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Embedded-Hacking/WEEK10/WEEK10.md
Kevin Thomas 612c0b5f3a Overhall w/ slides
2026-03-15 10:40:20 -04:00

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Week 10: Conditionals in Embedded Systems: Debugging and Hacking Static & Dynamic Conditionals w/ SG90 Servo Motor PWM Basics

🎯 What You'll Learn This Week

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

  • Understand the difference between static and dynamic conditionals in C
  • Know how if/else statements and switch/case blocks work at the assembly level
  • Understand Pulse Width Modulation (PWM) and how it controls servo motors
  • Calculate PWM timing from system clock to servo pulse width
  • Identify conditional branches in Ghidra (beq, bne instructions)
  • Hack string literals and timing delays in binary files
  • Modify branch targets to change program flow
  • Create "stealth" functionality by NOP-ing out print statements
  • Understand IEEE-754 floating-point for angle calculations

📚 Part 1: Understanding Conditionals in C

What Are Conditionals?

Conditionals are programming structures that make decisions. They let your program choose different paths based on whether a condition is true or false. Think of them like a fork in the road - the program checks a condition and decides which way to go.

Two Types of Conditionals

Type Description Example
Static Condition value is known/fixed at compile time if (choice == 1) where choice never changes
Dynamic Condition value changes based on runtime input if (choice == getchar()) where user types input

📚 Part 2: Static Conditionals

What Makes a Conditional "Static"?

A static conditional is one where the outcome is predetermined because the condition variable never changes during program execution:

int choice = 1;  // This NEVER changes!

while (true) {
    if (choice == 1) {
        printf("1\r\n");      // This ALWAYS runs
    } else if (choice == 2) {
        printf("2\r\n");      // This NEVER runs
    } else {
        printf("?\r\n");      // This NEVER runs
    }
}

┌─────────────────────────────────────────────────────────────────┐
│  Static Conditional Flow                                        │
│                                                                 │
│  choice = 1 (set once, never changes)                           │
│       │                                                         │
│       ▼                                                         │
│  ┌─────────────┐                                                │
│  │ choice == 1 │────YES────► printf("1")                        │
│  └─────────────┘                                                │
│       │NO (never taken)                                         │
│       ▼                                                         │
│  ┌─────────────┐                                                │
│  │ choice == 2 │────YES────► printf("2") (never reached)        │
│  └─────────────┘                                                │
│       │NO                                                       │
│       ▼                                                         │
│  printf("?") (never reached)                                    │
│                                                                 │
│  The branching logic EXISTS but only ONE path ever executes!    │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The if/else Statement

The if/else structure checks conditions in order:

if (choice == 1) {
    // Do something if choice is 1
} else if (choice == 2) {
    // Do something if choice is 2
} else {
    // Do something for all other values
}

The switch Statement

The switch statement is another way to handle multiple conditions:

switch (choice) {
    case 1:
        printf("one\r\n");
        break;
    case 2:
        printf("two\r\n");
        break;
    default:
        printf("??\r\n");
}

Key Differences:

Feature if/else switch
Condition Any boolean expression Single variable comparison
Values Ranges, complex logic Discrete values only
Fall-through No Yes (without break)
Readability Good for 2-3 conditions Better for many conditions

📚 Part 3: Dynamic Conditionals

What Makes a Conditional "Dynamic"?

A dynamic conditional is one where the condition variable changes based on runtime input:

uint8_t choice = 0;

while (true) {
    choice = getchar();  // User types a key - VALUE CHANGES!
    
    if (choice == '1') {
        printf("1\r\n");
    } else if (choice == '2') {
        printf("2\r\n");
    } else {
        printf("??\r\n");
    }
}

┌─────────────────────────────────────────────────────────────────┐
│  Dynamic Conditional Flow                                       │
│                                                                 │
│  ┌────────────────┐                                             │
│  │ choice=getchar │◄──── User types 'x' on keyboard             │
│  └────────────────┘                                             │
│         │                                                       │
│         ▼                                                       │
│  ┌─────────────┐                                                │
│  │ choice=='1' │────YES────► printf("1"), move servo            │
│  └─────────────┘                                                │
│         │NO                                                     │
│         ▼                                                       │
│  ┌─────────────┐                                                │
│  │ choice=='2' │────YES────► printf("2"), move servo            │
│  └─────────────┘                                                │
│         │NO                                                     │
│         ▼                                                       │
│  printf("??")                                                   │
│         │                                                       │
│         └──────────────────► Loop back to getchar()             │
│                                                                 │
│  EACH iteration can take a DIFFERENT path!                      │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The getchar() Function

getchar() reads a single character from the serial terminal:

uint8_t choice = getchar();  // Waits for user to type something
  • Returns the ASCII value of the key pressed
  • '1' = 0x31, '2' = 0x32, 'x' = 0x78, 'y' = 0x79
  • Blocks (waits) until a key is pressed

📚 Part 4: Understanding PWM (Pulse Width Modulation)

What is PWM?

