linux-kernel - Re: [PATCH 01/10] perf,x86: Fix event/group validation

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Message-ID: <CABPqkBSeLqMeGwcnaj2yss3dAY_0naQjTCWsjREFaV-h0WzWKQ@mail.gmail.com>
Date:	Thu, 21 May 2015 06:36:56 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Jiri Olsa <jolsa@...hat.com>,
	"Liang, Kan" <kan.liang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Hunter <ahh@...gle.com>,
	Maria Dimakopoulou <maria.n.dimakopoulou@...il.com>
Subject: Re: [PATCH 01/10] perf,x86: Fix event/group validation

On Thu, May 21, 2015 at 6:29 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2015-05-21 at 06:27 -0700, Stephane Eranian wrote:
>> Or are you talking about a preemption while executing x86_schedule_events()?
>
> That.
>
> And we can of course cure that by an earlier patch I send; but I find it
> a much simpler rule to just never allow modifying global state for
> validation.

I can see  validation being preempted, but not the context switch code path.
Is that what you are talking about?

You are saying validate_group() is in the middle of x86_schedule_events()
using fake_cpuc, when it gets preempted. The context switch code when it loads
the new thread's PMU state calls x86_schedule_events() which modifies the
cpuc->event_list[]->hwc. But this is cpuc vs. fake_cpuc again. So yes, the calls
nest but they do not touch the same state. And when you eventually come back
to validate_group() you are back to using the fake_cpuc. So I am still not clear
on how the corruption can happen.
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