Step 1:  CWE-190 vulnerability at line 108 is caused by an integer underflow. 

The variable `payload_size` is assigned the result of subtracting some value from `smh->submessageLength`. However, there is no check to ensure that the subtraction operation does not result in an underflow.

In C++, when you subtract a larger unsigned integer from a smaller one, the result wraps around to a large number. This can lead to unexpected behavior and potential security vulnerabilities.

In this specific case, if `smh->submessageLength` is less than the value being subtracted, `payload_size` will be assigned a large value, potentially leading to a buffer overflow or other security issues.

To fix this vulnerability, you should add a check to ensure that the subtraction operation does not result in an underflow. For example:

```cpp
if (smh->submessageLength >= /* value being subtracted */) {
    payload_size = smh->submessageLength - /* value being subtracted */;
} else {
    // handle underflow error
}
```

Alternatively, you can use a safer data type, such as `size_t` or `uint64_t`, to store the result of the subtraction operation, and check for overflow before assigning the result to `payload_size`.

It's also worth noting that the code has already checked for a minimum length at line 10:

```cpp
if (smh->submessageLength < RTPSMESSAGE_DATA_MIN_LENGTH)
```

However, this check is not sufficient to prevent the underflow vulnerability, as it only checks for a minimum length, but does not prevent the subtraction operation from resulting in an underflow.
Here are five possible patches for the vulnerability:

**Patch 1: Check for underflow**
```
<         payload_size = smh->submessageLength -
---
>         if (smh->submessageLength >= /* value being subtracted */) {
>             payload_size = smh->submessageLength - /* value being subtracted */;
>         } else {
>             // handle underflow error
>         }
```
**Patch 2: Use size_t to prevent underflow**
```
<         uint32_t payload_size;
---
>         size_t payload_size;
>         payload_size = smh->submessageLength - /* value being subtracted */;
```
**Patch 3: Use uint64_t to prevent underflow**
```
<         uint32_t payload_size;
---
>         uint64_t payload_size;
>         payload_size = smh->submessageLength - /* value being subtracted */;
```
**Patch 4: Check for underflow with a specific value**
```
<         payload_size = smh->submessageLength -
---
>         if (smh->submessageLength >= RTPSMESSAGE_OCTETSTOINLINEQOS_DATASUBMSG) {
>             payload_size = smh->submessageLength - RTPSMESSAGE_OCTETSTOINLINEQOS_DATASUBMSG;
>         } else {
>             // handle underflow error
>         }
```
**Patch 5: Use a safe subtraction function**
```
<         payload_size = smh->submessageLength -
---
>         payload_size = safe_subtract(smh->submessageLength, /* value being subtracted */);
>
>         // safe_subtract function
>         size_t safe_subtract(size_t a, size_t b) {
>             if (a < b) {
>                 // handle underflow error
>             }
>             return a - b;
>         }
```
Note that the `safe_subtract` function is just an example, and you may need to adapt it to your specific use case.