PWM (Pulse Width Modulation) is a technique for controlling power by rapidly switching a signal on and off. The ratio of "on time" to "off time" determines the average power delivered.

┌─────────────────────────────────────────────────────────────────┐
│  PWM Signal - 50% Duty Cycle                                    │
│                                                                 │
│  HIGH ─┐     ┌─────┐     ┌─────┐     ┌─────┐                    │
│        │     │     │     │     │     │     │                    │
│  LOW   └─────┘     └─────┘     └─────┘     └─────                │
│        ◄──T──►                                                  │
│        ON  OFF                                                  │
│                                                                 │
│  Duty Cycle = ON time / Total period = 50%                      │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

PWM for Servo Control

Servo motors use PWM differently - they care about the pulse width, not the duty cycle percentage:

┌─────────────────────────────────────────────────────────────────┐
│  Servo PWM Signal (50 Hz = 20ms period)                         │
│                                                                 │
│  0° Position (1ms pulse):                                       │
│  HIGH ─┐                                                        │
│        │ 1ms                                                    │
│  LOW   └────────────────────────────── (19ms) ──────────        │
│        ◄────────────── 20ms ─────────────────────────►          │
│                                                                 │
│  90° Position (1.5ms pulse):                                    │
│  HIGH ─────┐                                                    │
│            │ 1.5ms                                              │
│  LOW       └─────────────────────────── (18.5ms) ───────        │
│                                                                 │
│  180° Position (2ms pulse):                                     │
│  HIGH ─────────┐                                                │
│                │ 2ms                                            │
│  LOW           └───────────────────────── (18ms) ───────        │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The Magic Numbers

Angle Pulse Width PWM Ticks (at 1MHz)
0° 1000 µs 1000
90° 1500 µs 1500
180° 2000 µs 2000

📚 Part 5: PWM Timing Calculations

From 150 MHz to 50 Hz

The RP2350's system clock runs at 150 MHz (150 million cycles per second). A servo needs a 50 Hz signal (one pulse every 20 ms). How do we bridge this gap?

┌─────────────────────────────────────────────────────────────────┐
│  Clock Division Chain                                           │
│                                                                 │
│  System Clock: 150 MHz                                          │
│        │                                                        │
│        │ ÷ 150 (clock divider)                                  │
│        ▼                                                        │
│  PWM Tick Rate: 1 MHz (1 tick = 1 microsecond)                  │
│        │                                                        │
│        │ Count to 20,000 (wrap value = 19,999)                  │
│        ▼                                                        │
│  PWM Frequency: 50 Hz (20 ms period)                            │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The Math

Step 1: Clock Division

PWM Tick Rate = System Clock ÷ Divider
1,000,000 Hz = 150,000,000 Hz ÷ 150

Step 2: Frame Period

Period = (Wrap Value + 1) × Tick Duration
20 ms = 20,000 ticks × 1 µs/tick

Step 3: Pulse Width to Ticks

Ticks = Pulse Width (µs) × 1 tick/µs
1500 ticks = 1500 µs × 1

Worked Example: 90° Angle

Let's calculate what happens when we command 90°:

  1. Angle to Pulse Width:

    Pulse = MIN + (angle/180) × (MAX - MIN)
Pulse = 1000 + (90/180) × (2000 - 1000)
Pulse = 1000 + 0.5 × 1000
Pulse = 1500 µs

  2. Pulse to PWM Ticks:

    Level = 1500 µs × 1 tick/µs = 1500 ticks

  3. Hardware Timing:

    • Signal HIGH for 1500 ticks (1.5 ms)
    • Signal LOW for 18,500 ticks (18.5 ms)
    • Total period: 20,000 ticks (20 ms)

📚 Part 6: Understanding the SG90 Servo Motor

What is the SG90?

The SG90 is a small, inexpensive hobby servo motor commonly used in robotics projects:

┌─────────────────────────────────────────────────────────────────┐
│  SG90 Servo Motor                                               │
│                                                                 │
│     ┌───────────────┐                                           │
│     │    ┌─────┐    │                                           │
│     │    │ ARM │    │ ◄── Rotates 0° to 180°                    │
│     │    └──┬──┘    │                                           │
│     │       │       │                                           │
│     │  ┌────┴────┐  │                                           │
│     │  │  MOTOR  │  │                                           │
│     │  │  GEAR   │  │                                           │
│     │  │  BOX    │  │                                           │
│     │  └─────────┘  │                                           │
│     └───────┬───────┘                                           │
│             │                                                   │
│     ┌───────┼───────┐                                           │
│     │       │       │                                           │
│   ORANGE  RED    BROWN                                          │
│   Signal  VCC    GND                                            │
│   (PWM)   (5V)                                                  │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

