Embedded-Hacking/WEEK06/WEEK06.md

Week 6: Static Variables in Embedded Systems: Debugging and Hacking Static Variables w/ GPIO Input Basics

🎯 What You'll Learn This Week

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

• Understand the difference between regular (automatic) variables and static variables
• Know where different types of variables are stored (stack vs static storage)
• Configure GPIO pins as inputs and use internal pull-up resistors
• Read button states using gpio_get() and control LEDs based on input
• Use GDB to examine how the compiler handles static vs automatic variables
• Identify compiler optimizations by stepping through assembly
• Hack variable values and invert GPIO input/output logic using a hex editor
• Convert patched binaries to UF2 format for flashing

---

📚 Part 1: Understanding Static Variables

What is a Static Variable?

A static variable is a special kind of variable that "remembers" its value between function calls or loop iterations. Unlike regular variables that get created and destroyed each time, static variables persist for the entire lifetime of your program.

Think of it like this:

• Regular variable: Like writing on a whiteboard that gets erased after each class
• Static variable: Like writing in a notebook that you keep forever

┌─────────────────────────────────────────────────────────────────┐
│  Regular vs Static Variables                                    │
│                                                                 │
│  REGULAR (automatic):                                           │
│  ┌────────────────────────────────────────────────────────────┐ │
│  │ Loop 1: Create → Set to 42 → Increment to 43 → Destroy     │ │
│  │ Loop 2: Create → Set to 42 → Increment to 43 → Destroy     │ │
│  │ Loop 3: Create → Set to 42 → Increment to 43 → Destroy     │ │
│  │ Result: Always appears as 42!                              │ │
│  └────────────────────────────────────────────────────────────┘ │
│                                                                 │
│  STATIC:                                                        │
│  ┌────────────────────────────────────────────────────────────┐ │
│  │ Loop 1: Already exists → Read 42 → Increment → Store 43    │ │
│  │ Loop 2: Already exists → Read 43 → Increment → Store 44    │ │
│  │ Loop 3: Already exists → Read 44 → Increment → Store 45    │ │
│  │ Result: Keeps incrementing!                                │ │
│  └────────────────────────────────────────────────────────────┘ │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The static Keyword

In C, you declare a static variable by adding the static keyword:

uint8_t regular_fav_num = 42;       // Regular - recreated each time
static uint8_t static_fav_num = 42; // Static - persists forever

Where Do Variables Live in Memory?

Different types of variables are stored in different memory locations:

Variable Type	Storage Location	Lifetime	Example
Automatic (local)	Stack	Until function/block ends	uint8_t x = 5;
Static	Static Storage	Entire program lifetime	static uint8_t x = 5;
Global	Static Storage	Entire program lifetime	uint8_t x = 5; (outside functions)
Dynamic (heap)	Heap	Until free() is called	malloc(sizeof(int))

Stack vs Static Storage vs Heap

┌─────────────────────────────────────────────────────────────────┐
│  Memory Layout                                                  │
│                                                                 │
│  ┌───────────────────┐  High Address (0x20082000)               │
│  │      STACK        │  ← Automatic/local variables             │
│  │   (grows down)    │    Created/destroyed per function        │
│  ├───────────────────┤                                          │
│  │                   │                                          │
│  │    (free space)   │                                          │
│  │                   │                                          │
│  ├───────────────────┤                                          │
│  │       HEAP        │  ← Dynamic allocation (malloc/free)      │
│  │    (grows up)     │                                          │
│  ├───────────────────┤                                          │
│  │   .bss section    │  ← Uninitialized static/global vars      │
│  ├───────────────────┤                                          │
│  │   .data section   │  ← Initialized static/global vars        │
│  └───────────────────┘  Low Address (0x20000000)                │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Key Point: Static variables are NOT on the heap! They live in a fixed location in the .data section (if initialized) or .bss section (if uninitialized). This is different from heap memory which is dynamically allocated at runtime.

What Happens with Overflow?

Since static_fav_num is a uint8_t (unsigned 8-bit), it can only hold values 0-255. What happens when it reaches 255 and we add 1?

255 + 1 = 256... but that doesn't fit in 8 bits!
Binary: 11111111 + 1 = 100000000 (9 bits)
The 9th bit is lost, so we get: 00000000 = 0

This is called overflow or wrap-around. The value "wraps" back to 0 and starts counting again!

---

📚 Part 2: Understanding GPIO Inputs

Input vs Output

So far, we've used GPIO pins as outputs to control LEDs. Now we'll learn to use them as inputs to read button states!

