Re: [PATCH v4 1/5] perf/core: Add PERF_FORMAT_DROPPED

[Ravi Bangoria <ravi.bangoria@amd.com>]

>>> When a perf_event is dropped due to some kind of (SW-based) filter, it
>>> won't generate sample data.  For example, software events drops samples
>>> when it doesn't match to privilege from exclude_{user,kernel}.
>>>
>>> In order to account such dropped samples, add a new counter in the
>>> perf_event, and let users can read(2) the number with the new
>>> PERF_FORMAT_DROPPED like the lost sample count.
>>
>> Are we sure there's no scenario where exposing the dropped event count
>> gives an unprivileged user a way to probe what's happening in the
>> kernel, which is supposed to be prevented by exclude_kernel?
>>
>> Clearly it provides an attacker with some information, ie. the event
>> fired in the kernel and was dropped.
>>
>> For most events that's not very interesting, but for some maybe it could
>> be a useful signal?
> 
> Hmm.. good point.  It'd give some information to users.  I'm not sure
> how much impact it'd have, but there are some folks who want to know
> exact number of samples including dropped ones to reconstruct total
> period for the monitoring session.

Can we restrict PERF_FORMAT_DROPPED to perfmon_capable() if it's a
genuine problem? Or would it defeat the whole purpose of _DROPPED
count?

Thanks,
Ravi