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Date:   Sun, 4 Nov 2018 01:34:30 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
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Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces
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On Sat, 3 Nov 2018 09:13:41 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sat, 3 Nov 2018 22:00:12 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > On Fri, 2 Nov 2018 12:13:07 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> 
> > > Because that means if function graph tracer is active, then you can't
> > > do a kretprobe, and vice versa. I'd really like to have it working for
> > > multiple users, then we could trace different graph functions and store
> > > them in different buffers. It would also allow for perf to use function
> > > graph tracer too.  
> > 
> > Steve, how woul you allow multiple users on it? Something like this?
> > 
> > ret_trampoline_multiple(){
> >    list_for_each(handler, &shadow_entry[i].handlers, list)
> > 	handler(shadow_entry[i]);
> >    restore_retval_and_jump_to(shadow_entry[i].orig);
> > }
> >
> 
> Something like that. But since it's not a single mapping between shadow
> entries and handlers, that is we have multiple tasks with multiple
> shadow entries and multiple handlers, we can't use a link list (two
> different parents per handler).

Yes, I understand it.

> 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...

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.

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.)

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>