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Date:   Sat, 3 Nov 2018 13:30:21 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
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Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces
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On Sun, 4 Nov 2018 01:34:30 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > I was thinking of a bitmask that represents the handlers, and use that
> > to map which handler gets called for which shadow entry for a
> > particular task.  
> 
> Hmm, I doubt that is too complicated and not scalable. I rather like to see
> the open shadow entry...

It can scale and not too complex (I already played a little with it).
But that said, I'm not committed to it, and using the shadow stack is
also an interesting idea.

> 
> entry: [[original_retaddr][function][modified_retaddr]]
> 
> So if there are many users on same function, the entries will be like this 
> 
> [[original_return_address][function][trampoline_A]]
> [[trampline_A][function][trampoline_B]]
> [[trampline_B][function][trampoline_C]]
> 
> And on the top of the stack, there is trampline_C instead of original_return_address.
> In this case, return to trampoline_C(), it jumps back to trampline_B() and then
> it jumps back to trampline_A(). And eventually it jumps back to
> original_return_address.

Where are trampolines A, B, and C made? Do we also need to dynamically
create them? If I register multiple function tracing ones, each one
will need its own trampoline?

> 
> This way, we don't need allocate another bitmap/pages for the shadow stack.
> We only need a shadow stack for each task.
> Also, unwinder can easily find the trampline_C from the shadow stack and restores
> original_return_address. (of course trampline_A,B,C must be registered so that
> search function can skip it.)

What I was thinking was to store a count and the functions to be called:


	[original_return_address]
	[function_A]
	[function_B]
	[function_C]
	[ 3 ]

Then the trampoline that processes the return codes for ftrace (and
kretprobes and everyone else) can simply do:

	count = pop_shadow_stack();
	for (i = 0; i < count; i++) {
		func = pop_shadow_stack();
		func(...);
	}
	return_address = pop_shadow_stack();

That way we only need to register a function to the return handler and
it will be called, without worrying about making trampolines. There
will just be a single trampoline that handles all the work.

-- Steve