 in r1
Add them and store the result (8) in r2

The Stack Pointer (`r13` / SP)

The stack is a special area of memory that works like a stack of plates:

When you add something, you put it on top (called a PUSH)
When you remove something, you take it from the top (called a POP)

The Stack Pointer always points to the top of this stack. On ARM systems, the stack grows downward in memory. This means when you push something onto the stack, the address number gets smaller!

Higher Memory Address (0x20082000)
┌──────────────────┐
│                  │  ← Stack starts here (empty)
├──────────────────┤
│   Pushed Item 1  │  ← SP points here after 1 push
├──────────────────┤
│   Pushed Item 2  │  ← SP points here after 2 pushes
└──────────────────┘
Lower Memory Address (0x20081FF8)

The Link Register (`r14` / LR)

When you call a function, the processor needs to remember where to come back to. The Link Register stores this "return address."

Example:

main() calls print_hello()
   ↓
LR = address right after the call in main()
   ↓
print_hello() runs
   ↓
print_hello() finishes, looks at LR
   ↓
Jumps back to main() at the address stored in LR

The Program Counter (`r15` / PC)

The Program Counter always points to the next instruction the processor will execute. It's like your finger following along as you read a book - it always points to where you are.

📚 Part 3: Understanding Memory Layout

XIP - Execute In Place

The RP2350 uses something called XIP (Execute In Place). This means the processor can run code directly from the flash memory (where your program is stored) without copying it to RAM first.

Key Memory Address: 0x10000000

This is where your program code starts in flash memory. Remember this address - we'll use it a lot!

Memory Map Overview

┌─────────────────────────────────────┐
│  Flash Memory (XIP)                 │
│  Starts at:  0x10000000             │
│  Contains: Your program code        │
├─────────────────────────────────────┤
│  RAM                                │
│  Starts at: 0x20000000              │
│  Contains: Stack, Heap, Variables   │
└─────────────────────────────────────┘

Stack vs Heap

Stack	Heap
Automatic memory management	Manual memory management
Fast	Slower
Limited size	More flexible size
Used for function calls, local variables	Used for dynamic memory allocation
Grows downward	Grows upward

📚 Part 3.5: Reviewing Our Hello World Code

Before we start debugging, let's understand the code we'll be working with. Here's our 0x0001_hello-world.c program:

#include <stdio.h>
#include "pico/stdlib.h"

int main(void) {
    stdio_init_all();

    while (true)
        printf("hello, world\r\n");
}

Breaking Down the Code

The Includes

#include <stdio.h>
#include "pico/stdlib.h"

<stdio.h> - This is the standard input/output library. It gives us access to the printf() function that lets us print text.
"pico/stdlib.h" - This is the Pico SDK's standard library. It provides essential functions for working with the Raspberry Pi Pico hardware.

The Main Function

int main(void) {

Every C program starts running from the main() function. The void means it takes no arguments, and int means it returns an integer (though our program never actually returns).

Initializing Standard I/O

stdio_init_all();

This function initializes all the standard I/O (input/output) for the Pico. It sets up:

USB CDC (so you can see output when connected to a computer via USB)
UART (serial communication pins)

Without this line, printf() wouldn't have anywhere to send its output!

The Infinite Loop

while (true)
    printf("hello, world\r\n");

while (true) - This creates an infinite loop. The program will keep running forever (or until you reset/power off the Pico).
printf("hello, world\r\n") - This prints the text "hello, world" followed by a carriage return (\r) and newline (\n).

💡 Why \r\n instead of just \n?

In embedded systems, we often use both carriage return (\r) and newline (\n) together. The \r moves the cursor back to the beginning of the line, and \n moves to the next line. This ensures proper display across different terminal programs.

What Happens When This Runs?

Power on - The Pico boots up and starts executing code from flash memory
stdio_init_all() - Sets up USB and/or UART for communication
Infinite loop begins - The program enters the while(true) loop
Print forever - "hello, world" is sent over and over as fast as possible

Why This Code is Perfect for Learning

This simple program is ideal for reverse engineering practice because:

It has a clear, recognizable function call (printf)
It has an infinite loop we can observe
It's small enough to understand completely
It demonstrates real hardware interaction (USB/UART output)

When we debug this code, we'll be able to see how the C code translates to ARM assembly instructions!

Compiling and Flashing to the Pico 2

Now that we understand the code, let's get it running on our hardware:

Step 1: Compile the Code

In VS Code, look for the Compile button in the status bar at the bottom of the window. This is provided by the Raspberry Pi Pico extension. Click it to compile your project.

