linux-kernel - Re: [PATCH 0/2] Enable new kprobe event at boot

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Date:   Wed, 22 May 2019 17:02:46 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 0/2] Enable new kprobe event at boot

On Wed, 22 May 2019 16:30:21 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> On Wed, 22 May 2019 00:39:32 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > > Perhaps we could enable kprobes at early init?
> > 
> > It should be possible, I will try to find what blocks it. I guess after we
> > switch early_text_poke() to text_poke(), we can use kprobes on x86. But
> > for other archs, I need to investigate more.
> 
> OK, I just follow the kernel init related to kprobes
> 
> start_kernel()
>  -> trace_init()
>  -> rcu_init_nohz()
>  -> perf_event_init()
>  -> arch_call_rest_init()
>    -> rest_init()
>      -> rcu_scheduler_starting()
>      -> kernel_thread(kernel_init)
> kernel_init()
>  -> kernel_init_freeable()
>    -> wait_for_completion(&kthreadd_done);
>    -> workqueue_init()
>    -> smp_init()
>    -> do_basic_setup()
>      -> do_initcalls()
>        -> do_initcall_level()
>          (in subsys-level)
>          -> init_kprobes()
> 
> Since optprobe uses workqueue, we can not move it before workqueue_init().
> (but maybe we can disable it in early stage)
> Also, since kprobes depends on rcu, I guess we can not move it before
> rcu_scheduler_starting().

It seems rcu is not initialized yet at this point. It is initialized fully
at core_initcall.

> for kretprobe, we need to get the possible cpus, we need a fix if we move
> it before before smp_init().
> However, there is no reason we need to run it in subsys level. We can
> move init_kprobes() in core or pure level safely.

Also, I missed ftrace (function tracer) is initialized at core_initcall().
So we can move init_kprobes() at postcore_initcall() safely.

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>