┌─────────────────────────────────────────────────────────────────┐
│  GPIO Direction                                                 │
│                                                                 │
│  OUTPUT (what we've done before):                               │
│  ┌─────────┐                                                    │
│  │  Pico   │ ───────► LED                                       │
│  │ GPIO 16 │          (We control the LED)                      │
│  └─────────┘                                                    │
│                                                                 │
│  INPUT (new this week):                                         │
│  ┌─────────┐                                                    │
│  │  Pico   │ ◄─────── Button                                    │
│  │ GPIO 15 │          (We read the button state)                │
│  └─────────┘                                                    │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The Floating Input Problem

When a GPIO pin is set as an input but nothing is connected, it's called a floating input. The voltage on the pin is undefined and can randomly read as HIGH (1) or LOW (0) due to electrical noise.

┌─────────────────────────────────────────────────────────────────┐
│  Floating Input = Random Values!                                │
│                                                                 │
│  GPIO Pin (no connection):                                      │
│    Reading 1: HIGH                                              │
│    Reading 2: LOW                                               │
│    Reading 3: HIGH                                              │
│    Reading 4: HIGH                                              │
│    Reading 5: LOW                                               │
│    (Completely unpredictable!)                                  │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Pull-Up and Pull-Down Resistors

To solve the floating input problem, we use pull resistors:

Resistor Type	Default State	When Button Pressed
Pull-Up	HIGH (1)	LOW (0)
Pull-Down	LOW (0)	HIGH (1)

The Pico 2 has internal pull resistors that you can enable with software - no external components needed!

┌─────────────────────────────────────────────────────────────────┐
│  Pull-Up Resistor (what we're using)                            │
│                                                                 │
│     3.3V                                                        │
│       │                                                         │
│       ┴ (internal pull-up resistor)                             │
│       │                                                         │
│       ├──────► GPIO 15 (reads HIGH normally)                    │
│       │                                                         │
│     ┌─┴─┐                                                       │
│     │BTN│ ← Button connects GPIO to GND when pressed            │
│     └─┬─┘                                                       │
│       │                                                         │
│      GND                                                        │
│                                                                 │
│  Button NOT pressed: GPIO reads 1 (HIGH)                        │
│  Button PRESSED:     GPIO reads 0 (LOW)                         │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

GPIO Input Functions

Function	Purpose
gpio_init(pin)	Initialize a GPIO pin for use
gpio_set_dir(pin, GPIO_IN)	Set pin as INPUT
gpio_pull_up(pin)	Enable internal pull-up resistor
gpio_pull_down(pin)	Enable internal pull-down resistor
gpio_get(pin)	Read the current state (returns 0 or 1)

The Ternary Operator

The code uses a ternary operator to control the LED based on button state:

gpio_put(LED_GPIO, pressed ? 0 : 1);

This is a compact if-else statement:

• If pressed is true (1): output 0 (LED OFF... wait, that seems backwards!)
• If pressed is false (0): output 1 (LED ON)

Why is it inverted? Because of the pull-up resistor!

• Button released → GPIO reads 1 → pressed = 1 → output 0 → LED OFF
• Button pressed → GPIO reads 0 → pressed = 0 → output 1 → LED ON

A clearer way to write this:

gpio_put(LED_GPIO, !gpio_get(BUTTON_GPIO));

---

📚 Part 3: Understanding Compiler Optimizations

Why Does Code Disappear?

When you compile code, the compiler tries to make it faster and smaller. This is called optimization. Sometimes the compiler removes code that it thinks has no effect!

Example from our code:

while (true) {
    uint8_t regular_fav_num = 42; // Created
    regular_fav_num++;            // Incremented to 43
    // But then it's destroyed and recreated as 42 next loop!
}

The compiler sees that incrementing regular_fav_num has no lasting effect (because it's recreated as 42 each loop), so it may optimize away the increment operation entirely!

Function Inlining

Sometimes the compiler inlines functions, meaning it replaces a function call with the function's code directly.

Original code:

gpio_pull_up(BUTTON_GPIO);

What the compiler might do:

// Instead of calling gpio_pull_up, it calls the underlying function:
gpio_set_pulls(BUTTON_GPIO, true, false);

This is why when you look for gpio_pull_up in the binary, you might find gpio_set_pulls instead!

---

📚 Part 4: 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. OpenOCD installed and configured
4. GDB (arm-none-eabi-gdb) installed
5. Python installed (for UF2 conversion)
6. A serial monitor (PuTTY, minicom, or screen)
7. A push button connected to GPIO 15
8. An LED connected to GPIO 16 (or use the breadboard LED)
9. A hex editor (HxD, ImHex, or similar)
10. The sample project: 0x0014_static-variables

Hardware Setup

Connect your button like this:

• One side of button → GPIO 15
• Other side of button → GND

The internal pull-up resistor provides the 3.3V connection, so you only need to connect to GND!

