Re: [PATCH v1 0/2] perf: Drop leaked kernel samples

[Jin, Yao]

On 6/16/2018 8:56 AM, Kyle Huey wrote:
> On Fri, Jun 15, 2018 at 5:50 PM, Jin, Yao <yao.jin@xxxxxxxxxxxxxxx> wrote:
> > Hi All,
> >
> > This patch raised many questions, I was prepared. :)
> >
> > I'd like to try another proposal that it adds a special flag in the returned
> > perf_sample_data to indicate the perf binary that this sample is a leaked
> > sample.
> >
> > For example, create a new PERF_SAMPLE_RETURN_LEAKAGE in
> > perf_event_sample_format.
> >
> > In perf_prepare_sample(),
> >
> > if (event->attr.exclude_kernel && !user_mode(regs))
> >          data->type |= PERF_SAMPLE_RETURN_LEAKAGE;
> >
> > Now all the samples are kept and the leaked kernel samples are tagged with
> > PERF_SAMPLE_RETURN_LEAKAGE.
> >
> > In perf binary, it filters out the samples with PERF_SAMPLE_RETURN_LEAKAGE.
> > It needs perf binary modification but rr doesn't need to be changed.
> >
> > I don't 0-stuffing some fields because:
> >
> > 1. Keeping the skid info should allow us to look at that if we have
> > interesting later.
> >
> > 2. Not sure if 0-stuffing some fields has potential conflicts with some
> > applications.
> >
> > Is this proposal reasonable?
> >
> > Thanks
> > Jin Yao
> >
> >
> > On 6/16/2018 1:34 AM, Robert O'Callahan wrote:
> > > On Fri, Jun 15, 2018 at 10:16 AM, Kyle Huey <me@xxxxxxxxxxxx> wrote:
> > > > If you want a sysctl for your own reasons that's fine. But we don't
> > > > want a sysctl. We want to work without any further configuration.
> > >
> > >
> > > Also toggling a sysctl would require root privileges, but rr does not
> > > currently need to run as root. Thus rr users would have to either
> > > permanently change their system configuration (and every extra
> > > configuration step is a pain), or run rr as root so rr can toggle the
> > > sysctl itself. Running rr as root is highly undesirable.
> > >
> > > Rob
>
> If the problem you're trying to fix is an inappropriate disclosure of
> kernel-space information to user-space, how does filtering the samples
> in user space solve anything?
>
> - Kyle

In theory it is. But actually we just use perf tool to look at the 
sampling result.

And suppose a case, if we want to estimate the skid window, we still 
need the skid info.

Thanks
Jin Yao