Re: [PATCH 0/5][RFC] kprobes/ftrace: Have kprobes use ftrace on ftracenops

From: Masami Hiramatsu
Date: Thu Aug 11 2011 - 02:28:59 EST

(2011/08/11 9:34), Steven Rostedt wrote:
> On Thu, 2011-08-11 at 09:21 +0900, Masami Hiramatsu wrote:
>> Hi Steven,
>> Thanks for this nice feature!
>> (2011/08/11 1:22), Steven Rostedt wrote:
>>> Hi All,
>>> I started working on adding the -mfentry switch to ftrace, which
>>> allows us to remove the frame pointers requirement from function tracing
>>> as well as makes mcount (fentry) work just better.
>>> But when I did this in another branch, I noticed that I broke kprobes
>>> in its most common usage. The attaching a probe at the beginning of
>>> a function to use get to its parameters.
>>> So I started this branch. This branch is to have kprobes use ftrace
>>> directly when a probe is attached to a ftrace nop. Currently, kprobes
>>> will just error when that happens. With this patch set, it will hook
>>> into the ftrace infrastructure and use ftrace instead. This is more
>>> like an optimized probe as no breakpoints need to be set. A call to
>>> the function is done directly via the mcount trampoline. If ftrace
>>> pt_regs is implemented for an arch, kprobes gets this feature for free.
>> I agreed this idea, this looks good to me too :)
>> With -fentry, this can improve dynamic trace events very much.
>> BTW (OT), it seems that current kprobe data structure becomes a bit
>> fat. Maybe what we need is just a "holder of hooking handler" as
>> what ftrace provides, not a full storage data structure of copied
>> instrucutions. Perhaps, we'd better diet the kprobe structure for
>> transparency of hooking infrastructure.
> Sure, I can make the ftrace_ops field in kprobes dynamically allocated
> instead. That shouldn't be an issue.

By the way (again), perhaps, much simpler solution is using ftrace
not in kprobe, but in the trace_kprobe. Of course, there are several
pros and cons...

The pros:
- Arch independent solution (anyway, ftrace still needs passing pt_regs
to their handler)
- Don't need to introduce more complexity into kprobes itself.
- Maybe systemtap also can catch up with this as using same method.

The cons:
- Native kprobes users will be disappointed... anyway, they just need to
move their probes to the next instruction (usually addr+=5 is OK).

... are there any other cons? :)

Thank you,

Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
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