Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf_event is mapped

[Andy Lutomirski]

On Mon, Oct 20, 2014 at 1:33 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Sun, Oct 19, 2014 at 03:57:54PM -0700, Andy Lutomirski wrote:
>> > Maybe, but at that point we commit to yet another ABI... I'd rather just
>> > put a 'sane' implementation in a library or so.
>>
>> This cuts both ways, though.  For vdso timekeeping, the underlying
>> data structure has changed repeatedly, sometimes to add features, and
>> sometimes for performance, and the vdso has done a good job insulating
>> userspace from it.  (In fact, until 3.16, even the same exact kernel
>> version couldn't be relied on to have the same data structure with
>> different configs, and even now, no one really wants to teach user
>> libraries how to parse the pvclock data structures.)
>
> Fair enough, but as it stands we've already committed to the data
> structure exposed to userspace.

True.

OTOH, if a vdso function gets added, a few releases go by, and all the
userspace tools get updated, then the old data structure could be
dropped if needed by clearing cap_user_rdpmc.

Anyway, this is so far out of scope for the current project that I'm
going to ignore it.

>> FWIW, something should probably specify exactly when it's safe to try
>> a userspace rdpmc.  I think that the answer is that, for a perf event
>> watching a pid, only that pid can do it (in particular, other threads
>> must not try).  For a perf event monitoring a whole cpu, the answer is
>> less clear to me.
>
> This all was really only meant to be used for self-monitoring, so where
> an event is attached to the very same task, anything else and I'm find
> disabling it.

Actually implementing this might be a touch awkward.  I can check
whether an event has a task context that matches the creating task,
but that's not necessarily the same thing as the task that mmaps it.

--Andy
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