Let's analyze the code step by step to identify the CWE-787 (Out-of-bounds Write) vulnerability:

1. Input analysis:
   - The function takes a `page` pointer, `len` (length of page), a function pointer `fun`, `priv` pointer, and `size`.
   - `bufsize` is calculated based on `size` (line 6).

2. Buffer allocation:
   - A buffer `buf` is allocated with `bufsize` (line 14).
   - `bufsize` is 1 + size * 8 + 1 + 1, which seems to be a reasonable size calculation.

3. Main loop (line 18-33):
   - Iterates through `page` until `pg - page < len`.

4. Inner loop (lines 20-22):
   - Condition: `i < sizeof(buf) && (pg - page < len) && *pg != '\0' && *pg != '\n'`
   - Action: Copies characters from `pg` to `buf[i]`

5. Vulnerability (line 20):
   - The condition `i < sizeof(buf)` is incorrect.
   - `sizeof(buf)` returns the size of the pointer (likely 4 or 8 bytes), not the allocated size of the buffer.
   - This means the loop could write far beyond the allocated buffer size.

6. Consequence:
   - The loop can write data beyond the end of the allocated buffer.
   - This can lead to memory corruption, potentially overwriting other data structures or executable code.

Conclusion:
The vulnerability is an out-of-bounds write (CWE-787) caused by using `sizeof(buf)` instead of the actual allocated size `bufsize`. This allows the loop to write beyond the buffer's bounds, potentially corrupting memory or leading to arbitrary code execution. To fix this, the condition should use `bufsize` instead of `sizeof(buf)`.