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

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Date:   Wed, 17 May 2017 10:47:07 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] kprobes: Document how optimized kprobes are removed
 from module unload

On Tue, 16 May 2017 14:58:35 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> 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.
> 

OK, and I prefer this comment to move into kill_kprobe() right
before calling kill_optimized_kprobe() because that actually
does it.

Thank you,

> 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);
>  			}


-- 
Masami Hiramatsu <mhiramat@...nel.org>