The CWE-787 vulnerability is a "Out-of-bounds Write" vulnerability. 

The root cause of this vulnerability is that the `copy_from_sockptr` function does not properly validate the `optlen` parameter before copying data from the `optval` pointer to the `opt` variable.

In the given code slice, the `copy_from_sockptr` function is called with `sizeof(u32)` as the length parameter, which is 4 bytes. However, the `optlen` parameter is not checked to ensure that it is at least 4 bytes. 

If `optlen` is less than 4 bytes, the `copy_from_sockptr` function will still attempt to copy 4 bytes from the `optval` pointer to the `opt` variable, resulting in an out-of-bounds write. This can cause undefined behavior, including crashes, data corruption, or even code execution.

To fix this vulnerability, the code should check the `optlen` parameter before calling `copy_from_sockptr` to ensure that it is at least `sizeof(u32)` bytes. Here is an example of how the code could be modified to fix the vulnerability:

```c
if (optlen < sizeof(u32)) {
    // Handle error: optlen is too small
    return -EINVAL;
}

if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
    // Handle error: copy_from_sockptr failed
}
```

By adding this check, the code ensures that the `copy_from_sockptr` function is not called with an invalid length parameter, preventing the out-of-bounds write vulnerability.