┌─────────────────────────────────────────────────────────────────┐
│  Breadboard Wiring                                              │
│                                                                 │
│  Pico 2                                                         │
│  ┌──────────┐                                                   │
│  │          │                                                   │
│  │ GPIO 15  │────────┐                                          │
│  │          │        │                                          │
│  │ GPIO 16  │────────┼───► LED (with resistor to GND)           │
│  │          │        │                                          │
│  │   GND    │────────┼───┐                                      │
│  │          │        │   │                                      │
│  └──────────┘      ┌─┴─┐ │                                      │
│                    │BTN│─┘                                      │
│                    └───┘                                        │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Project Structure

Embedded-Hacking/
├── 0x0014_static-variables/
│   ├── build/
│   │   ├── 0x0014_static-variables.uf2
│   │   └── 0x0014_static-variables.elf
│   └── 0x0014_static-variables.c
└── uf2conv.py

---

🔬 Part 5: Hands-On Tutorial - Static Variables and GPIO Input

Step 1: Review the Source Code

Let's examine the static variables code:

File: 0x0014_static-variables.c

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

int main(void) {
    stdio_init_all();
    
    const uint BUTTON_GPIO = 15;
    const uint LED_GPIO = 16;
    bool pressed = 0;
    
    gpio_init(BUTTON_GPIO);
    gpio_set_dir(BUTTON_GPIO, GPIO_IN);
    gpio_pull_up(BUTTON_GPIO);
    
    gpio_init(LED_GPIO);
    gpio_set_dir(LED_GPIO, GPIO_OUT);
    
    while (true) {
        uint8_t regular_fav_num = 42;
        static uint8_t static_fav_num = 42;
        
        printf("regular_fav_num: %d\r\n", regular_fav_num);
        printf("static_fav_num: %d\r\n", static_fav_num);
        
        regular_fav_num++;
        static_fav_num++;
        
        pressed = gpio_get(BUTTON_GPIO);
        gpio_put(LED_GPIO, pressed ? 0 : 1);
    }
}

What this code does:

1. Line 6-8: Defines constants for button (GPIO 15) and LED (GPIO 16) pins
2. Line 10-12: Sets up GPIO 15 as input with internal pull-up resistor
3. Line 14-15: Sets up GPIO 16 as output for the LED
4. Line 18-19: Creates two variables:
   • regular_fav_num - a normal local variable (recreated each loop)
   • static_fav_num - a static variable (persists across loops)
5. Line 21-22: Prints both values to the serial terminal
6. Line 24-25: Increments both values
7. Line 27-28: Reads button and controls LED accordingly

Step 2: 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 0x0014_static-variables.uf2 onto the drive
5. The Pico will reboot and start running!

Step 3: Open Your Serial Monitor

Open PuTTY, minicom, or screen and connect to your Pico's serial port.

You should see output like this:

...
regular_fav_num: 42
static_fav_num: 42
regular_fav_num: 42
static_fav_num: 43
regular_fav_num: 42
static_fav_num: 44
regular_fav_num: 42
static_fav_num: 45
...

Notice the difference:

• regular_fav_num stays at 42 every time (it's recreated each loop)
• static_fav_num increases each time (it persists and remembers its value)

Step 4: Test the Button

Now test the button behavior:

• Button NOT pressed: LED should be ON (because of the inverted logic)
• Button PRESSED: LED should turn OFF

Wait... that seems backwards from what you'd expect! That's because of the pull-up resistor and the ternary operator. We'll hack this later to make it more intuitive!

Step 5: Watch for Overflow

Keep the program running and watch static_fav_num. After 255, you'll see:

static_fav_num: 254
static_fav_num: 255
static_fav_num: 0      ← Wrapped around!
static_fav_num: 1
static_fav_num: 2
...

This demonstrates unsigned integer overflow!

---

🔬 Part 6: Debugging with GDB (Dynamic Analysis)

🔄 REVIEW: This setup is identical to previous weeks. If you need a refresher on OpenOCD and GDB connection, refer back to Week 3 Part 6.