SG90 Specifications

Parameter Value
Voltage 4.8V - 6V (typically 5V)
Rotation 0° to 180°
Pulse Width 1000µs - 2000µs
Frequency 50 Hz (20ms period)
Stall Current ~650mA (can spike to 1A+)

Wire Colors

Wire Color Function Connect To
Brown GND Ground
Red VCC 5V Power (VBUS)
Orange Signal GPIO Pin (PWM)

📚 Part 7: Power Supply Safety

⚠️ CRITICAL WARNING ⚠️

NEVER power the servo directly from the Pico's 3.3V pin!

Servos can draw over 1000mA during movement spikes. The Pico's 3.3V regulator cannot handle this and you will:

  • Cause brownouts (Pico resets)
  • Damage the Pico's voltage regulator
  • Potentially damage your USB port

Correct Power Setup

┌─────────────────────────────────────────────────────────────────┐
│  CORRECT Power Wiring                                           │
│                                                                 │
│  USB ────► VBUS (5V) ───┬──► Servo VCC (Red)                    │
│                         │                                       │
│                         └──► Capacitor (+)                      │
│                              │                                  │
│  Pico GND ──────────────┬────┴──► Capacitor (-)                 │
│                         │                                       │
│                         └──────► Servo GND (Brown)              │
│                                                                 │
│  Pico GPIO 6 ──────────────────► Servo Signal (Orange)          │
│                                                                 │
│  IMPORTANT: Use a 1000µF 25V capacitor across the servo         │
│  power to absorb current spikes!                                │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Why the Capacitor?

The 1000µF capacitor acts as a tiny battery:

  • Absorbs sudden current demands when servo moves
  • Prevents voltage drops that could reset the Pico
  • Smooths out electrical noise

📚 Part 8: Setting Up Your Environment

Prerequisites

Before we start, make sure you have:

  1. A Raspberry Pi Pico 2 board
  2. A Raspberry Pi Pico Debug Probe
  3. Ghidra installed (for static analysis)
  4. Python installed (for UF2 conversion)
  5. A serial monitor (PuTTY, minicom, or screen)
  6. An SG90 servo motor
  7. A 1000µF 25V capacitor
  8. The sample projects: 0x001d_static-conditionals and 0x0020_dynamic-conditionals

Hardware Setup

Connect your servo like this:

Servo Wire Pico 2 Pin
Brown (GND) GND
Red (VCC) VBUS (5V)
Orange (Signal) GPIO 6 
┌─────────────────────────────────────────────────────────────────┐
│  Servo Wiring with Capacitor                                    │
│                                                                 │
│  Pico 2                              SG90 Servo                 │
│  ┌──────────┐                       ┌──────────┐                │
│  │          │                       │          │                │
│  │ GPIO 6   │─────── Orange ───────►│ Signal   │                │
│  │          │                       │          │                │
│  │ VBUS(5V) │───┬─── Red ──────────►│ VCC      │                │
│  │          │   │                   │          │                │
│  │ GND      │───┼─── Brown ────────►│ GND      │                │
│  │          │   │                   └──────────┘                │
│  └──────────┘   │                                               │
│                 │    ┌─────────┐                                │
│                 └────┤ + CAP - ├──── GND                        │
│                      │ 1000µF │                                 │
│                      │  25V   │                                 │
│                      └─────────┘                                │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Project Structure

Embedded-Hacking/
├── 0x001d_static-conditionals/
│   ├── build/
│   │   ├── 0x001d_static-conditionals.uf2
│   │   └── 0x001d_static-conditionals.bin
│   ├── main/
│   │   └── 0x001d_static-conditionals.c
│   └── servo.h
├── 0x0020_dynamic-conditionals/
│   ├── build/
│   │   ├── 0x0020_dynamic-conditionals.uf2
│   │   └── 0x0020_dynamic-conditionals.bin
│   ├── main/
│   │   └── 0x0020_dynamic-conditionals.c
│   └── servo.h
└── uf2conv.py

🔬 Part 9: Hands-On Tutorial - Static Conditionals Code

Step 1: Review the Source Code

Let's examine the static conditionals code:

File: 0x001d_static-conditionals.c

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

#define SERVO_GPIO 6

int main(void) {
    stdio_init_all();
    
    int choice = 1;  // STATIC - never changes!
    
    servo_init(SERVO_GPIO);
    
    while (true) {
        // if/else conditional
        if (choice == 1) {
            printf("1\r\n");
        } else if (choice == 2) {
            printf("2\r\n");
        } else {
            printf("?\r\n");
        }
        
