On Thu, Jun 29, 2017 at 10:12:33AM +0200, Ingo Molnar wrote: > > * Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > It still seems wrong to make up data, though. > > So what we have here is a hardware quirk: we asked for user-space samples, but > didn't get them and we cannot expose the kernel-internal address. > > The question is, how do we handle the hardware quirk. Since we cannot fix the > hardware on existing systems there's really just two choices: > > - Lose the sample (and signal it as a lost sample) > > - Keep the sample but change the sensitive kernel-internal address to something > that is not sensitive: 0 or -1 works, but we could perhaps also return a > well-known user-space address such as the vDSO syscall trampoline or such? > > there's no other option really. > > I'd lean towards Vince's take: losing samples is more surprising than getting the > occasional sample with some sanitized data in it. > > If we make the artificial data still a meaningful user-space address, related to > kernel entries, then it might even be a bonus, as users would learn to recognize > it as: 'oh, skid artifact, I know about that'. So while we could easily fake SAMPLE_IP to do as you suggest, other entries might be much harder to fake. That said, I have no problems with just 0 stuffing them. The only real problem is determining how much to stuff I suppose.