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

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Date:	Thu, 12 May 2011 23:30:16 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [BUG] perf: bogus correlation of kernel symbols

The other contradiction, I see, is that you have perf_event paranoia level
and this new kptr masquerading feature which conflict with each
other.

You can be allowed to monitor at the kernel level (paranoid=1, default)
but you cannot correlate symbols:

$ perf record -e cycles:k foo

I suspect if you have this kptr thing turned on, then you need to disallow
monitoring at the kernel level too.



On Thu, May 12, 2011 at 11:07 PM, Stephane Eranian <eranian@...gle.com> wrote:
> On Thu, May 12, 2011 at 10:31 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> On Thu, May 12, 2011 at 7:48 AM, Stephane Eranian <eranian@...gle.com> wrote:
>> >
>> > I think there is a serious problem with kernel symbol correlation
>> > with the latest perf in 2.6.39-rc7-tip.
>>
>> Yeah. It's annoying. It's a "perf" bug, though - triggered by
>> /proc/sys/kernel/kptr_restrict being set to 1.
>>
> I did not know about this new masquerading of pointers in /proc/kallsyms.
> That certainly explains the problem.
>
>>
>> The bug is that perf doesn't say "I can't match kernel symbols", but
>> instead does some crazy matching and gives total crap module
>> information (I think it just picks the one that shows up last in
>> /proc/kallsyms).
>>
> But I agree perf must not silently return bogus information. It
> should print a big warning message and/or fallback to printing the raw
> addresses. So much for having perf in the kernel source tree to
> keep things in sync...
>
>>
>> That said, I have considered just reverting the thing that makes
>> kptr_restrict be 1 by default. I do like the security implications of
>> restricting visibility into kernel pointers, but I also think that
>> security rules that make the system less usable are dubious. So I
>> dunno.
>>
> I am not clear as to what people could actually do with the addresses
> taken out of /proc/kallsyms. Looks to me like we've lost functionality
> for the vast majority of users. So maybe the default should be inverted.
>
> I know of a somewhat similar issue with the file descriptor limit which
> people are hitting frequently these days when monitoring apps with lots
> of threads or lots of events in one run on large smp systems.
> That can easily be corrected by again requires root privilege to regain
> the functionality.
>
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