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

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Date:   Tue, 16 May 2017 14:58:35 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH] kprobes: Document how optimized kprobes are removed from
 module unload

From: Steven Rostedt (VMware) <rostedt@...dmis.org>

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@...utronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
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.
 				 */
 				kill_kprobe(p);
 			}