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

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Date:	Fri, 13 May 2011 00:00:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] perf: bogus correlation of kernel symbols


* Stephane Eranian <eranian@...gle.com> wrote:

> On Thu, May 12, 2011 at 11:50 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Stephane Eranian <eranian@...gle.com> wrote:
> >
> >> On Thu, May 12, 2011 at 11:35 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >> >
> >> > * Stephane Eranian <eranian@...gle.com> wrote:
> >> >
> >> >> 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.
> >> >
> >> > The better (and consistent) solution would be to turn the kptr_restrict thing
> >> > off - see the patch i sent.
> >>
> >> I saw that. But I think that when someone turns it back on, then you need to
> >> increase the perf_events paranoia level to disallow kernel monitoring to
> >> regular users such that you maintain consistency across the board.
> >
> > Dunno, i would not couple them necessarily - certain users might still have
> > access to kernel symbols via some other channel - for example the System.map.
>
> Ok, that's true, but then you'd need to have perf print a message or refuse to
> use /proc/kallsyms and suggest that the user provides a System.map.

Correct - the right approach would be to just use what we had in earlier 
versions, an 'unknown symbol' kind of catch-all entry that shows how much
stuff we did not recognize.

Thanks,

	Ingo
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