Date: Wed, 28 Jun 2017 18:49:01 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Kyle Huey <me@kylehuey.com>, Vince Weaver <vincent.weaver@maine.edu>
Cc: "Jin, Yao" <yao.jin@linux.intel.com>, Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        stable@vger.kernel.org,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Thomas Gleixner <tglx@linutronix.de>, acme@kernel.org,
        jolsa@kernel.org, kan.liang@intel.com,
        Will Deacon <will.deacon@arm.com>, gregkh@linuxfoundation.org,
        "Robert O'Callahan" <robert@ocallahan.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: 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)
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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.