Starting the Debug Session

Terminal 1 - 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"

Terminal 2 - Start GDB:

arm-none-eabi-gdb build/0x0014_static-variables.elf

Connect to target:

(gdb) target remote :3333
(gdb) monitor reset halt

Step 6: Examine Main Function

Let's examine the main function at its entry point. First, disassemble from the start:

x/38i 0x10000234

You should see output like:

   0x10000234 <+0>:     push    {r4, lr}
   0x10000236 <+2>:     bl      0x10003014 <stdio_init_all>
   0x1000023a <+6>:     movs    r0, #15
   0x1000023c <+8>:     bl      0x10000300 <gpio_init>
   0x10000240 <+12>:    movs    r0, #15
   0x10000242 <+14>:    mov.w   r3, #0
   0x10000246 <+18>:    mcrr    0, 4, r0, r3, cr4
   0x1000024a <+22>:    movs    r2, #0
   0x1000024c <+24>:    movs    r1, #1
   0x1000024e <+26>:    bl      0x100002d8 <gpio_set_pulls>
   0x10000252 <+30>:    movs    r0, #16
   0x10000254 <+32>:    bl      0x10000300 <gpio_init>
   0x10000258 <+36>:    movs    r3, #16
   0x1000025a <+38>:    mov.w   r2, #1
   0x1000025e <+42>:    mcrr    0, 4, r3, r2, cr4
   0x10000262 <+46>:    ldr     r4, [pc, #44]   @ (0x10000290 <main+92>)
   0x10000264 <+48>:    movs    r1, #42 @ 0x2a
   0x10000266 <+50>:    ldr     r0, [pc, #44]   @ (0x10000294 <main+96>)
   0x10000268 <+52>:    bl      0x100031a4 <__wrap_printf>
   0x1000026c <+56>:    ldrb    r1, [r4, #0]
   0x1000026e <+58>:    ldr     r0, [pc, #40]   @ (0x10000298 <main+100>)
   0x10000270 <+60>:    bl      0x100031a4 <__wrap_printf>
   0x10000274 <+64>:    mov.w   r1, #3489660928 @ 0xd0000000
   0x10000278 <+68>:    ldrb    r3, [r4, #0]
   0x1000027a <+70>:    movs    r2, #16
   0x1000027c <+72>:    adds    r3, #1
   0x1000027e <+74>:    strb    r3, [r4, #0]
   0x10000280 <+76>:    ldr     r3, [r1, #4]
   0x10000282 <+78>:    ubfx    r3, r3, #15, #1
   0x10000286 <+82>:    eor.w   r3, r3, #1
   0x1000028a <+86>:    mcrr    0, 4, r2, r3, cr0
   0x1000028e <+90>:    b.n     0x10000264 <main+48>
   0x10000290 <+92>:    lsls    r0, r5, #22
   0x10000292 <+94>:    movs    r0, #0
   0x10000294 <+96>:    adds    r5, #96 @ 0x60
   0x10000296 <+98>:    asrs    r0, r0, #32
   0x10000298 <+100>:   adds    r5, #120        @ 0x78
   0x1000029a <+102>:   asrs    r0, r0, #32

Step 7: Set a Breakpoint at Main

b *0x10000234
c

GDB responds:

Breakpoint 1 at 0x10000234: file C:/Users/flare-vm/Desktop/Embedded-Hacking-main/0x0014_static-variables/0x0014_static-variables.c, line 5.
Note: automatically using hardware breakpoints for read-only addresses.
(gdb) c
Continuing.

Thread 1 "rp2350.cm0" hit Breakpoint 1, main ()
    at C:/Users/flare-vm/Desktop/Embedded-Hacking-main/0x0014_static-variables/0x0014_static-variables.c:5
5           stdio_init_all();

⚠️ Note: If GDB says The program is not being run. when you type c, the target hasn't been started yet. Use monitor reset halt first, then c to continue to your breakpoint.

Step 8: Examine the Static Variable Location

Static variables live at fixed RAM addresses. But how do we find that address? Look at the first instruction in the disassembly from Step 6:

0x10000262:    ldr r4, [pc, #44]   @ (0x10000290 <main+92>)

This loads r4 from the literal pool at address 0x10000290. The literal pool stores constants that are too large for immediate encoding — in this case, a 32-bit RAM address. Let's examine what's stored there:

(gdb) x/1wx 0x10000290
0x10000290 <main+92>:   0x200005a8

That's 0x200005a8 — the RAM address of static_fav_num! The compiler placed this address in the literal pool because it can't encode a full 32-bit address in a single Thumb instruction.

💡 Why did the disassembly at 0x10000290 show lsls r0, r5, #22 instead? Because x/i (disassemble) interprets raw data as instructions. The bytes A8 05 00 20 at that address are the little-endian encoding of 0x200005A8, but GDB's disassembler doesn't know it's data — it tries to decode it as a Thumb instruction. Using x/wx (examine as word) shows the actual value.