        // switch/case conditional
        switch (choice) {
            case 1:
                printf("one\r\n");
                break;
            case 2:
                printf("two\r\n");
                break;
            default:
                printf("??\r\n");
        }
        
        // Servo movement
        servo_set_angle(0.0f);
        sleep_ms(500);
        servo_set_angle(180.0f);
        sleep_ms(500);
    }
}

Step 2: Understand the Program Flow

Since choice = 1 and NEVER changes:

┌─────────────────────────────────────────────────────────────────┐
│  Static Conditional Execution                                   │
│                                                                 │
│  Every loop iteration:                                          │
│                                                                 │
│  1. Check if (choice == 1) → TRUE → print "1"                   │
│  2. Check switch case 1 → MATCH → print "one"                   │
│  3. Move servo to 0°                                            │
│  4. Wait 500ms                                                  │
│  5. Move servo to 180°                                          │
│  6. Wait 500ms                                                  │
│  7. Repeat forever...                                           │
│                                                                 │
│  Output always: "1" then "one" (forever)                        │
│  Servo: sweeps 0° → 180° → 0° → 180° (forever)                  │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Step 3: Flash the Binary to Your Pico 2

  1. Hold the BOOTSEL button on your Pico 2
  2. Plug in the USB cable (while holding BOOTSEL)
  3. Release BOOTSEL - a drive called "RPI-RP2" appears
  4. Drag and drop 0x001d_static-conditionals.uf2 onto the drive
  5. The Pico will reboot and start running!

Step 4: Verify It's Working

Check the serial monitor (PuTTY at 115200 baud):

1
one
1
one
1
one
...

Watch the servo:

  • It should sweep from 0° to 180° every second
  • The movement is continuous and repetitive

🔬 Part 10: Debugging with GDB (Static Conditionals)

Step 5: Start OpenOCD (Terminal 1)

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"

You should see output indicating OpenOCD connected successfully to your Pico 2 via the Debug Probe.

Step 6: Start GDB (Terminal 2)

Open a new terminal and launch GDB with the binary:

arm-none-eabi-gdb build\0x001d_static-conditionals.elf

Step 7: Connect to the Remote Target

In GDB, connect to OpenOCD:

target remote :3333

Step 8: Halt the Running Binary

Stop the processor:

monitor halt

Step 9: Examine Main Function

Disassemble around main to see the conditionals:

disassemble 0x10000234,+200

Look for comparison and branch instructions that implement the if/else and switch logic.

Step 10: Set a Breakpoint at Main

break *0x10000234

Reset and run to hit the breakpoint:

monitor reset halt
continue

Step 11: Find the Comparison Instructions

Step through and examine the comparison:

stepi 20
info registers

Look for cmp instructions that compare the choice variable (value 1).

Step 12: Examine the Printf Arguments

Before printf calls, check r0 for the format string address:

x/s $r0

You should see strings like "1\r\n" or "one\r\n".

Step 13: Watch the Servo Commands

Set a breakpoint on servo_set_angle:

break *0x10000280
continue

Check the floating-point register for the angle:

info registers s0

Step 14: Examine the Timing Delay

Set a breakpoint on sleep_ms and check the delay value:

break *0x10000290
continue
info registers r0

You should see 0x1f4 (500 decimal) for the 500ms delay.

Step 15: Step Through the Loop

Watch one complete iteration:

stepi 100
info registers

Notice how the static conditional always takes the same branch path.

Step 16: Exit GDB

When done exploring:

quit

🔬 Part 11: Setting Up Ghidra for Static Conditionals

Step 17: Start Ghidra

Open a terminal and type:

ghidraRun

Step 18: Create a New Project

  1. Click File → New Project
  2. Select Non-Shared Project
  3. Click Next
  4. Enter Project Name: 0x001d_static-conditionals
  5. Click Finish

Step 19: Import the Binary

  1. Open your file explorer
  2. Navigate to the 0x001d_static-conditionals/build/ folder
  3. Drag and drop the .bin file into Ghidra's project window

Step 20: Configure the Binary Format

Click the three dots (…) next to "Language" and:

  1. Search for "Cortex"
  2. Select ARM Cortex 32 little endian default
  3. Click OK

Click the "Options…" button and:

  1. Change Block Name to .text
  2. Change Base Address to 10000000
  3. Click OK

Step 21: Analyze the Binary

  1. Double-click on the file in the project window
  2. A dialog asks "Analyze now?" - Click Yes
  3. Use default analysis options and click Analyze

Wait for analysis to complete.

