To generate the patch for the given code slice with a CWE-787 vulnerability, we can follow these reasoning steps:

### Step 1: Root Cause Analysis

- 1: Validate `size`:
  - `size` is obtained from the function parameter and then rounded up to the nearest page size using `roundup(size, PAGE_SIZE)`.
  - We need to consider what values `size` might take after rounding up.

- 2: Potential Vulnerability:
  - The vulnerability arises if `size` becomes 0 after rounding up. This could happen if the original `size` is less than `PAGE_SIZE` and rounding up results in an integer overflow or if the rounding up function has a bug that returns 0 for certain inputs.
  - If `size` is 0, then `i915_gem_alloc_object(dev, size)` might allocate an object of size 0, which could lead to an out-of-bounds write when trying to access the object.

- 3: Array Allocation:
  - The `i915_gem_alloc_object(dev, size)` function allocates memory for the object.
  - If `size` is 0, this function might return a null pointer or allocate a very small amount of memory, which could lead to an out-of-bounds write when trying to access the object.

### Conclusion:
The vulnerable behavior in this code is a potential out-of-bounds write. It occurs because the code does not check if `size` is 0 after rounding up. If `size` is 0, the `i915_gem_alloc_object(dev, size)` function might allocate an object of size 0, leading to potential memory corruption and security vulnerabilities. Proper bounds checking should be added to ensure `size` is not 0 before allocating the object.

### Step 2: Fixing Strategy

- A common fixing strategy is to add a check to ensure that `size` is not 0 before allocating the object.
- This can be done by adding a simple if statement to check if `size` is 0 and return an error code if it is.

Therefore, the patch should be:
```
<     size = roundup(size, PAGE_SIZE);
<     obj = i915_gem_alloc_object(dev, size);
---
>     size = roundup(size, PAGE_SIZE);
>     if (size == 0)
>     {
>         return -EINVAL;
>     }
>     obj = i915_gem_alloc_object(dev, size);
```
This patch adds a check to ensure that `size` is not 0 before allocating the object, preventing a potential out-of-bounds write and CWE-787 vulnerability.