Step 9: Step Through the Loop

Set a breakpoint at the start of the loop and step through:

(gdb) b *0x10000264
(gdb) c

Now use si (step instruction) to execute one instruction at a time:

(gdb) si

Watch how the static variable gets loaded (ldrb), incremented (adds), and stored back (strb).

Step 10: Examine Register Values

After stepping to 0x10000262 or later, check the registers:

(gdb) i r

Pay attention to:

• r4 — Should hold 0x200005a8 (static variable's RAM address, loaded from literal pool)
• r1 — Used for printf arguments (holds 42 or the static variable value)
• r3 — Used for load/increment/store of the static variable
• pc — Program counter (current instruction address)

Step 11: Watch the Static Variable Change

Now that we know the static variable lives at 0x200005a8, examine it directly:

(gdb) x/1db 0x200005a8
0x200005a8:     42

Step through a full loop iteration (back to 0x10000264) and re-examine:

(gdb) c
(gdb) x/1db 0x200005a8
0x200005a8:     43

The value incremented from 42 to 43! Each loop iteration, the adds r3, #1 at 0x1000027c bumps it by 1, and strb r3, [r4, #0] at 0x1000027e writes it back to RAM.

Step 12: Examine GPIO State

Read the GPIO input register to see the button state:

(gdb) x/1wx 0xd0000004

The SIO GPIO input register at 0xd0000004 shows the current state of all GPIO pins. Bit 15 corresponds to our button on GPIO 15. To extract just bit 15:

(gdb) p/x (*(unsigned int *)0xd0000004 >> 15) & 1

• Returns 1 when button is not pressed (pull-up holds it HIGH)
• Returns 0 when button is pressed (connected to GND)

TRY IT!

---

🔬 Part 7: Understanding the Assembly

Now that we've explored the binary in GDB, let's make sense of the key patterns.

Step 13: Analyze the Regular Variable

In GDB, examine the code at the start of the loop:

(gdb) x/5i 0x10000262

Look for this instruction:

0x10000264:    movs r1, #42 @ 0x2a

This loads the value 0x2a (42 in decimal) directly into register r1 for the first printf call.

Key insight: The compiler optimized away the regular_fav_num variable entirely! Since it's always 42 when printed, the compiler just uses the constant 42 directly. The regular_fav_num++ after the print is also removed because it has no observable effect.

Step 14: Analyze the Static Variable

Examine the static variable operations in the second half of the loop body:

(gdb) x/10i 0x10000274

Look for the load-increment-store pattern using r4 (which holds the static variable's RAM address):

   0x10000274 <main+64>:        mov.w   r1, #3489660928 @ 0xd0000000
   0x10000278 <main+68>:        ldrb    r3, [r4, #0]
   0x1000027a <main+70>:        movs    r2, #16
   0x1000027c <main+72>:        adds    r3, #1
   0x1000027e <main+74>:        strb    r3, [r4, #0]
   0x10000280 <main+76>:        ldr     r3, [r1, #4]
   0x10000282 <main+78>:        ubfx    r3, r3, #15, #1
   0x10000286 <main+82>:        eor.w   r3, r3, #1
   0x1000028a <main+86>:        mcrr    0, 4, r2, r3, cr0
   0x1000028e <main+90>:        b.n     0x10000264 <main+48>

Note that r4 was loaded earlier at 0x10000262 via ldr r4, [pc, #44] — this pulled the static variable's RAM address (0x200005a8) from the literal pool at 0x10000290.

Key insight: The static variable lives at a fixed RAM address (0x200005a8). It's loaded, incremented, and stored back — unlike the regular variable which was optimized away!

Verify the static variable value which should be 43:

(gdb) x/1db 0x200005a8

Step 15: Analyze the GPIO Logic

Examine the GPIO input/output code:

(gdb) x/10i 0x10000274

Look for this sequence:

   0x10000274 <main+64>:        mov.w   r1, #3489660928 @ 0xd0000000
   0x10000278 <main+68>:        ldrb    r3, [r4, #0]
   0x1000027a <main+70>:        movs    r2, #16
   0x1000027c <main+72>:        adds    r3, #1
   0x1000027e <main+74>:        strb    r3, [r4, #0]
   0x10000280 <main+76>:        ldr     r3, [r1, #4]
   0x10000282 <main+78>:        ubfx    r3, r3, #15, #1
   0x10000286 <main+82>:        eor.w   r3, r3, #1
   0x1000028a <main+86>:        mcrr    0, 4, r2, r3, cr0
   0x1000028e <main+90>:        b.n     0x10000264 <main+48>

Breaking this down:

