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

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Date:	Wed, 31 Jul 2013 18:42:28 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 Wed, 2013-07-31 at 22:40 +0200, Oleg Nesterov wrote:
> 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.

Actually I meant while in unregister_trace_probe(), it gets by the
trace_probe_is_enabled() part first, then the write succeeds (as the
event_mutex isn't taken till unregister_probe_event()). The the
unregister_probe_event fails, but the tp was freed. The event files
still reference the tp and this is where a crash can happen without this
patch set.

-- Steve


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