Re: [PATCH 0/5][RFC] kprobes/ftrace: Have kprobes use ftrace onftrace nops

From: Steven Rostedt
Date: Thu Aug 11 2011 - 09:01:38 EST

On Thu, 2011-08-11 at 15:28 +0900, Masami Hiramatsu wrote:
> (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.

Note that systemtap and others will be hooking into kprobes version, not
the trace_kprobe one. If we do it in trace_kprobe, then everyone else
needs to reimplement it too. I have bigger ideas for the future of
this, and I really want to get this working. If it doesn't work for
kprobes, then it won't work for anything else.

> 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).

I've been told that doing the addr+=5 (which is also arch specific) can
break things for other tools.

-- Steve

> ... are there any other cons? :)
> Thank you,

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