Root Cause Analysis: The CWE-787 (Out-of-bounds Write) vulnerability exists in the given code because the `offset` value is calculated by adding `file->f_pos` without any bounds checking. If `file->f_pos` is a large value, the addition could cause an integer overflow, resulting in a small `offset` value. This small `offset` value could then be used to write data beyond the allocated buffer boundaries, leading to an out-of-bounds write vulnerability (CWE-787).

To mitigate this vulnerability, the code should perform proper bounds checking and handle integer overflow conditions before updating the `offset` value.