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Date:   Wed, 7 Jul 2021 19:15:10 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
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        ast@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
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        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 10:20:41 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> > > Steve, can you clarify the ftrace side here? Afaict return_to_handler()
> > > is similarly affected.
> > 
> > I'm not exactly sure what the issue is. As Masami stated, kretprobe
> > uses a ret to return to the calling function, but ftrace uses a jmp.
> 
> I'll have to re-read the ftrace bits, but from the top of my head you
> cannot do an indirect jump and preserve all registers at the same time,
> so a return stub must use jump from stack aka. ret.
> 
> > kretprobe return tracing is more complex than the function graph return
> > tracing is (which is one of the issues I need to overcome to unify
> > them),
> 
> I'm not sure it is. IIRC the biggest pain point with kretprobe is that
> 'silly' property that the kretprobe_instance are not the same between
> kretprobes. Luckily, that's not actually used anywhere, so we can simply
> rip that out.

I actually don't want to keep this feature because no one use it.
(only systemtap needs it?)

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.

Thank you,


-- 
Masami Hiramatsu <mhiramat@kernel.org>