Step 1:  root cause of the CWE-787 (Out-of-bounds Write) vulnerability in this code slice is:

The function copy_from_sockptr() is called to copy data from user space (optval) to kernel space (&opt) without proper size validation. Specifically:

1. The function takes sizeof(u32) as the size parameter, which is fixed at 4 bytes.

2. However, the actual size of the user-provided data (optlen) is not checked against this fixed size before the copy operation.

3. If optlen is larger than sizeof(u32), this could lead to an out-of-bounds write, potentially overwriting memory beyond the 'opt' variable.

To fix this vulnerability, the code should:

1. Check if optlen is exactly equal to sizeof(u32) before performing the copy operation.
2. Return an error if the sizes don't match.

This ensures that only the expected amount of data is copied, preventing potential buffer overflows and memory corruption.
Here are five possible patches for the vulnerability:

Patch 1:
```
<     if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
---
>     if (optlen != sizeof(u32) || copy_from_sockptr(&opt, optval, sizeof(u32))) {
```

Patch 2:
```
<     if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
---
>     if (optlen < sizeof(u32) || copy_from_sockptr(&opt, optval, sizeof(u32))) {
```

Patch 3:
```
<     if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
---
>     if (optlen != sizeof(u32)) {
>         return -EINVAL;
>     }
>     if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
```

Patch 4:
```
<     if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
---
>     if (optlen != sizeof(u32)) {
>         return -EFAULT;
>     }
>     if (copy_from_sockptr(&opt, optval, min(optlen, sizeof(u32)))) {
```

Patch 5:
```
<     if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
---
>     if (optlen < sizeof(u32)) {
>         return -EINVAL;
>     }
>     if (copy_from_sockptr(&opt, optval, sizeof(u32))) {
```

Each of these patches adds a check to ensure that the size of the input (optlen) matches or is at least as large as the size of the destination buffer (sizeof(u32)) before performing the copy operation. This prevents potential buffer overflows and ensures that only the expected amount of data is copied.