From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty
Date: Thu, 30 Mar 2017 08:53:32 +0200
Message-ID: <20170330065332.GA30148@gmail.com>
References: <149076484118.24574.7083269903420611708.stgit@devbox> <149076498222.24574.679546540523044200.stgit@devbox> <20170329063005.GA12220@gmail.com> <20170329172510.e012406497fd38a54d5069b3@kernel.org>
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Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,	Alban Crequy <alban.crequy@gmail.com>,	Alban Crequy <alban@kinvolk.io>,	Alexei Starovoitov <ast@kernel.org>,	Jonathan Corbet <corbet@lwn.net>,	Arnaldo Carvalho de Melo <acme@redhat.com>,	Omar Sandoval <osandov@fb.com>, linux-doc@vger.kernel.org,	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,	iago@kinvolk.io, michael@kinvolk.io,	Dorau Lukasz <lukasz.dorau@intel.com>, systemtap@sourceware.org
To: Masami Hiramatsu <mhiramat@kernel.org>
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* Masami Hiramatsu <mhiramat@kernel.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!

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