linux-kernel - Re: [BUG] perf: bogus correlation of kernel symbols

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Date:	Thu, 12 May 2011 23:06:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	eranian@...gle.com, acme@...hat.com, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [BUG] perf: bogus correlation of kernel symbols

* David Miller <davem@...emloft.net> wrote:

> From: Stephane Eranian <eranian@...gle.com>
> Date: Thu, 12 May 2011 16:48:46 +0200
> 
> > I think there is a serious problem with kernel symbol correlation
> > with the latest perf in 2.6.39-rc7-tip.
> 
> The behavior seems to be intentional, so that we don't expose internal
> kernel addresses to userspace.
> 
> I hate this too, and I think it's absolutely rediculous.
> 
> Also, like you, I lost an entire afternoon trying to figure out why
> this started happening.

I lost about an hour with Arnaldo on IRC to help me until we figured out that 
/proc/kallsyms started having zero value entries ... I'm too running perf as an 
unprivileged user.

Zero is a valid symbol address so nothing within perf tripped up explicitly, 
but perf report and perf top results were nonsensical.

There was another problem with it: perf is caching and storing known kernel 
buildid addresses in ~/.debug, under the (previously correct) assumption that 
kernel symbols do not change for one given kernel build. But with kptr_restrict 
it would cache the zero values - which were cached even after kptr_restrict was 
set back to 0.

Thanks,

	Ingo
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