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Date:   Wed, 7 Jul 2021 19:45:30 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
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        ast@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
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        linux-ia64@vger.kernel.org,
        Abhishek Sagar <sagar.abhishek@gmail.com>,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        wuqiang.matt@bytedance.com
Subject: Re: [PATCH -tip v8 11/13] x86/unwind: Recover kretprobe trampoline
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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.
> 
> > 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 (*).

* note that this frame pointer might be used for fixing up the
stack trace, but the fixup method depends on the architecture.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>