# 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 2: Invert the Temperature Reading

#### Objective
Using GDB to locate the IEEE-754 scaling constant `0.1f` at file offset `0x42C`, patch it to `-0.1f` using a hex editor, and verify on hardware that the serial output now displays negative temperature values.

#### Prerequisites
- Completed Week 9 tutorial (GDB, Ghidra, 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 DHT11 driver uses a scaling constant of `0.1f` stored at address `0x1000042C` (file offset `0x42C`) to convert raw sensor data into human-readable values. By changing this constant to `-0.1f`, you will invert the decimal component of the temperature calculation, causing the reported temperature to drop. This exercise teaches IEEE-754 float encoding and how a single 4-byte patch can dramatically change sensor behavior.

#### 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: Verify the Current Scaling Constant

Examine the float constant at the known address:

```gdb
(gdb) x/wx 0x1000042c
```

Output:
```
0x1000042c: 0x3dcccccd
```

This is `0.1f` in IEEE-754 encoding (approximately — the repeating binary fraction makes it `0x3dcccccd`).

##### Step 3: Understand the IEEE-754 Encoding

**Current value (0.1f):**

| Field    | Bits       | Value |
| -------- | ---------- | ----- |
| Sign     | `0`        | Positive |
| Exponent | `01111011` | 123 (biased) |
| Mantissa | `10011001100110011001101` | ~1.6 |

**New value (-0.1f):**
- Flip only the sign bit (bit 31) from `0` to `1`
- `0x3dcccccd` → `0xbdcccccd`

| Value  | Hex          | Little-Endian Bytes |
| ------ | ------------ | ------------------- |
| 0.1f   | `0x3dcccccd` | `cd cc cc 3d`       |
| -0.1f  | `0xbdcccccd` | `cd cc cc bd`       |

> 💡 **Key insight:** To negate an IEEE-754 float, you only need to flip the most significant bit. In little-endian, this is the **last** byte — change `3d` to `bd`.

##### Step 4: 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 offset: `42C`
3. You should see: `cd cc cc 3d` (or `cc cc cc 3d`)
4. Replace with: `cd cc cc bd` (or `cc cc cc bd`)

> ⚠️ **Note:** The exact bytes may be `cc cc cc 3d` or `cd cc cc 3d` depending on compiler rounding. Just change the last byte from `3d` to `bd`.

##### Step 5: 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 6: 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:**
- All operator values remain unchanged (50, 5, 0, 0, 12, 11)
- Temperature should now display with an inverted decimal component
- Humidity will also be affected (same constant is shared)

#### Expected Output

After completing this exercise, you should be able to:
- Decode and encode IEEE-754 floating-point values
- Understand that flipping one bit (sign bit) negates a float
- Patch floating-point constants in compiled binaries
- Predict how a constant change propagates through calculations

#### Questions for Reflection

###### Question 1: Why does changing one byte (`3d` → `bd`) negate the entire float value? What does the sign bit (bit 31) control in IEEE-754?

###### Question 2: The scaling constant `0.1f` is used by BOTH the humidity and temperature `vfma.f32` instructions. Why does patching this single constant affect both readings?

###### Question 3: If you wanted to change the constant to `0.5f` (`0x3f000000`, little-endian `00 00 00 3f`) instead of `-0.1f`, how would the temperature reading change? If the raw decimal part is 8, what would the new output be?

###### Question 4: Could you achieve negative temperature by patching the `vfma.f32` instruction itself instead of the constant? What instruction might you replace it with?

#### Tips and Hints
- IEEE-754 sign bit is the MSB (bit 31) — `0` = positive, `1` = negative
- In little-endian, the sign bit is in the **last** (highest address) byte
- Use an online IEEE-754 converter to verify your understanding
- If the output looks completely wrong (NaN, inf), you may have changed the wrong byte — start over with a fresh copy of the `.bin` file
