Re: [RFC][PATCH 00/14] function_graph: Rewrite to allow multiple users

[Steven Rostedt]

On Fri, 30 Nov 2018 11:26:58 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> On Thu, 29 Nov 2018 11:46:52 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > On Thu, 29 Nov 2018 23:29:27 +0900
> > Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >   
> > > > One way to solve this is to also have a counter array that gets updated
> > > > every time the index array gets updated. And save the counter to the
> > > > shadow stack index as well. This way, we only call the return if the
> > > > counter on the stack matches what's in the counter on the counter array
> > > > for the index.    
> > > 
> > > Hmm, but we already know the current stack "header" entry when calling
> > > handlers, don't we? I thought we just calcurate out from curr_ret_stack.  
> > 
> > Basically we have this:
> > 
> >  array: | &fgraph_ops_1 | &fgraph_ops_2 | &fgraph_ops_stub | ...
> > 
> > On entry of function we do:
> >   
> 	push header(including original ret_addr) onto ret_stack

We can't put the ret_addr of the callback on the stack. What if that
ret_addr is a module, and it gets unloaded? We must not call it.

> 
> > 	for (i = 0; i < array_entries; i++) {
> > 		if (array[i]->entryfunc(...)) {
> > 			push i onto ret_stack;
> > 		}
> > 	}
> > 
> > On the return side, we do:
> > 
> > 	idx = pop ret_stack;
> > 
> > 	array[idx]->retfunc(...);  
> 
> So at this point we have the header on ret_stack, don't we? :)
> 
> Anyway, I think we may provide an API for unwinder to find correct
> original return address form ret_stack. That is OK for me.

Yes. In fact, I have something that worked for that. I'll have to test
it some more.


> > > I need only sizeof(unsigned long). If the kretprobe user requires more,
> > > it will be fall back to current method -- get an "instance" and store
> > > its address to the entry :-)  
> > 
> > Awesome, then this shouldn't be too hard to implement.  
> 
> Oops, anyway I noticed that I must store a value on each area so that we can
> identify which kretprobe is using that if there are several kretprobes on same
> function. So, kretprobe implementation will be something like below.
> 
> kretprobe_retfunc(trace, regs)
> {
> 	kp = get_kprobe(trace->func);
> 
> 	if (private == to_kretprobe(kp)) // this is directly mapped to current kprobe
> 		goto found_kretprobe;
> 
> 	if (!list_empty(&kp->list)) {	// we need to find from multiple kretprobes
> 		list_for_each_entry(kp, &kp->list, list)
> 			if (private == kp)
> 				goto found_kretprobe;
> 	}
> 
> 	// Or this must be an instance
> 	struct kretprobe_instance *ri = trace->private;
> 	rp = ri->rp;
> 	if (valid_kretprobe(rp))
> 		rp->handler(ri, regs);
> 	kretprobe_recycle_instance(ri);
> 	goto out;
> 
> found_kretprobe:
> 	struct kretprobe_instance rii = {.rp = to_kretprobe(kp),
> 		.ret_addr=trace->ret, .task = current}
> 	rp->handler(&rii, regs);
> 
> out:
> 	return 0;
> }
> 
> I think we talked about pt_regs, which is redundant for return probe, so it should
> be just a return value. (but how we pass it? trace->retval?)

Yeah, we can add that.

> That is OK for ftrace (but the transition needs more code).
> And I would like to ask ebpf and systemtap people that is OK since it will change
> the kernel ABI.

I agree.

-- Steve