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 `len`:
  - `len` is obtained from `get_kobj_path_length(kobj)`. We need to consider what values `len` might take.
  - If `len` is 0, this could lead to an out-of-bounds access or incorrect memory allocation in the `kmalloc` function.

- 2: Memory Allocation:
  - The `kmalloc` function allocates memory of size `len`.
  - If `len` is 0, `kmalloc` will return a pointer to a zero-sized allocation, which may not be what the programmer intended. This could lead to unexpected behavior or crashes when trying to access the allocated memory.

- 3: Potential Vulnerability:
  - The vulnerability arises if `len` is 0, which results in an incorrect memory allocation.
  - This is a classic example of a buffer overflow vulnerability due to incorrect allocation size. Allocating zero bytes can lead to unexpected behavior or crashes when trying to access the allocated memory.

### Conclusion:
The vulnerable behavior in this code is a buffer overflow due to incorrect allocation size. It occurs because the code does not check if `len` is 0 before allocating memory with `kmalloc`. If `len` is 0, the allocation will be incorrect, leading to potential crashes or security vulnerabilities. Proper bounds checking should be added to ensure `len` is not 0 before allocating memory.

### Step 2: Fixing Strategy

A common fixing strategy is to add a check to ensure that `len` is not 0 before allocating memory with `kmalloc`. This can be done by adding a conditional statement to check if `len` is 0 and return an error or a null pointer if it is. Therefore, the patch should be:
```
<     len = get_kobj_path_length(kobj);
<     path = kmalloc(len, gfp_mask);
---
>     len = get_kobj_path_length(kobj);
>     if (len == 0)
>     {
>         return NULL;
>     }
>     path = kmalloc(len, gfp_mask);
```
This patch adds a check to ensure that `len` is not 0 before allocating memory, preventing the potential buffer overflow vulnerability.