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

[Masami Hiramatsu]

Frederic Weisbecker wrote:
> On Mon, Nov 02, 2009 at 10:02:23AM -0500, Masami Hiramatsu wrote:
> > Frederic Weisbecker wrote:
> > > I'm not sure I've well understood how is performed the call to the jprobe
> > > handler.
> > > But if I understand well we have:
> > >
> > > 	func() {
> > > 		int3() {
> > > 			jprobe_handler() {
> > > 				(-)
> > > 				set ip after iret to user_handler()
> > > 			}
> > > 		}
> > > 		user_handler() {
> > > 			jprobe_return() {
> > > 				(+)
> > > 				int3() {
> > > 					set ip after iret to func+...()
> > > 				}
> > > 			       |
> > > 			       |
> > > 			       |
> > >                   <--------------
> > >                   (execute the rest of func())
> > > 	}
> > >
> > > If we replace (-) with pause_graph_tracing() and (+) with
> > > unpause_graph_tracing(), this should do the trick...I hope.
> >
> > I'm not so sure about pause_graph_tracing(), however, it seems that
> > int3() and jprobe_handler() already pushed on the stack of the
> > func graph tracer at (-). If it's true, where are those entries
> > popped up?
>
>
> pause_graph_tracing() will disable the tracing for the current task
> but it won't disable the address pop from stack.
>
> If the above jprobe scheme is right, the scenario will be:
>
>        func() {
>                /* push func ret */
>                int3() {
>                        /* push do_trap ret */
>                        jprobe_handler() {
>                                /* push jprobe_handler ret */
>                                pause_graph_tracing();
>                                set ip after iret to user_handler()
>                        } /* pop jprobe_handler ret */
>                } /* pop do_trap ret */
>                user_handler() {
>                        jprobe_return() {
>                                unpause_graph_tracing()
>                                int3() {
>                                       /* push do_trap ret */
>                                        set ip after iret to func+...()
>                                } /* pop do_trap ret */
>                               |
>                               |
>                               |
>                 <--------------
>                 (execute the rest of func())
>        } /* pop func ret */
>
>
> Hmm?

Oh, I see. That should work. :-)

Thank you,

--
Masami Hiramatsu

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

e-mail: mhiramat@xxxxxxxxxx

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