[PATCH] kprobes: Document how optimized kprobes are removed from module unload

From: Steven Rostedt
Date: Tue May 16 2017 - 14:58:42 EST

From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

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.

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 7367e0e..ac386f6 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -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.