Address	Instruction	Purpose
0x10000274	mov.w r1, #0xd0000000	Load SIO (Single-cycle I/O) base address into r1
0x10000278	ldrb r3, [r4, #0]	Load static_fav_num from RAM into r3
0x1000027a	movs r2, #16	Load LED pin number (16) into r2 for later
0x1000027c	adds r3, #1	Increment static_fav_num by 1
0x1000027e	strb r3, [r4, #0]	Store incremented value back to RAM
0x10000280	ldr r3, [r1, #4]	Read GPIO input state (SIO_GPIO_IN at offset 0x04)
0x10000282	ubfx r3, r3, #15, #1	Extract bit 15 (GPIO 15 = button)
0x10000286	eor.w r3, r3, #1	XOR with 1 to invert (implements ? 0 : 1)
0x1000028a	mcrr 0, 4, r2, r3, cr0	Write r3 (button) and r2 (pin 16) to GPIO output
0x1000028e	b.n 0x10000264	Loop back to start (while (true))

💡 Notice how the compiler interleaves the static variable increment with the GPIO logic. It loads the SIO base address (r1) before doing the increment, and sets up r2 = 16 (LED pin) in between. This is called instruction scheduling — the compiler reorders instructions to avoid pipeline stalls while waiting for memory reads.

Step 16: Find the Infinite Loop

The last instruction at 0x1000028e is already covered in the table above:

0x1000028e:    b.n 0x10000264

This is an unconditional branch back to 0x10000264 (the movs r1, #42 at the top of the loop) — this is the while (true) in our code! There is no pop or bx lr to return from main because the loop never exits.

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🔬 Part 8: Hacking the Binary with a Hex Editor

Now for the fun part — we'll patch the .bin file directly using a hex editor!

💡 Why a hex editor? GDB cannot write to flash memory — the 0x10000000+ address range where program instructions live. Trying set *(char *)0x10000264 = 0x2b in GDB gives Writing to flash memory forbidden in this context. To make permanent patches that survive a power cycle, we edit the .bin file directly with a hex editor and re-flash it.

Step 17: Open the Binary in a Hex Editor

1. Open HxD (or your preferred hex editor: ImHex, 010 Editor, etc.)
2. Click File → Open
3. Navigate to C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x0014_static-variables\build\
4. Open 0x0014_static-variables.bin

Step 18: Calculate the File Offset

The binary is loaded at base address 0x10000000. To find the file offset of any address:

file_offset = address - 0x10000000

For example:

• Address 0x10000264 → file offset 0x264 (612 in decimal)
• Address 0x10000286 → file offset 0x286 (646 in decimal)

Step 19: Hack #1 — Change regular_fav_num from 42 to 43

From our GDB analysis, we know the instruction at 0x10000264 is:

movs r1, #0x2a    →    bytes: 2a 21

To change the value from 42 (0x2a) to 43 (0x2b):

1. In HxD, open C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x0014_static-variables\build\0x0014_static-variables.bin
2. Press Ctrl+G (Go to offset)
3. Enter offset: 264
4. You should see the byte 2A at this position
5. Change 2A to 2B
6. The instruction is now movs r1, #0x2b (43 in decimal)

🔍 How Thumb encoding works: In movs r1, #imm8, the immediate value is the first byte, and the opcode 21 is the second byte. So the bytes 2a 21 encode movs r1, #0x2a.

Step 20: Hack #2 — Invert the Button Logic

Understand the Encoding

From GDB, we found the eor.w r3, r3, #1 instruction at 0x10000286 that inverts the button value. Examine the exact bytes:

(gdb) x/4bx 0x10000286
0x10000286 <main+82>:   0x83    0xf0    0x01    0x03

This is the 32-bit Thumb-2 encoding of eor.w r3, r3, #1. The bytes break down as:

┌─────────────────────────────────────────────────────────────────┐
│  eor.w r3, r3, #1  →  bytes: 83 F0 01 03                        │
│                                                                 │
│  Byte 0: 0x83  ─┐                                               │
│  Byte 1: 0xF0  ─┘  First halfword (opcode + source register)    │
│  Byte 2: 0x01  ──── Immediate value (#1) ← CHANGE THIS          │
│  Byte 3: 0x03  ──── Destination register (r3)                   │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

To change eor.w r3, r3, #1 to eor.w r3, r3, #0 (making XOR do nothing):

The file offset is 0x10000286 - 0x10000000 = 0x286. The immediate byte is the 3rd byte of the instruction, so: 0x286 + 2 = 0x288.

