netdev - Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty

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Date:   Thu, 30 Mar 2017 15:03:11 +0200
From:   Alban Crequy <alban@...volk.io>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alban Crequy <alban.crequy@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Omar Sandoval <osandov@...com>, linux-doc@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Iago López Galeiras <iago@...volk.io>,
        Michael Schubert <michael@...volk.io>,
        Dorau Lukasz <lukasz.dorau@...el.com>, systemtap@...rceware.org
Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance
 if its free list is empty

On Thu, Mar 30, 2017 at 8:53 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
>> > So this is something I missed while the original code was merged, but the concept
>> > looks a bit weird: why do we do any "allocation" while a handler is executing?
>> >
>> > That's fundamentally fragile. What's the maximum number of parallel
>> > 'kretprobe_instance' required per kretprobe - one per CPU?
>>
>> It depends on the place where we put the probe. If the probed function will be
>> blocked (yield to other tasks), then we need a same number of threads on
>> the system which can invoke the function. So, ultimately, it is same
>> as function_graph tracer, we need it for each thread.
>
> So then put it into task_struct (assuming there's no kretprobe-inside-kretprobe
> nesting allowed). There's just no way in hell we should be calling any complex
> kernel function from kernel probes!

Some kprobes are called from an interruption context. We have a kprobe
on tcp_set_state() and this is sometimes called when the network card
receives a packet.

> I mean, think about it, a kretprobe can be installed in a lot of places, and now
> we want to call get_free_pages() from it?? This would add a massive amount of
> fragility.
>
> Instrumentation must be _simple_, every patch that adds more complexity to the
> most fundamental code path of it should raise a red flag ...
>
> So let's make this more robust, ok?
>
> Thanks,
>
>         Ingo

Thanks,
Alban