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

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Date:	Fri, 13 May 2011 09:24:17 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Kees Cook <kees.cook@...onical.com>
Cc:	David Miller <davem@...emloft.net>, davej@...hat.com,
	eranian@...gle.com, acme@...hat.com, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [BUG] perf: bogus correlation of kernel symbols

Hi Kees,

On Fri, May 13, 2011 at 9:12 AM, Kees Cook <kees.cook@...onical.com> wrote:
> Hi Pekka,
>
> On Thu, May 12, 2011 at 10:58:53PM +0300, Pekka Enberg wrote:
>> On Thu, May 12, 2011 at 10:01 PM, David Miller <davem@...emloft.net> wrote:
>> > From: Dave Jones <davej@...hat.com>
>> > Date: Thu, 12 May 2011 14:37:41 -0400
>> >
>> >> On Thu, May 12, 2011 at 02:06:30PM -0400, David Miller wrote:
>> >>  > 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 wish we could revert this change.
>> >>
>> >> At least it can be permanently disabled..
>> >>
>> >> echo kernel.kptr_restrict = 0 >> /etc/sysctl.conf
>> >
>> > Regardless, what to do about all of the "perf is broken" reports?
>>
>> Lets revert the commit 9f36e2c448007b54851e7e4fa48da97d1477a175
>> ("printk: use %pK for /proc/kallsyms and /proc/modules"), please! I
>> too have been wondering what's going on with perf reporting insane
>> symbols and this should definitely not be enabled by default.
>
> No, reverting that is not the answer. If perf has a problem with the
> kptr_restrict feature, it should just disable it in /proc/sys when it
> runs and restore it when finished. Since our defaults should be secure
> for the average user (who does not use perf), it's fine the way it
> is. Anyone using perf can adjust this for their use-case (that is why
> there is a /proc/sys tunable).

No, it's the other way around. See commit
411f05f123cbd7f8aa1edcae86970755a6e2a9d9 ("vsprintf: Turn
kptr_restrict off by default") in Linus' tree for details.

                        Pekka
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