Re: [PATCH 0/2] kprobes: Fix kretprobe incorrect stacking order problem

[Andrea Righi]

On Mon, Jan 07, 2019 at 04:28:33PM -0500, Steven Rostedt wrote:
> On Mon, 7 Jan 2019 22:19:04 +0100
> Andrea Righi <righi.andrea@xxxxxxxxx> wrote:
> 
> > > > If we put a kretprobe to raw_spin_lock_irqsave() it looks like
> > > > kretprobe is going to call kretprobe...  
> > > 
> > > Right, but we should be able to add some recursion protection to stop
> > > that. I have similar protection in the ftrace code.  
> > 
> > If we assume that __raw_spin_lock/unlock*() are always inlined a
> 
> I wouldn't assume that.
> 
> > possible way to prevent this recursion could be to use directly those
> > functions to do locking from the kretprobe trampoline.
> > 
> > But I'm not sure if that's a safe assumption... if not I'll see if I can
> > find a better solution.
> 
> All you need to do is have a per_cpu variable, where you just do:
> 
> 	preempt_disable_notrace();
> 	if (this_cpu_read(kprobe_recursion))
> 		goto out;
> 	this_cpu_inc(kprobe_recursion);
> 	[...]
> 	this_cpu_dec(kprobe_recursion);
> out:
> 	preempt_enable_notrace();
> 
> And then just ignore any kprobes that trigger while you are processing
> the current kprobe.
> 
> Something like that. If you want (or if it already happens) replace
> preempt_disable() with local_irq_save().

Oh.. definitely much better. I'll work on that and send a new patch.
Thanks for the suggestion!

-Andrea