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Date:   Sun, 11 Jul 2021 23:09:09 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Matt Wu <wuqiang.matt@bytedance.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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        linux-ia64@vger.kernel.org,
        Abhishek Sagar <sagar.abhishek@gmail.com>,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>
Subject: Re: [PATCH -tip v8 11/13] x86/unwind: Recover kretprobe trampoline
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On Wed, 7 Jul 2021 22:42:47 +0800
Matt Wu <wuqiang.matt@bytedance.com> wrote:

> On 2021/7/7 PM9:29, Masami Hiramatsu wrote:
> > On Wed, 7 Jul 2021 19:45:30 +0900
> > Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > 
> >> On Wed, 7 Jul 2021 12:20:57 +0200
> >> Peter Zijlstra <peterz@infradead.org> wrote:
> >>
> >>> On Wed, Jul 07, 2021 at 07:15:10PM +0900, Masami Hiramatsu wrote:
> >>>
> >>>> I actually don't want to keep this feature because no one use it.
> >>>> (only systemtap needs it?)
> >>>
> >>> Yeah, you mentioned systemtap, but since that's out-of-tree I don't
> >>> care. Their problem.
> > 
> > Yeah, maybe it is not hard to update.
> > 
> >>>
> >>>> Anyway, if we keep the idea-level compatibility (not code level),
> >>>> what we need is 'void *data' in the struct kretprobe_instance.
> >>>> User who needs it can allocate their own instance data for their
> >>>> kretprobes when initialising it and sets in their entry handler.
> >>>>
> >>>> Then we can have a simple kretprobe_instance.
> >>>
> >>> When would you do the alloc? When installing the retprobe, but that
> >>> might be inside the allocator, which means you can't call the allocator
> >>> etc.. :-)
> >>
> >> Yes, so the user may need to allocate a pool right before register_kretprobe().
> >> (whether per-kretprobe or per-task or global pool, that is user's choice.)
> >>
> >>>
> >>> If we look at struct ftrace_ret_stack, it has a few fixed function
> >>> fields. The calltime one is all that is needed for the kretprobe
> >>> example code.
> >>
> >> kretprobe consumes 3 fields, a pointer to 'struct kretprobe' (which
> >> stores callee function address in 'kretprobe::kp.addr'), a return
> >> address and a frame pointer (*).
> >  > Oops, I forgot to add "void *data" for storing user data.
> > 
> 
> Should use "struct kretprobe_holder *rph", since "struct kretprobe" belongs
> to 3rd-party module (which might be unloaded any time).

Good catch. Yes, instead of 'struct kretprobe', we need to use the holder.

> User's own pool might not work if the module can be unloaded. Better manage
> the pool in kretprobe_holder, which needs no changes from user side.

No, since the 'data' will be only refered from user handler. If the kretprobe
is released, then the kretprobe_holder will clear the refernce to the 'struct
kretprobe'. Then, the user handler is never called. No one access the 'data'.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>