Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty

From: Ingo Molnar <[email-masked]>
Date: Thu, 30 Mar 2017 08:53:32 +0200

* Masami Hiramatsu <[email-masked]> wrote:

> > So this is something I missed while the original code was merged, but the concept 
> > looks a bit weird: why do we do any "allocation" while a handler is executing?
> > 
> > That's fundamentally fragile. What's the maximum number of parallel 
> > 'kretprobe_instance' required per kretprobe - one per CPU?
> 
> It depends on the place where we put the probe. If the probed function will be
> blocked (yield to other tasks), then we need a same number of threads on
> the system which can invoke the function. So, ultimately, it is same
> as function_graph tracer, we need it for each thread.

So then put it into task_struct (assuming there's no kretprobe-inside-kretprobe 
nesting allowed). There's just no way in hell we should be calling any complex 
kernel function from kernel probes!

I mean, think about it, a kretprobe can be installed in a lot of places, and now 
we want to call get_free_pages() from it?? This would add a massive amount of 
fragility.

Instrumentation must be _simple_, every patch that adds more complexity to the 
most fundamental code path of it should raise a red flag ...

So let's make this more robust, ok?

Thanks,

	Ingo