linux-kernel - Re: [RFC][PATCH 3/4] tracing/kprobes: Fail to unregister if probe event files are open

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Message-ID: <20130731204003.GA30188@redhat.com>
Date:	Wed, 31 Jul 2013 22:40:03 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 3/4] tracing/kprobes: Fail to unregister if probe
	event files are open

On 07/31, Steven Rostedt wrote:
>
> On Wed, 2013-07-03 at 23:33 -0400, Steven Rostedt wrote:
> > The above will corrupt the kprobe system, as the write to the enable
> > file will happen after the kprobe was deleted.
>
> Oleg,
>
> The above no longer triggers the bug due to your changes. The race is
> much tighter now

Yes, the changelog should be updated...

> and requires a process with the enable file opened and
> races with a write to enable it where the removal of the trace file
> checks the trace disabled, sees that it is, continues, but then the
> write enables it just as it gets deleted.

This should be fine. Either event_remove() path takes event_mutex
first and then ->write() fails, or ftrace_event_enable_disable()
actually disables this even successfully.

> Do you know of a way to trigger this bug?

I'll try to think more tomorrow, but most probably no. The race is
unlikely.

We need perf_trace_event_init() or ":enable_event:this-event" right
before trace_remove_event_call() takes the mutex.

And right after the caller (kprobes) checks "disabled".

> Hmm, what happens without this patch now? If it is active, and we delete
> it? It will call back into the kprobes and access a tracepoint that does
> not exist? Would this cause a crash?

I think yes, the crash is possible.

perf or FL_SOFT_MODE, this call/file has the external references, and
we are going to free it.

Oleg.

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