To generate this patch, we can follow these reasoning steps:

Step 1. Root Cause Analysis:

1. Input validation:
   - The function receives `nbuf`, `doff`, and `descsz` as parameters.
   - There's no explicit check on the size of `descsz` relative to the size of `pi`.

2. Buffer size:
   - `pi` is declared as a struct NetBSD_elfcore_procinfo.
   - Its size is fixed and determined by sizeof(pi).

3. Memory copy operation:
   - memcpy() is used to copy data from nbuf + doff to &pi.
   - The size of the copy is determined by descsz.

4. Potential vulnerability:
   - If descsz is larger than sizeof(pi), it will write beyond the bounds of pi.
   - This is a classic buffer overflow vulnerability (CWE-787).

Conclusion: The vulnerability occurs because the code assumes descsz is always less than or equal to sizeof(pi), which may not be true.

Step 2. Fixing Strategy:

To fix this vulnerability, we need to ensure that we never copy more data into pi than it can hold. A common strategy is to use the minimum of the source size and the destination buffer size.

The fix involves using the MIN macro to choose the smaller of descsz and sizeof(pi):

```c
memcpy(&pi, nbuf + doff, MIN(descsz, sizeof(pi)));
```

This ensures that:
1. If descsz is smaller than sizeof(pi), we copy only descsz bytes.
2. If descsz is larger than sizeof(pi), we copy only sizeof(pi) bytes.

This prevents any potential buffer overflow, as we never write more data than pi can hold.