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

From: Masami Hiramatsu
Date: Wed May 17 2017 - 00:00:42 EST


On Tue, 16 May 2017 22:46:02 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Wed, 17 May 2017 10:47:07 +0900
> Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> > On Tue, 16 May 2017 14:58:35 -0400
> > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > > 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.
> > >
> >
> > 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.

OK, I see.

Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thanks,

>
> -- Steve
>
>
> >
> > Thank you,
> >
> > > 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.
> > > */
> > > kill_kprobe(p);
> > > }
> >
> >
>


--
Masami Hiramatsu <mhiramat@xxxxxxxxxx>