The root cause of the CWE-787 (Out-of-bounds Write) vulnerability lies in the condition within the `while` loop on line 20:

```c
while (i < sizeof(buf) && (pg - page < len) &&
      buf[i++] = *pg++;
```

Let's break down the issue:

1. **Buffer Size Calculation:** The buffer `buf` is allocated with a size of `bufsize` on line 14. However, the calculation of `bufsize` on line 6 seems illogical and potentially erroneous, as it tries to accommodate 8 times the provided `size` parameter plus some constant overheads. This might lead to an inadequate buffer size depending on the input.

2. **Loop Condition:** The loop iterates as long as three conditions are met:
   - `i < sizeof(buf)`: This part appears correct, preventing `i` from exceeding the bounds of `buf`.
   - `(pg - page < len)`: This ensures that the code doesn't read beyond the bounds of the input `page` buffer.
   - `buf[i++] = *pg++`: **This is where the problem lies.** The assignment occurs **before** the index `i` is incremented. If `i` reaches `sizeof(buf) - 1`, the assignment will write to `buf[sizeof(buf)]`, which is one byte beyond the allocated buffer.

3. **Missing Boundary Check After Assignment:**  Even if the buffer size calculation was correct, the code lacks a check after the assignment within the loop. This means that if `i` becomes equal to `sizeof(buf)` due to the assignment, the code would still proceed to the next iteration and attempt to access `buf[sizeof(buf)]` in the subsequent comparison `i < sizeof(buf)`, resulting in an out-of-bounds read.

**In essence, the vulnerability arises from the potential for the loop to perform a write operation (the assignment) when the index `i` is already at the boundary of the buffer, leading to an out-of-bounds write.**

**To fix the vulnerability:**

- Carefully re-evaluate the logic behind the `bufsize` calculation to ensure it accurately reflects the required buffer size based on the input parameters.
- Modify the loop condition to increment `i` **before** the assignment:
  ```c
  while ((pg - page < len) && (i < sizeof(buf)) && 
         (buf[i++] = *pg++));
  ```
  By moving `i++` to the end of the conditional expression, you guarantee that the assignment `buf[i++] = *pg++` will only happen if `i` is strictly less than `sizeof(buf)` **before** the increment. 
