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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