netdev - Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY maps trace data output when perf sampling

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Message-ID: <20151021140921.GI3604@twins.programming.kicks-ass.net>
Date:	Wed, 21 Oct 2015 16:09:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	pi3orama <pi3orama@....com>
Cc:	"Wangnan (F)" <wangnan0@...wei.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	xiakaixu <xiakaixu@...wei.com>, davem@...emloft.net,
	acme@...nel.org, mingo@...hat.com, masami.hiramatsu.pt@...achi.com,
	jolsa@...nel.org, daniel@...earbox.net,
	linux-kernel@...r.kernel.org, hekuang@...wei.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY
 maps trace data output when perf sampling

On Wed, Oct 21, 2015 at 10:01:46PM +0800, pi3orama wrote:
> > 在 2015年10月21日，下午9:49，Peter Zijlstra <peterz@...radead.org> 写道：
> > 
> >> On Wed, Oct 21, 2015 at 09:42:12PM +0800, Wangnan (F) wrote:
> >> How can an eBPF program access a !local event:
> >> 
> >> when creating perf event array we don't care which perf event
> >> is for which CPU, so perf program can access any perf event in
> >> that array.
> > 
> > So what is stopping the eBPF thing from calling perf_event_read_local()
> > on a !local event and triggering a kernel splat?
> 
> I can understand the perf_event_read_local() case, but I really can't understand
> what is stopping us to write to an atomic field belong to a !local perf event.
> Could you please give a further explanation?

I simply do not get how this eBPF stuff works.

Either I have access to !local events and I can hand one to
perf_event_read_local() and cause badness, or I do not have access to
!local events and the whole 'soft enable/disable' thing is simple.

They cannot be both true.

So explain; how does this eBPF stuff work.
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