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

[Masami Hiramatsu <mhiramat@redhat.com>]

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@redhat.com