The extension will run CMake and build your code, creating a .uf2 file that can be loaded onto the Pico 2.

Step 2: Put the Pico 2 in Flash Loading Mode

To flash new code to your Pico 2, you need to put it into BOOTSEL mode:

Press and hold the right-most button on your breadboard (the BOOTSEL button)
While holding BOOTSEL, press the white Reset button
Release the Reset button first
Then release the BOOTSEL button

When done correctly, your Pico 2 will appear as a USB mass storage device (like a flash drive) on your computer. This means it's ready to receive new firmware!

💡 Tip: You'll see a drive called "RP2350" appear in your file explorer when the Pico 2 is in flash loading mode.

Step 3: Flash and Run

Back in VS Code, click the Run button in the status bar. The extension will:

Copy the compiled .uf2 file to the Pico 2
The Pico 2 will automatically reboot and start running your code

Once flashed, your Pico 2 will immediately start executing the hello-world program, printing "hello, world" continuously when we open PuTTY!

📚 Part 4: Dynamic Analysis with GDB

Prerequisites

Before we start, make sure you have:

A Raspberry Pi Pico 2 board
GDB (GNU Debugger) installed
OpenOCD or another debug probe connection
The sample "hello-world" binary loaded on your Pico 2

Connecting to Your Pico 2 with OpenOCD

Open a terminal and start OpenOCD:

openocd ^
  -s "C:\Users\flare-vm\.pico-sdk\openocd\0.12.0+dev\scripts" ^
  -f interface/cmsis-dap.cfg ^
  -f target/rp2350.cfg ^
  -c "adapter speed 5000"

Connecting to Your Pico 2 with GDB

Open another terminal and start GDB with your binary:

arm-none-eabi-gdb -q build/0x0001_hello-world.elf

Connect to your target:

(gdb) target extended-remote localhost:3333
(gdb) monitor reset halt

Basic GDB Commands: Your First Steps

Now that we're connected, let's learn three essential GDB commands that you'll use constantly in embedded reverse engineering.

Setting a Breakpoint with `b main`

A breakpoint tells the debugger to pause execution when it reaches a specific point. Let's set one at our main function:

(gdb) b main
Breakpoint 1 at 0x10000234: file ../0x0001_hello-world.c, line 5.

What this tells us:

GDB found our main function
It's located at address 0x10000234 in flash memory
The source file and line number are shown (because we have debug symbols)

Now let's run to that breakpoint:

(gdb) c
Continuing.

Breakpoint 1, main () at ../0x0001_hello-world.c:5
5       stdio_init_all();

The program has stopped right at the beginning of main!

Disassembling with `disas`

The disas (disassemble) command shows us the assembly instructions for the current function:

(gdb) disas
Dump of assembler code for function main:
=> 0x10000234 <+0>:     push    {r3, lr}
   0x10000236 <+2>:     bl      0x1000156c <stdio_init_all>
   0x1000023a <+6>:     ldr     r0, [pc, #8]    @ (0x10000244 <main+16>)
   0x1000023c <+8>:     bl      0x100015fc <__wrap_puts>
   0x10000240 <+12>:    b.n     0x1000023a <main+6>
   0x10000242 <+14>:    nop
   0x10000244 <+16>:    adds    r4, r1, r7
   0x10000246 <+18>:    asrs    r0, r0, #32
End of assembler dump.

Understanding the output:

The => arrow shows where we're currently stopped
Each line shows: address <offset>: instruction operands
We can see the calls to stdio_init_all and __wrap_puts (printf was optimized to puts)
The b.n 0x1000023a at the end is our infinite loop - it jumps back to reload the string!

Viewing Registers with `i r`

The i r (info registers) command shows the current state of all CPU registers:

(gdb) i r
r0             0x0                 0
r1             0x10000235          268436021
r2             0x80808080          -2139062144
r3             0xe000ed08          -536810232
r4             0x100001d0          268435920
r5             0x88526891          -2007865199
r6             0x4f54710           83183376
r7             0x400e0014          1074659348
r8             0x43280035          1126694965
r9             0x0                 0
r10            0x10000000          268435456
r11            0x62707361          1651536737
r12            0xed07f600          -318245376
sp             0x20082000          0x20082000
lr             0x1000018f          268435855
pc             0x10000234          0x10000234 <main>
xpsr           0x69000000          1761607680

Key registers to watch:

Register	Value	Meaning
`pc`	`0x10000234`	Program Counter - we're at the start of `main`
`sp`	`0x20081fc8`	Stack Pointer - top of our stack in RAM
`lr`	`0x100002d5`	Link Register - where we return after `main`
`r0-r3`	Various	Will hold function arguments and return values

💡 Tip: You can also use i r pc sp lr to show only specific registers you care about.