🔬 Part 12: Resolving Functions in Ghidra (Static)

Step 22: Navigate to Main

  1. Press G (Go to address) and type 10000234
  2. Right-click → Edit Function Signature
  3. Change to: int main(void)
  4. Click OK

Step 23: Resolve stdio_init_all

At address 0x10000236:

  1. Double-click on the called function
  2. Right-click → Edit Function Signature
  3. Change to: bool stdio_init_all(void)
  4. Click OK

Step 24: Resolve servo_init

Look for a function call where r0 is loaded with 0x6 (GPIO pin 6):

movs r0, #0x6      ; GPIO pin 6
bl   FUN_xxxxx     ; servo_init
  1. Right-click → Edit Function Signature
  2. Change to: void servo_init(uint pin)
  3. Click OK

Step 25: Resolve puts

Look for function calls that load string addresses into r0:

ldr  r0, =0x10001c54   ; Address of "1" string
bl   FUN_xxxxx          ; puts

How do we know it's puts?

  • It takes a single string argument
  • The hex 0x31 is ASCII "1"
  • The hex 0x0d is carriage return "\r"
  • We see "1" echoed in PuTTY
  1. Right-click → Edit Function Signature
  2. Change to: int puts(char *s)
  3. Click OK

Step 26: Resolve servo_set_angle

Look for a function that loads float constants. Inside the function, you'll find:

  • 0x7D0 (2000 decimal) - maximum pulse width
  • 0x3E8 (1000 decimal) - minimum pulse width

These are the servo pulse limits!

  1. Right-click → Edit Function Signature
  2. Change to: void servo_set_angle(float degrees)
  3. Click OK

Step 27: Resolve sleep_ms

Look for a function where r0 is loaded with 0x1f4 (500 decimal):

ldr  r0, =0x1f4    ; 500 milliseconds
bl   FUN_xxxxx     ; sleep_ms
  1. Right-click → Edit Function Signature
  2. Change to: void sleep_ms(uint ms)
  3. Click OK

🔬 Part 13: Hacking Static Conditionals

Step 28: Open the Bytes Editor

  1. Click Window → Bytes
  2. A new panel appears showing raw hex bytes
  3. Click the pencil icon to enable editing

Step 29: Hack #1 - Change "1" to "2"

Find the string "1" in memory:

  1. Double-click on the address reference for "1" (around 0x10001c54)
  2. You'll see 31 (ASCII for "1")
  3. Change 31 to 32 (ASCII for "2")

Step 30: Hack #2 - Change "one" to "fun"

Find the string "one":

  1. Navigate to the "one" string address
  2. Find bytes 6f 6e 65 ("one" in ASCII)
  3. Change to 66 75 6e ("fun" in ASCII)

ASCII Reference:

Character Hex
o 0x6f
n 0x6e
e 0x65
f 0x66
u 0x75
n 0x6e

Step 31: Hack #3 - Speed Up the Servo

Find the sleep_ms delay value:

  1. Look for 0x1f4 (500) in the code
  2. This appears TWICE (once for each sleep_ms call)
  3. Change both from f4 01 to 64 00 (100 in little-endian)

Before: 500ms delay (servo moves slowly) After: 100ms delay (servo moves FAST!)

Step 32: Export and Flash

  1. Click File → Export Program
  2. Set Format to Binary
  3. Name: 0x001d_static-conditionals-h.bin
  4. Click OK

Convert and flash:

cd C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x001d_static-conditionals
python ..\uf2conv.py build\0x001d_static-conditionals-h.bin --base 0x10000000 --family 0xe48bff59 --output build\hacked.uf2

Step 33: Verify the Hacks

Serial output now shows:

2
fun
2
fun
...

The servo now moves 5x faster! It's spinning like crazy!

🔬 Part 14: Dynamic Conditionals - The Source Code

Step 34: Review the Dynamic Code

File: 0x0020_dynamic-conditionals.c

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

#define SERVO_GPIO 6

int main(void) {
    stdio_init_all();
    
    uint8_t choice = 0;  // DYNAMIC - changes with user input!
    
    servo_init(SERVO_GPIO);
    
    while (true) {
        choice = getchar();  // Wait for keyboard input
        
        if (choice == 0x31) {       // '1'
            printf("1\r\n");
        } else if (choice == 0x32) { // '2'
            printf("2\r\n");
        } else {
            printf("??\r\n");
        }
        
        switch (choice) {
            case '1':
                printf("one\r\n");
                servo_set_angle(0.0f);
                sleep_ms(500);
                servo_set_angle(180.0f);
                sleep_ms(500);
                break;
            case '2':
                printf("two\r\n");
                servo_set_angle(180.0f);
                sleep_ms(500);
                servo_set_angle(0.0f);
                sleep_ms(500);
                break;
            default:
                printf("??\r\n");
        }
    }
}

Step 35: Understand the Dynamic Behavior

User Types Output Servo Action
'1' (0x31) "1" + "one" 0° → 180°
'2' (0x32) "2" + "two" 180° → 0°
Anything else "??" + "??" No movement

Step 36: Flash and Test

  1. Flash 0x0020_dynamic-conditionals.uf2
  2. Open PuTTY
  3. Press '1' - servo sweeps one direction
  4. Press '2' - servo sweeps the other direction
  5. Press 'x' - prints "??" and no movement

🔬 Part 15: Debugging with GDB (Dynamic Conditionals)

Step 37: Start OpenOCD (Terminal 1)

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"

You should see output indicating OpenOCD connected successfully to your Pico 2 via the Debug Probe.

