Vsprintf

How kernel stack buffer overflow happens in C vsprintf() and how to fix it

A critical stack buffer overflow vulnerability was discovered in `sys/kern/debug.c` where the kernel's `printf()` function called a custom `vsprintf()` implementation without any length constraint on the output buffer `db_msg`. By replacing the unbounded `vsprintf()` call with a size-aware `vsnprintf()` implementation, the fix prevents crafted format strings or oversized arguments from overwriting kernel stack memory, closing a path to arbitrary kernel code execution.