Re: [PATCH 0/3][RFC] tracing/kprobes: prevent jprobes from crashingfunction graph tracer

[Masami Hiramatsu]

Frank Ch. Eigler wrote:
> Steven Rostedt<rostedt@xxxxxxxxxxx>  writes:
>
> > [...]  Jprobes and the function graph tracer use the same mechanism
> > to trace the exit of a function. Unfortunately, only one can be done
> > at a time.  The function graph tracer replaces the return address
> > with its own handler, but so does jprobes. The two are not
> > compatible. [...]
>
> What about kretprobes?  It too uses the same mechanism.

Exactly, kretprobe uses similar mechanism with func-graph tracer.

Fortunately, it doesn't cause any problem, because kretprobe doesn't
skip any function-return, and func-graph tracer(mcount) always intrudes
a function after kretprobe.

---
call func
	<-- kretprobe prehandler changes return address
	func()
	<-- func-graph prehandler changes return address
	...
	return
	--> func-graph trampoline sets ip to "kretprobe trampoline"
	--> kretprobe trampoline sets ip to "real return address"
(return address)
---

So that kretprobe handler doesn't conflict with function graph tracer.

Thank you,

--
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx

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