kprobes: Document how optimized kprobes are removed from module unload

Thomas discovered a bug where the kprobe trace tests had a race
condition where the kprobe_optimizer called from a delayed work queue
that does the optimizing and "unoptimizing" of a kprobe, can try to
modify the text after it has been freed by the init code.

The kprobe trace selftest is a special case, and Thomas and myself
investigated to see if there's a chance that this could also be a bug
with module unloading, as the code is not obvious to how it handles
this. After adding lots of printks, I figured it out. Thomas suggested
that this should be commented so that others will not have to go
through this exercise again.

Link: http://lkml.kernel.org/r/20170516145835.3827d3aa@gandalf.local.homeAcked-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
Suggested-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent b172296b
......@@ -2183,6 +2183,12 @@ static int kprobes_module_callback(struct notifier_block *nb,
* The vaddr this probe is installed will soon
* be vfreed buy not synced to disk. Hence,
* disarming the breakpoint isn't needed.
*
* Note, this will also move any optimized probes
* that are pending to be removed from their
* corresponding lists to the freeing_list and
* will not be touched by the delayed
* kprobe_optimizer work handler.
*/
kill_kprobe(p);
}
......
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