linux-kernel - Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf

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Date:	Sun, 19 Oct 2014 13:23:17 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Erik Bosman <ebn310@....vu.nl>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf_event is mapped

On Thu, Oct 16, 2014 at 5:00 PM, Andy Lutomirski <luto@...capital.net> wrote:
> The current cap_user_rdpmc code seems rather confused to me.  On x86,
> *all* events set cap_user_rdpmc if the global rdpmc control is set.
> But only x86_pmu events define .event_idx, so amd uncore events won't
> actually expose their rdpmc index to userspace.
>
> Would it make more sense to add a flag PERF_X86_EVENT_RDPMC_PERMITTED
> that gets set on all events created while rdpmc == 1, to change
> x86_pmu_event_idx to do something like:
>
> if (event->hw.flags & PERF_X86_EVENT_RDPMC_PERMITTED)
>   return event->hw.event_base_rdpmc + 1;
> else
>   return 0;
>
> and to change arch_perf_update_userpage cap_user_rdpmc to match
> PERF_X86_EVENT_RDPMC_PERMITTED?
>
> Then we could ditch the static key and greatly simplify writes to the
> rdpmc flag by just counting PERF_X86_EVENT_RDPMC_PERMITTED events.
>
> This would be a user-visible change on AMD, and I can't test it.
>
>
> On a semi-related note: would this all be nicer if there were vdso
> function __u64 __vdso_perf_event__read_count(int fd, void *userpage)?
> This is very easy to do nowadays.  If we got *really* fancy, it would
> be possible to have an rdpmc_safe in the vdso, which has some
> benefits, although it would be a bit evil and wouldn't work if
> userspace tracers like pin are in use.
>

Also, I don't understand the purpose of cap_user_time.  Wouldn't it be
easier to just record the last CLOCK_MONOTONIC time and let the user
call __vdso_clock_gettime if they need an updated time?

--Andy
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