linux-kernel - Re: Q: perf_event && task->ptrace

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Message-ID: <20110117210115.GC16856@nowhere>
Date:	Mon, 17 Jan 2011 22:01:17 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_event && task->ptrace_bps[]

On Mon, Jan 17, 2011 at 09:52:56PM +0100, Peter Zijlstra wrote:
> 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.

Yeah sorry I lost track on this and left it unanswered in the middle.
Just lemme rewalk the thread and I'm back :)
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