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)

From	"Jin, Yao" <yao.jin@linux.intel.com>
Newsgroups	linux.kernel
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)
Date	2017-06-29 02:30 +0200
Message-ID	<tXvvk-rh-11@gated-at.bofh.it> (permalink)
References	(4 earlier) <tXieJ-5vN-11@gated-at.bofh.it> <tXiRs-5KR-23@gated-at.bofh.it> <tXppW-t5-33@gated-at.bofh.it> <tXppW-t5-31@gated-at.bofh.it> <tXu6e-1eI-13@gated-at.bofh.it>
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On 6/29/2017 6:55 AM, Kyle Huey wrote:
> On Wed, Jun 28, 2017 at 10:49 AM, Mark Rutland <mark.rutland@arm.com> wrote:
>> 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.
> I'm not very familiar with the kernel internals, but the reason I
> didn't suggest this originally is it seems like it will be difficult
> to determine what the "correct" userspace registers are.  For example,
> what happens if a performance counter is fixed to a given tid, the
> interrupt fires during a context switch from that task to another that
> is not being monitored, and the kernel is far enough along in the
> context switch that the current task struct has been switched out?
> Reporting the new task's registers seems as bad as reporting the
> kernel's registers.  But maybe this is easier than I imagine for
> whatever reason.
>
> Something to think about.
>
> - Kyle

Yes, I think so.

The skid interrupt may be triggered at a wrong context and return wrong 
indications (e.g. wrong regs) to userspace.

So that's why I think the *skid* interrupt had better be dropped.

Thanks
Jin Yao

Thread

Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Kyle Huey <me@kylehuey.com> - 2017-06-28 03:10 +0200
  Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region "Jin, Yao" <yao.jin@linux.intel.com> - 2017-06-28 04:10 +0200
    Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Kyle Huey <me@kylehuey.com> - 2017-06-28 07:00 +0200
      Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region "Jin, Yao" <yao.jin@linux.intel.com> - 2017-06-28 07:40 +0200
        Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Kyle Huey <me@kylehuey.com> - 2017-06-28 09:40 +0200
      Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Mark Rutland <mark.rutland@arm.com> - 2017-06-28 12:20 +0200
        [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> - 2017-06-28 13:00 +0200
          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) Vince Weaver <vincent.weaver@maine.edu> - 2017-06-28 14:50 +0200
            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> - 2017-06-28 15:10 +0200
              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) Alexey Budankov <alexey.budankov@linux.intel.com> - 2017-06-29 10:20 +0200
                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) Alexey Budankov <alexey.budankov@linux.intel.com> - 2017-06-29 10:30 +0200
          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> - 2017-06-28 20:00 +0200
            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) Kyle Huey <me@kylehuey.com> - 2017-06-29 01:00 +0200
              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) "Jin, Yao" <yao.jin@linux.intel.com> - 2017-06-29 02:30 +0200
              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) Kyle Huey <me@kylehuey.com> - 2017-06-30 19:50 +0200
            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) Ingo Molnar <mingo@kernel.org> - 2017-06-29 10:20 +0200
          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) Kyle Huey <me@kylehuey.com> - 2017-06-28 20:00 +0200
        Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region "Robert O'Callahan" <robert@ocallahan.org> - 2017-06-28 20:00 +0200
        Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Kyle Huey <me@kylehuey.com> - 2017-06-28 20:00 +0200
          Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Mark Rutland <mark.rutland@arm.com> - 2017-06-28 20:00 +0200
        Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Mark Rutland <mark.rutland@arm.com> - 2017-06-28 20:00 +0200
        Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the  kernel in the "skid" region Kyle Huey <me@kylehuey.com> - 2017-06-28 20:00 +0200

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