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Subject: Re: Q: perf_event && task->ptrace_bps[]
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Alan Stern <stern@rowland.harvard.edu>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>,
        Paul Mackerras <paulus@samba.org>, Prasad <prasad@linux.vnet.ibm.com>,
        Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org
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	 <20110117203459.GA32700@redhat.com>
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Date: Mon, 17 Jan 2011 21:52:56 +0100
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On Mon, 2011-01-17 at 21:34 +0100, Oleg Nesterov wrote:
> On 11/08, Oleg Nesterov wrote:
> >
> > I am trying to understand the usage of hw-breakpoints in arch_ptrace().
> > ptrace_set_debugreg() and related code looks obviously racy. Nothing
> > protects us against flush_ptrace_hw_breakpoint() called by the dying
> > tracee. Afaics we can leak perf_event or use the already freed memory
> > or both.
> >
> > Am I missed something?
> >
> > Looking into the git history, I don't even know which patch should be
> > blamed (if I am right), there were too many changes. I noticed that
> > 2ebd4ffb6d0cb877787b1e42be8485820158857e "perf events: Split out task
> > search into helper" moved the PF_EXITING check from find_get_context().
> > This check coould help if sys_ptrace() races with SIGKILL, but it was
> > racy anyway.
> 
> Ping.
> 
> Any idea how to fix this cleanly? May be we can reuse perf_event_mutex,
> but this looks soooo ugly. And do_exit()->flush_ptrace_hw_breakpoint()
> has the strange "FIXME:" comment which doesn't help me to understand
> what can we do.
> 
> Probably the best fix is to change this code so that the tracer owns
> ->ptrace_bps[], not the tracee. But this is not trivial, and needs a
> lot of changes in ptrace code.

Wasn't this sorted by: 8882135bcd332f294df5455747ea43ba9e6f77ad?

Or is this purely related to the ptrace muck? in which case I guess
Frederic is you man, I never looked at the hw_breakpoint stuff in
general and the ptrace bits in particular.

> I am reading perf_event.c, but all I found so far is a couple of trivial
> methods to crash the kernel via sys_perf_event_open(), will report
> tomorrow...

Ow, that's not too pretty..