Step 38: Start GDB (Terminal 2)

Open a new terminal and launch GDB with the binary:

arm-none-eabi-gdb build\0x0020_dynamic-conditionals.elf

Step 39: Connect to the Remote Target

In GDB, connect to OpenOCD:

target remote :3333

Step 40: Halt the Running Binary

Stop the processor:

monitor halt

Step 41: Examine Main Function

Disassemble around main to see the dynamic conditionals:

disassemble 0x10000234,+250

Look for the getchar call followed by comparison and branch instructions.

Step 42: Set a Breakpoint After getchar

Find where choice gets its value:

break *0x10000250

Reset and continue:

monitor reset halt
continue

Step 43: Watch the Input Value

When you press a key in PuTTY, the breakpoint hits. Check the return value:

info registers r0

If you pressed '1', you should see 0x31. If you pressed '2', you should see 0x32.

Step 44: Trace the Comparison Logic

Step through the comparison instructions:

stepi 10
info registers

Watch for cmp r0, #0x31 and cmp r0, #0x32 instructions.

Step 45: Examine the Branch Decisions

Look at the condition flags after comparison:

info registers cpsr

The zero flag (Z) determines if the branch is taken.

Step 46: Watch Different Input Paths

Continue and press different keys to see how the program takes different branches:

continue

Press '2' in PuTTY, then examine registers again.

Step 47: Examine Servo Control

Set a breakpoint on servo_set_angle:

break *0x10000280
continue

Check the angle value:

info registers s0

Step 48: Exit GDB

When done exploring:

quit

🔬 Part 16: Setting Up Ghidra for Dynamic Conditionals

Step 49: Create New Project

  1. Create project: 0x0020_dynamic-conditionals
  2. Import the .bin file
  3. Configure as ARM Cortex, base address 10000000
  4. Analyze

Step 50: Navigate to Main

Press G and go to 10000234.

Step 51: Resolve Functions

Follow the same process:

  1. main at 0x10000234 → int main(void)
  2. stdio_init_all → bool stdio_init_all(void)
  3. servo_init → void servo_init(uint pin)
  4. puts → int puts(char *s)
  5. servo_set_angle → void servo_set_angle(float degrees)
  6. sleep_ms → void sleep_ms(uint ms)

Step 52: Identify getchar

Look for a function that:

  • Returns a value in r0
  • That value is then compared against 0x31 ("1")
bl   FUN_xxxxx      ; This is getchar!
mov  r4, r0         ; Save return value
cmp  r4, #0x31      ; Compare to '1'
beq  LAB_xxxxx      ; Branch if equal
  1. Right-click → Edit Function Signature
  2. Change to: int getchar(void)
  3. Click OK

Step 53: Identify Hardware Addresses

Double-click into stdio_init_all and look for hardware addresses:

ldr  r0, =0x40070000   ; UART0 base address

Check the RP2350 datasheet Section 2.2 (Address Map):

  • 0x40070000 = UART0

This confirms it's a UART initialization function!

🔬 Part 17: Understanding Branch Instructions

ARM Branch Instructions

Instruction Meaning Condition
b Branch (always) Unconditional jump
beq Branch if Equal Zero flag set
bne Branch if Not Equal Zero flag clear
bgt Branch if Greater Than Signed greater
blt Branch if Less Than Signed less

How Conditionals Become Branches

if (choice == 0x31) {
    printf("1");
}

Becomes:

cmp  r4, #0x31      ; Compare choice to '1'
bne  skip_printf    ; If NOT equal, skip the printf
; ... printf code here ...
skip_printf:

┌─────────────────────────────────────────────────────────────────┐
│  Conditional Branch Flow                                        │
│                                                                 │
│  cmp r4, #0x31                                                  │
│       │                                                         │
│       │ (Sets flags based on r4 - 0x31)                         │
│       ▼                                                         │
│  beq target_address                                             │
│       │                                                         │
│       ├── If r4 == 0x31: Jump to target_address                 │
│       │                                                         │
│       └── If r4 != 0x31: Continue to next instruction           │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

