To generate the patch for the CWE-190 vulnerability in the given code slice, we can follow these reasoning steps:

### Step 1: Root Cause Analysis

- 1: Validate `bson_buffer_size` function:
  - The function `bson_buffer_size` is supposed to return the size of the BSON buffer.
  - The calculation `(b->cur - b->data + 1)` is used to determine the size.

- 2: Potential Integer Overflow:
  - The calculation involves subtracting two pointers (`b->cur` and `b->data`) and adding 1.
  - If the result of the subtraction is close to the maximum value that can be represented by an `int`, adding 1 could cause an integer overflow.
  - This is because the result of the subtraction is assigned to an `int` variable, which has a limited range.

- 3: Vulnerability:
  - The vulnerability arises if the result of the calculation exceeds the maximum value that can be represented by an `int`.
  - This could lead to incorrect results or undefined behavior.

### Conclusion:
The vulnerable behavior in this code is an integer overflow. It occurs because the result of the calculation is assigned to an `int` variable, which has a limited range. If the result exceeds this range, an integer overflow occurs, leading to incorrect results or undefined behavior.

### Step 2: Fixing Strategy

- A common fixing strategy for integer overflow vulnerabilities is to use a data type that can represent larger values.
- In this case, we can change the return type of the `bson_buffer_size` function from `int` to `size_t`.
- `size_t` is an unsigned integer type that is large enough to represent the size of any object in memory.
- By using `size_t`, we can avoid the integer overflow vulnerability and ensure that the function returns the correct size of the BSON buffer.

Therefore, the patch should be:
```
< MONGO_EXPORT int bson_buffer_size( const bson *b ) {
---
> MONGO_EXPORT size_t bson_buffer_size( const bson *b ) {
```
This change ensures that the function returns a value that can represent the size of the BSON buffer without overflowing.