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SubjectRe: [PATCH v3 1/2] kretprobe: produce sane stack traces
On Sat, 3 Nov 2018 13:30:21 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> 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?
>

No, I think tramplines are very limited. currently we will only have ftrace
and kretprobe trampolines.


> > 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();

Ah, that's a good idea. I think we also have to store the called function
entry address with the number header, but basically I agree with you.

If we have a space to store a data with the function address, that is also
good to kretprobe. systemtap heavily uses "entry data" for saving some data
at function entry for exit handler.

> 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.

OK, and could you make it independent from func graph tracer, so that
CONFIG_KPROBES=y but CONFIG_FUNCTION_GRAPH_TRACER=n kernel can support
kretprobes too.

Thank you,


--
Masami Hiramatsu <mhiramat@kernel.org>

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