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

[Masami Hiramatsu <mhiramat@redhat.com>]

Steven Rostedt wrote:
> On Thu, 2009-10-29 at 18:02 -0400, Masami Hiramatsu wrote:\
>>> The solution I am proposing with this patch set is to add a call in
>>> ftrace that lets other code in the kernel permanently disable functions from
>>> being traced by the function and function graph tracer. As a probe function
>>> is registered with jprobes, it calls this new function and that entry
>>> will be removed from being traced.
>>>
>>> I tested this with this patch series and it does solve the problem.
>>>
>>> Some issues though:
>>>
>>> 1) this only works when DYNAMIC_FTRACE is enabled. We can prevent
>>> function graph tracing with jprobes when DYNAMIC_FTRACE is not
>>> enabled through Kconfig dependencies. Or have the registering of
>>> a jprobe permanently disable function graph tracing.
>>
>> IMHO, those *probe handler should be tagged as __kprobes and notrace.
> 
> Yeah, I agree. But how do you guarantee that it does. If one forgets,
> than we still have the issue. We can perhaps test to make sure the
> function is in the kprobes section. But that does not mean they will not
> be notraced. The __kprobes and notrace are no longer in the same set.
> 
>>
>>> 2) This also prevents the function tracer from being able to trace a
>>> function probe, even though the function tracer is not at issue
>>> with this bug.
>>
>> I think we can skip those user handlers, because those are irregular
>> functions and user can control (enable/disable) it.
> 
> True, but it may be nice to still trace them.

Hm, in that case, I think we can change jprobe_return() to call
f-g-tracer's return handler if needed as below;
---
static inline jprobe_return(void)
{
	implicit_function_return();	/* This executes f-g-tracer prologue */
	__jprobe_return();	/* This should be notraced */
}
---

Thank you,

-- 
Masami Hiramatsu

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

e-mail: mhiramat@redhat.com