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

[Namhyung Kim <namhyung@kernel.org>]

Hello Ravi,

On Thu, Oct 24, 2024 at 10:13:24AM +0530, Ravi Bangoria wrote:
> >>> 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?

Right, that's the purpose of the PERF_FORMAT_DROPPED.  But I think
we can discuss this interface later and just focus on the IBS swfilt
first.  I'll remove this part now and add it back separately.

Thanks,
Namhyung