[$] A canary for timer-expiration functions

<p>A bug that allows an attacker to overwrite a function pointer in the kernelopens up a relativelyeasy way to compromise the kernel-doubly so, if an attacker simplyneeds to wait for the kernel use the compromised pointer. There are varioustechniques that can be used to protect kernel function pointers that areset at either compile or initialization time, but there are some pointersthat are routinely set as the kernel runs; timer completion functions are agood example. An RFC patch posted to the kernel-hardening mailing listwould add a way to detect that those function pointers have been changedin an unexpected way and to stop the kernel from executing that code.