# Embedded Systems Reverse Engineering
[Repository](https://github.com/mytechnotalent/Embedded-Hacking)

## Week 9
Operators in Embedded Systems: Debugging and Hacking Operators w/ DHT11 Temperature & Humidity Sensor Single-Wire Protocol Basics

### Exercise 1: Change the Sleep Duration

#### Objective
Find the `sleep_ms(2000)` call in the `0x001a_operators` binary using GDB, identify the immediate value `0x7d0` (2000) being loaded into `r0`, calculate the file offset, patch it to `0x1388` (5000) using a hex editor, and verify on hardware that the serial output now prints every 5 seconds instead of every 2 seconds.

#### Prerequisites
- Completed Week 9 tutorial (GDB and hex editor sections)
- `0x001a_operators.elf` and `0x001a_operators.bin` available in your build directory
- GDB (`arm-none-eabi-gdb`) and OpenOCD installed
- A hex editor (HxD, ImHex, or similar)
- Python installed (for UF2 conversion)
- Raspberry Pi Pico 2 with DHT11 sensor connected

#### Task Description
The program calls `sleep_ms(2000)` at the end of its main loop, causing a 2-second delay between each set of serial output. The value `2000` (`0x7D0`) is loaded into register `r0` before the `bl sleep_ms` call. You will locate this value in the disassembly, find the corresponding bytes in the `.bin` file, and patch them to `5000` (`0x1388`) so the loop runs every 5 seconds instead.

#### Step-by-Step Instructions

##### Step 1: Start the Debug Session

**Terminal 1 - Start OpenOCD:**

```powershell
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:**

```powershell
arm-none-eabi-gdb build\0x001a_operators.elf
```

**Connect to target:**

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

##### Step 2: Find the sleep_ms Call

Disassemble main and look for the `sleep_ms` call:

```gdb
(gdb) x/60i 0x10000234
```

Look for an instruction pattern like:

```
ldr  r0, =0x7d0     ; 2000 milliseconds
bl   sleep_ms
```

The value `0x7d0` will be loaded from the literal pool.

##### Step 3: Examine the Literal Pool Value

Once you find the `ldr r0, [pc, #offset]` instruction, examine the literal pool entry it references:

```gdb
(gdb) x/wx <literal_pool_address>
```

You should see `0x000007d0` (2000 in hex).

##### Step 4: Calculate the File Offset

```
file_offset = literal_pool_address - 0x10000000
```

Note the file offset of the 4-byte word containing `0x7d0`.

##### Step 5: Encode the New Value

The new value `5000` in hex is `0x1388`. In little-endian byte order:

| Value | Hex        | Little-Endian Bytes |
| ----- | ---------- | ------------------- |
| 2000  | `0x000007D0` | `D0 07 00 00`       |
| 5000  | `0x00001388` | `88 13 00 00`       |

##### Step 6: Patch with HxD

1. In HxD, open `C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x001a_operators\build\0x001a_operators.bin`
2. Press **Ctrl+G** and enter the file offset you calculated
3. You should see: `D0 07 00 00`
4. Replace with: `88 13 00 00`

##### Step 7: Save and Convert

1. Click **File** → **Save As** → `0x001a_operators-h.bin`

```powershell
cd C:\Users\flare-vm\Desktop\Embedded-Hacking-main\0x001a_operators
python ..\uf2conv.py build\0x001a_operators-h.bin --base 0x10000000 --family 0xe48bff59 --output build\hacked.uf2
```

##### Step 8: Flash and Verify

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

**Check the serial monitor:**
- Output should now appear every **5 seconds** instead of every 2 seconds
- All operator values should remain unchanged (50, 5, 0, 0, 12, 11)

#### Expected Output

After completing this exercise, you should be able to:
- Locate literal pool values referenced by `ldr` instructions
- Understand little-endian byte ordering for 32-bit values
- Patch timing constants in embedded firmware
- Calculate file offsets from memory addresses

#### Questions for Reflection

###### Question 1: The value `2000` is stored in the literal pool as a 32-bit word, not as an immediate in the instruction. Why can't `2000` be encoded as an immediate in a single `movs` instruction?

###### Question 2: If you wanted to change the delay to exactly 1 second (1000ms = `0x3E8`), what bytes would you write in little-endian order?

###### Question 3: The literal pool value is shared — could other code in the binary also reference this same `0x7D0` value? How would you check?

###### Question 4: What would happen if you changed the sleep value to `0` (`00 00 00 00`)? Would the program crash or just run extremely fast?

#### Tips and Hints
- `movs` can only encode immediates 0-255; values larger than 255 must be loaded from the literal pool via `ldr`
- Always verify the bytes BEFORE patching — make sure you see `D0 07 00 00` at your calculated offset
- The literal pool is typically right after the function's `b.n` (loop branch) instruction
- Use a stopwatch or count "one-one-thousand" to verify the timing change