Quick Reference: Essential GDB Commands

Command	Short Form	What It Does
`break main`	`b main`	Set a breakpoint at main
`continue`	`c`	Continue execution until breakpoint
`disassemble`	`disas`	Show assembly for current function
`info registers`	`i r`	Show all register values
`stepi`	`si`	Execute one assembly instruction
`nexti`	`ni`	Execute one instruction (skip calls)
`x/10i $pc`		Examine 10 instructions at PC
`monitor reset halt`		Reset the target and halt

💡 What's Next? In Week 2, we'll put these GDB commands to work with hands-on debugging exercises! We'll step through code, examine the stack, watch registers change, and ultimately use these skills to modify a running program. The commands you learned here are the foundation for everything that follows.

🔬 Part 5: Static Analysis with Ghidra

Setting Up Your First Ghidra Project

Before we dive into GDB debugging, let's set up Ghidra to analyze our hello-world binary. Ghidra is a powerful reverse engineering tool that will help us visualize the disassembly and decompiled code.

Step 1: Create a New Project

Launch Ghidra
A window will appear - select File → New Project
Choose Non-Shared Project and click Next
Enter the Project Name: 0x0001_hello-world
Click Finish

Step 2: Import the Binary

Open your file explorer and navigate to the Embedded-Hacking folder
Drag and drop the 0x0001_hello-world.elf file into the folder panel within the Ghidra application

Step 3: Understand the Import Dialog

In the small window that appears, you will see the file identified as an ELF (Executable and Linkable Format).

💡 What is an ELF file?

ELF stands for Executable and Linkable Format. This format includes symbols - human-readable names for functions and variables. These symbols make reverse engineering much easier because you can see function names like main and printf instead of just memory addresses.

In future weeks, we will work with stripped binaries (.bin files) that do not contain these symbols. This is more realistic for real-world reverse engineering scenarios where symbols have been removed to make analysis harder.

Click Ok to import the file
Double-click on the file within the project window to open it in the CodeBrowser

Step 4: Auto-Analyze the Binary

When prompted, click Yes to auto-analyze the binary. Accept the default analysis options and click Analyze.

Ghidra will now process the binary, identifying functions, strings, and cross-references. This may take a moment.

Reviewing the Main Function in Ghidra

Once analysis is complete, let's find our main function:

In the Symbol Tree panel on the left, expand Functions
Look for main in the list (you can also use Search → For Address or Label and type "main")
Click on main to navigate to it

What You'll See

Ghidra shows you two views of the code:

Listing View (Center Panel) - The disassembled ARM assembly:

                             *************************************************************
                             *                           FUNCTION                          
                             *************************************************************
                             int  main (void )
                               assume LRset = 0x0
                               assume TMode = 0x1
             int               r0:4           <RETURN>
                             main                                            XREF[3]:     Entry Point (*) , 
                                                                                          _reset_handler:1000018c (c) , 
                                                                                          .debug_frame::00000018 (*)   
                             0x0001_hello-world.c:4 (2)
                             0x0001_hello-world.c:5 (2)
        10000234 08  b5           push       {r3,lr}
                             0x0001_hello-world.c:5 (4)
        10000236 01  f0  99  f9    bl         stdio_init_all                                   _Bool stdio_init_all(void)
                             LAB_1000023a                                    XREF[1]:     10000240 (j)   
                             0x0001_hello-world.c:7 (6)
                             0x0001_hello-world.c:8 (6)
        1000023a 02  48           ldr        r0=>__EH_FRAME_BEGIN__ ,[DAT_10000244 ]           = "hello, world\r"
                                                                                             = 100019CCh
        1000023c 01  f0  de  f9    bl         __wrap_puts                                      int __wrap_puts(char * s)
                             0x0001_hello-world.c:7 (8)
        10000240 fb  e7           b          LAB_1000023a
        10000242 00              ??         00h
        10000243 bf              ??         BFh
                             DAT_10000244                                    XREF[1]:     main:1000023a (R)   
        10000244 cc  19  00  10    undefine   100019CCh                                        ?  ->  100019cc

Decompile View (Right Panel) - The reconstructed C code:

int main(void) {
    stdio_init_all();
    do {
        __wrap_puts("hello, world");
    } while (true);
}

🎯 Notice how Ghidra reconstructed our original C code! The decompiler recognized the infinite loop and the puts call (the compiler optimized printf to puts since we're just printing a simple string).