🔬 Part 18: Advanced Hacking - Creating Stealth Commands

The Goal

We want to create secret commands that:

  1. Respond to 'x' and 'y' instead of '1' and '2'
  2. Move the servo WITHOUT printing anything
  3. Leave NO trace in the terminal

Step 54: Plan the Patches

Original behavior:

  • '1' (0x31) → prints "1" and "one", moves servo
  • '2' (0x32) → prints "2" and "two", moves servo

Hacked behavior:

  • 'x' (0x78) → moves servo SILENTLY
  • 'y' (0x79) → moves servo SILENTLY
  • '1' (0x31) → prints "1" and "one" (normal)
  • '2' (0x32) → prints "2" and "two" (normal)

Step 55: Change Comparison Values

Find the compare instructions:

cmp  r4, #0x31      ; Change to #0x78 ('x')
...
cmp  r4, #0x32      ; Change to #0x79 ('y')

Step 56: Redirect Branches to Skip Prints

For the stealth keys, we need to jump PAST the printf calls directly to the servo code.

Original flow:

compare → branch → printf("1") → printf("one") → servo code

Hacked flow:

compare 'x' → branch → [skip prints] → servo code

Change the beq target addresses:

  • Original: beq 0x10000270 (goes to printf)
  • Hacked: beq 0x1000027c (skips to servo)

Step 57: NOP Out Print Calls

NOP (No Operation) is an instruction that does nothing. We use it to "erase" code without changing the size of the binary.

ARM Thumb NOP encoding: 00 bf (2 bytes)

To NOP out a bl puts instruction (4 bytes), use 00 bf 00 bf.

Step 58: Apply All Patches

Here's the complete patch list:

Offset Original Patched Purpose
Compare1 31 78 Check for 'x' instead of '1'
Compare2 32 79 Check for 'y' instead of '2'
Branch1 target=0x270 target=0x27c Skip printf for 'x'
Branch2 target=0x29a target=0x2a6 Skip printf for 'y'
puts1 bl puts nop nop Remove print
puts2 bl puts nop nop Remove print

Step 59: Hack the Angle Value

Let's also change 180° to 30° for fun!

Original: 0x43340000 (180.0f in IEEE-754) New: 0x41f00000 (30.0f in IEEE-754)

Calculation for 30.0f:

30.0 = 1.875 × 2^4
Sign = 0
Exponent = 127 + 4 = 131 = 0x83
Mantissa = 0.875 = 0x700000

Binary: 0 10000011 11100000000000000000000
Hex: 0x41f00000
Little-endian: 00 00 f0 41

Step 60: Export and Test

  1. Export as 0x0020_dynamic-conditionals-h.bin
  2. Convert to UF2:
cd C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x0020_dynamic-conditionals
python ..\uf2conv.py build\0x0020_dynamic-conditionals-h.bin --base 0x10000000 --family 0xe48bff59 --output build\hacked.uf2
  1. Flash and test:
    • Press '1' → prints "1" and "one", servo moves
    • Press '2' → prints "2" and "two", servo moves
    • Press 'x' → NO OUTPUT, but servo moves silently!
    • Press 'y' → NO OUTPUT, but servo moves silently!

📊 Part 19: Summary and Review

What We Accomplished

  1. Learned static vs dynamic conditionals - Fixed vs runtime-determined values
  2. Understood if/else and switch/case - Two ways to branch in C
  3. Mastered PWM calculations - 150MHz to 50Hz servo signal
  4. Identified conditional branches in assembly - beq, bne, cmp instructions
  5. Hacked string literals - Changed "one" to "fun"
  6. Modified timing values - Sped up servo from 500ms to 100ms
  7. Created stealth commands - Hidden 'x' and 'y' keys
  8. NOPed out print statements - Removed logging for stealth
  9. Redirected branch targets - Changed program flow

Static vs Dynamic Summary

┌─────────────────────────────────────────────────────────────────┐
│  Static Conditionals                                            │
│  ───────────────────                                            │
│  • Variable set once, never changes                             │
│  • Same path taken every iteration                              │
│  • Compiler may optimize out dead branches                      │
│  • Example: int choice = 1; if (choice == 1)                    │
├─────────────────────────────────────────────────────────────────┤
│  Dynamic Conditionals                                           │
│  ────────────────────                                           │
│  • Variable changes based on input/sensors                      │
│  • Different paths taken based on runtime state                 │
│  • All branches must remain in binary                           │
│  • Example: choice = getchar(); if (choice == '1')              │
└─────────────────────────────────────────────────────────────────┘