To change eor.w r3, r3, #1 to eor.w r3, r3, #0:

1. In HxD, press Ctrl+G (Go to offset)
2. Enter offset: 288 (the third byte of the 4-byte instruction)
3. You should see the byte 01 at this position
4. Change 01 to 00

🔍 Why offset 0x288 and not 0x286? The immediate value #1 is in the third byte of the 4-byte instruction. The instruction starts at file offset 0x286, so the immediate byte is at 0x286 + 2 = 0x288.

Now the logic is permanently changed:

• Button released (input = 1): 1 XOR 0 = 1 → LED ON
• Button pressed (input = 0): 0 XOR 0 = 0 → LED OFF

This is the opposite of the original behavior!

Step 21: Save the Patched Binary

1. Click File → Save As
2. Save as 0x0014_static-variables-h.bin in the build directory
3. Close the hex editor

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🔬 Part 9: Converting and Flashing the Hacked Binary

Step 22: Convert to UF2 Format

Open a terminal and navigate to your project directory:

cd C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x0014_static-variables

Run the conversion command:

python ..\uf2conv.py build\0x0014_static-variables-h.bin --base 0x10000000 --family 0xe48bff59 --output build\hacked.uf2

What this command means:

• uf2conv.py = the conversion script (in the parent Embedded-Hacking-main directory)
• --base 0x10000000 = the XIP base address where code runs from
• --family 0xe48bff59 = the RP2350 family ID
• --output build\hacked.uf2 = the output filename

Step 23: Flash the Hacked Binary

1. Hold BOOTSEL and plug in your Pico 2
2. Drag and drop hacked.uf2 onto the RPI-RP2 drive
3. Open your serial monitor

Step 24: Verify the Hacks

Check the serial output:

regular_fav_num: 43    ← Changed from 42!
static_fav_num: 42
regular_fav_num: 43
static_fav_num: 43
...

Check the LED behavior:

• LED should now be ON by default (when button is NOT pressed)
• LED should turn OFF when you press the button

🎉 BOOM! We successfully:

1. Changed the printed value from 42 to 43
2. Inverted the LED/button logic

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📊 Part 10: Summary and Review

What We Accomplished

1. Learned about static variables - How they persist across function calls and loop iterations
2. Understood memory layout - Stack vs static storage vs heap
3. Configured GPIO inputs - Using pull-up resistors and reading button states
4. Analyzed compiled code in GDB - Saw how the compiler optimizes code
5. Discovered function inlining - gpio_pull_up became gpio_set_pulls
6. Hacked variable values - Changed 42 to 43 using a hex editor
7. Inverted GPIO logic - Made LED behavior opposite

Static vs Automatic Variables

Aspect	Automatic (Regular)	Static
Storage	Stack	Static storage (.data/.bss)
Lifetime	Block/function scope	Entire program
Initialization	Every time block entered	Once at program start
Persistence	Lost when scope exits	Retained between calls
Compiler view	May be optimized away	Always has memory location

GPIO Input Configuration

┌─────────────────────────────────────────────────────────────────┐
│  GPIO Input Setup Steps                                         │
│                                                                 │
│  1. gpio_init(pin)                   - Initialize the pin       │
│  2. gpio_set_dir(pin, GPIO_IN)       - Set as input             │
│  3. gpio_pull_up(pin)                - Enable pull-up           │
│     OR gpio_pull_down(pin)           - OR enable pull-down      │
│  4. gpio_get(pin)                    - Read the state           │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

The Binary Hacking Workflow

┌─────────────────────────────────────────────────────────────────┐
│  1. Analyze the binary with GDB                                 │
│     - Disassemble functions with x/Ni                           │
│     - Identify key instructions and addresses                   │
├─────────────────────────────────────────────────────────────────┤
│  2. Understand compiler optimizations                           │
│     - Some functions get inlined (gpio_pull_up → gpio_set_pulls)│
│     - Some variables are optimized away                         │
├─────────────────────────────────────────────────────────────────┤
│  3. Calculate file offsets                                      │
│     - file_offset = address - 0x10000000                        │
├─────────────────────────────────────────────────────────────────┤
│  4. Patch the .bin file with a hex editor                       │
│     - Open the .bin file in HxD / ImHex                         │
│     - Go to the calculated offset                               │
│     - Change the target byte(s)                                 │
├─────────────────────────────────────────────────────────────────┤
│  5. Convert to UF2                                              │
│     python uf2conv.py file.bin --base 0x10000000                │
│       --family 0xe48bff59 --output hacked.uf2                   │
├─────────────────────────────────────────────────────────────────┤
│  6. Flash and verify                                            │
│     - Hold BOOTSEL, plug in, drag UF2                           │
│     - Check serial output and button/LED behavior               │
└─────────────────────────────────────────────────────────────────┘

Key Memory Addresses

Address	Description
0x10000234	Typical main() entry point
0x10003014	stdio_init_all() function
0x200005a8	Static variable storage (example)
0xd0000000	SIO (Single-cycle I/O) base address

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✅ Practice Exercises

Exercise 1: Change Static Variable Initial Value

The static variable starts at 42. Hack the binary to make it start at 100 instead.