Why We Start with .elf Files

We're using the .elf file because it contains symbols that help us learn:

Function names are visible (main, stdio_init_all, puts)
Variable names may be preserved
The structure of the code is easier to understand

In future weeks, we'll work with .bin files that have been stripped of symbols. This will teach you how to identify functions and understand code when you don't have these helpful hints!

📊 Part 6: Summary and Review

What We Learned

Registers: The ARM Cortex-M33 has 13 general-purpose registers (r0-r12), plus special registers for the stack pointer (r13/SP), link register (r14/LR), and program counter (r15/PC).
The Stack:
- Grows downward in memory
- PUSH adds items (SP decreases)
- POP removes items (SP increases)
- Used to save return addresses and register values
Memory Layout:
- Code lives in flash memory starting at 0x10000000
- Stack lives in RAM around 0x20080000
GDB Basics: We learned the essential commands for connecting to hardware and examining code:

Command	What It Does
`target remote :3333`	Connect to OpenOCD debug server
`monitor reset halt`	Reset and halt the processor
`b main`	Set breakpoint at main function
`c`	Continue running until breakpoint
`disas`	Disassemble current function
`i r`	Show all register values

Ghidra Static Analysis: We set up a Ghidra project and analyzed our binary:
- Imported the ELF file with symbols
- Found the main function
- Saw the decompiled C code
- Understood how assembly maps to C
Little-Endian: The RP2350 stores multi-byte values with the least significant byte at the lowest address, making them appear "backwards" when viewed as a single value.

The Program Flow

┌─────────────────────────────────────────────────────┐
│  1. push {r3, lr}                                   │
│     Save registers to stack                         │
├─────────────────────────────────────────────────────┤
│  2. bl stdio_init_all                               │
│     Initialize standard I/O                         │
├─────────────────────────────────────────────────────┤
│  3. ldr r0, [pc, #8]      ────────────────┐         │
│     Load address of "hello, world" into r0│         │
├─────────────────────────────────────────────────────┤
│  4. bl __wrap_puts                        │         │
│     Print the string                      │         │
├─────────────────────────────────────────────────────┤
│  5. b.n (back to step 3)  ────────────────┘         │
│     Infinite loop!                                  │
└─────────────────────────────────────────────────────┘

✅ Practice Exercises

Try these on your own to reinforce what you learned:

Exercise 1: Explore in Ghidra

Open your 0x0001_hello-world project in Ghidra
Find the stdio_init_all function in the Symbol Tree
Look at its decompiled code - can you understand what it's setting up?

Exercise 2: Find Strings in Ghidra

In Ghidra, go to Window → Defined Strings
Look for "hello, world" - what address is it at?
Double-click to navigate to it in the listing

Exercise 3: Cross-References

In Ghidra, navigate to the main function
Find the ldr r0, [DAT_...] instruction that loads the string
Right-click on DAT_10000244 and select References → Show References to
This shows you where this data is used!

Exercise 4: Connect GDB (Preparation for Week 2)

Start OpenOCD and connect GDB as shown in Part 4
Set a breakpoint at main: b main
Continue: c
Use disas to see the assembly
Use i r to see register values

💡 Note: The detailed hands-on GDB debugging (stepping through code, watching the stack, examining memory) will be covered in Week 2!

🎓 Key Takeaways

Reverse engineering combines static and dynamic analysis - we look at the code (static with Ghidra) and run it to see what happens (dynamic with GDB).
The stack is fundamental - understanding how push/pop work is essential for following function calls.
GDB and Ghidra work together - Ghidra helps you understand the big picture, GDB lets you watch it happen live.
Assembly isn't scary - each instruction does one simple thing. Put them together and you understand the whole program!
Everything is just numbers - whether it's code, data, or addresses, it's all stored as numbers in memory.

📖 Glossary

Term	Definition
Assembly	Human-readable representation of machine code
Breakpoint	A marker that tells the debugger to pause execution
GDB	GNU Debugger - a tool for examining running programs
Hex/Hexadecimal	Base-16 number system (0-9, A-F)
Little-Endian	Storing the least significant byte at the lowest address
Microcontroller	A small computer on a single chip
Program Counter	Register that points to the next instruction
Register	Fast storage inside the processor
Stack	Memory region for temporary storage during function calls
Stack Pointer	Register that points to the top of the stack
XIP	Execute In Place - running code directly from flash

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