Re: [PATCH] perf/core: generate overflow signal when samples are dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region)

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Jun 28, 2017 at 09:48:27AM -0700, Kyle Huey wrote:
> On Wed, Jun 28, 2017 at 3:56 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > @@ -6101,6 +6116,12 @@ void perf_prepare_sample(struct perf_event_header *header,
> >         struct perf_output_handle handle;
> >         struct perf_event_header header;
> >
> > +       /*
> > +        * For security, drop the skid kernel samples if necessary.
> > +        */
> > +       if (!sample_is_allowed(event, regs))
> > +               return ret;
> 
> Just a bare return here.

Ugh, yes. Sorry about that. I'll fix that up.

[...]

> I can confirm that with that fixed to compile, this patch fixes rr.

Thanks for giving this a go.

Having thought about this some more, I think Vince does make a good
point that throwing away samples is liable to break stuff, e.g. that
which only relies on (non-sensitive) samples.

It still seems wrong to make up data, though.

Maybe for exclude_kernel && !exclude_user events we can always generate
samples from the user regs, rather than the exception regs. That's going
to be closer to what the user wants, regardless. I'll take a look
tomorrow.

Thanks,
Mark.