Hint: Find where DAT_200005a8 is initialized in the .data section.

Exercise 2: Make the LED Blink

Instead of responding to button presses, hack the binary to make the LED blink continuously.

Hint: You'll need to change the GPIO output logic to toggle instead of following button state.

Exercise 3: Reverse Engineer gpio_set_pulls

Using GDB, disassemble the gpio_set_pulls function and figure out what registers it writes to.

Hint: Look for writes to addresses around 0x40038000 (PADS_BANK0).

Exercise 4: Add a Second Static Variable

If you had two static variables, where would they be stored in memory? Would they be next to each other?

Hint: Static variables in the same compilation unit are typically placed consecutively in the .data section.

Exercise 5: Overflow Faster

The static variable overflows after 255 iterations. Can you hack it to overflow sooner?

Hint: Change the increment from +1 to +10 by modifying the adds r3,#0x1 instruction.

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🎓 Key Takeaways

1. Static variables persist - They keep their value between function calls and loop iterations.

2. Static storage ≠ heap - Static variables are in a fixed location, not dynamically allocated.

3. Compilers optimize aggressively - Regular variables may be optimized away if the compiler sees no effect.

4. Function inlining is common - gpio_pull_up becomes gpio_set_pulls in the binary.

5. Pull-up resistors invert logic - Button pressed = LOW, button released = HIGH.

6. XOR is useful for inverting - eor r3,r3,#0x1 flips a bit between 0 and 1.

7. Static variables have fixed addresses - You can find them in the .data section at known RAM addresses.

8. Overflow wraps around - A uint8_t at 255 becomes 0 when incremented.

9. UBFX extracts bits - Used to read a single GPIO pin from a register.

10. Binary patching is powerful - Change values and logic without source code!

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📖 Glossary

Term	Definition
Automatic	Variable that's created and destroyed automatically (local vars)
eor/XOR	Exclusive OR - flips bits where operands differ
Floating Input	GPIO input with undefined voltage (reads random values)
Function Inlining	Compiler replaces function call with the function's code
gpio_get	Function to read the current state of a GPIO pin
Heap	Memory area for dynamic allocation (malloc/free)
Overflow	When a value exceeds its type's maximum and wraps around
Pull-Down	Resistor that holds a pin LOW when nothing drives it
Pull-Up	Resistor that holds a pin HIGH when nothing drives it
SIO	Single-cycle I/O - fast GPIO access on RP2350
Stack	Memory area for local variables and function call frames
Static Storage	Fixed memory area for static and global variables
Static Variable	Variable declared with static that persists across calls
Ternary Operator	condition ? value_if_true : value_if_false
UBFX	Unsigned Bit Field Extract - extracts bits from a register
Varargs	Variable arguments - functions that take unlimited parameters

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🔗 Additional Resources

GPIO Input Reference

Function	Purpose
gpio_init(pin)	Initialize GPIO pin
gpio_set_dir(pin, GPIO_IN)	Set pin as input
gpio_set_dir(pin, GPIO_OUT)	Set pin as output
gpio_pull_up(pin)	Enable internal pull-up
gpio_pull_down(pin)	Enable internal pull-down
gpio_disable_pulls(pin)	Disable all pull resistors
gpio_get(pin)	Read pin state (0 or 1)
gpio_put(pin, value)	Set pin output (0 or 1)

Key Assembly Instructions

Instruction	Description
movs rN, #imm	Move immediate value to register
ldrb rN, [rM, #off]	Load byte from memory
strb rN, [rM, #off]	Store byte to memory
adds rN, #imm	Add immediate value to register
eor rN, rM, #imm	Exclusive OR (XOR) with immediate
ubfx rN, rM, #lsb, #w	Extract unsigned bit field
mcrr p0, ...	Move to coprocessor (GPIO control on RP2350)
b LABEL	Unconditional branch (jump)

Memory Map Quick Reference

Address Range	Description
0x10000000	XIP Flash (code execution)
0x20000000-200005xx	SRAM (.data section)
0x20082000	Stack top (initial SP)
0x40038000	PADS_BANK0 (pad configuration)
0xd0000000	SIO (single-cycle I/O)

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Remember: Static variables are your friends when you need to remember values across function calls. But they also make your program's behavior more complex to analyze - which is exactly why we practice reverse engineering!

Happy hacking! 🔧