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

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Date:   Tue, 16 May 2017 22:46:02 -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: Re: [PATCH] kprobes: Document how optimized kprobes are removed
 from module unload

On Wed, 17 May 2017 10:47:07 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> 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.

Actually, I can add a comment in both places. I didn't put it into
kill_kprobe() because, yes that's where it is done. I placed it in the
module handler because that's where one will look to see if module
unloading doesn't have any race conditions with the delayed handling.

-- Steve


> 
> 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);
> >  			}  
> 
>