PWM Calculation Summary

┌─────────────────────────────────────────────────────────────────┐
│  Servo PWM Calculation Chain                                    │
│                                                                 │
│  Angle (degrees) → Pulse Width (µs) → PWM Ticks → Servo Motion  │
│                                                                 │
│  0°   → 1000 µs → 1000 ticks → Fully counter-clockwise          │
│  90°  → 1500 µs → 1500 ticks → Center position                  │
│  180° → 2000 µs → 2000 ticks → Fully clockwise                  │
│                                                                 │
│  Formula: pulse = 1000 + (angle/180) × 1000                     │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Key Memory Addresses

Memory Address Description
0x10000234 main() function
0x40070000 UART0 hardware registers
0x1f4 500 (sleep_ms delay)
0x7D0 2000 (max pulse width)
0x3E8 1000 (min pulse width)
0x43340000 180.0f (max angle)

✅ Practice Exercises

Exercise 1: Change Servo Angle Range

Modify the servo to sweep from 45° to 135° instead of 0° to 180°.

Hint: Calculate IEEE-754 values for 45.0f and 135.0f.

Exercise 2: Add a Third Command

Add support for key '3' that moves the servo to 90° (center position).

Hint: You'll need to find space in the binary or modify existing code.

Exercise 3: Reverse the Servo Direction

Make '1' do what '2' does and vice versa.

Hint: Swap the branch targets.

Exercise 4: Speed Profile

Create fast movement for '1' (100ms) and slow movement for '2' (1000ms).

Hint: Find both sleep_ms calls and patch them differently.

Exercise 5: Complete Stealth Mode

Make ALL servo movements silent - remove ALL printf and puts calls.

Hint: NOP out every output function call.

🎓 Key Takeaways

  1. Static conditionals have fixed outcomes - The same path always executes

  2. Dynamic conditionals respond to input - Different paths based on runtime state

  3. PWM frequency = 50Hz for servos - One pulse every 20ms

  4. Pulse width encodes position - 1ms=0°, 1.5ms=90°, 2ms=180°

  5. beq = branch if equal - Jumps when comparison matches

  6. bne = branch if not equal - Jumps when comparison doesn't match

  7. NOP erases code without changing size - 00 bf in ARM Thumb

  8. Branch targets can be redirected - Change where code jumps to

  9. IEEE-754 is needed for angles - Floats have specific bit patterns

  10. Stealth requires removing ALL output - NOP out printf AND puts

📖 Glossary

Term Definition
beq Branch if Equal - ARM conditional jump
bne Branch if Not Equal - ARM conditional jump
Dynamic Conditional Condition that changes based on runtime input
Duty Cycle Percentage of time signal is HIGH
getchar() C function that reads one character from input
NOP No Operation - instruction that does nothing
PWM Pulse Width Modulation - variable duty cycle signal
SG90 Common hobby servo motor model
Static Conditional Condition with fixed/predetermined outcome
switch/case C structure for multiple discrete value comparisons
Wrap Value PWM counter maximum before reset

🔗 Additional Resources

ASCII Reference Table

Character Hex Decimal
'0' 0x30 48
'1' 0x31 49
'2' 0x32 50
'x' 0x78 120
'y' 0x79 121
'\r' 0x0d 13
'\n' 0x0a 10

IEEE-754 Common Angles

Angle IEEE-754 Hex Little-Endian Bytes
0.0 0x00000000 00 00 00 00
30.0 0x41f00000 00 00 f0 41
45.0 0x42340000 00 00 34 42
90.0 0x42b40000 00 00 b4 42
135.0 0x43070000 00 00 07 43
180.0 0x43340000 00 00 34 43

ARM Thumb NOP Encodings

Instruction Encoding Size
nop 00 bf 2 bytes
nop.w 00 f0 00 80 4 bytes

RP2350 Key Addresses

Address Peripheral
0x40070000 UART0
0x40078000 UART1
0x40050000 PWM

🚨 Real-World Implications

Why Stealth Commands Matter

The ability to create hidden commands has serious implications:

Legitimate Uses:

  • Factory test modes
  • Debugging interfaces
  • Emergency recovery features

Malicious Uses:

  • Backdoors in firmware
  • Hidden surveillance features
  • Unauthorized control of systems

Real-World Example

Imagine a drone with hacked firmware:

  • Normal keys ('1', '2') control it visibly with logging
  • Hidden keys ('x', 'y') control it with NO log entries
  • An attacker could operate the drone while security monitors show nothing

The Nuclear Fuel Rod Analogy

A fast-moving servo is like a nuclear fuel rod:

  • Both are small components with immense power
  • Both require precise control to prevent damage
  • Both can "go critical" if pushed beyond limits
  • Both teach the importance of safety margins

Remember: The techniques you learned today demonstrate how conditional logic can be manipulated at the binary level. Understanding these attacks helps us build more secure embedded systems. Always use your skills ethically and responsibly!

Happy hacking! 🔧

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