CVE-2024-38576: Buffer Copy without Checking Size of Input

In the Linux kernel, the following vulnerability has been resolved:

rcu: Fix buffer overflow in print_cpu_stall_info()

The rcuc-starvation output from print_cpu_stall_info() might overflow the buffer if there is a huge difference in jiffies difference. The situation might seem improbable, but computers sometimes get very confused about time, which can result in full-sized integers, and, in this case, buffer overflow.

Also, the unsigned jiffies difference is printed using %ld, which is normally for signed integers. This is intentional for debugging purposes, but it is not obvious from the code.

This commit therefore changes sprintf() to snprintf() and adds a clarifying comment about intention of %ld format.

Found by Linux Verification Center (linuxtesting.org) with SVACE.