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

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Message-ID: <CALCETrXRGNeUipujcGiT30SV+MDHCFpTxG7LOTXj4Z4CbtxdJQ@mail.gmail.com>
Date:	Thu, 16 Oct 2014 17:00:56 -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

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.

--Andy
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