linux-kernel - 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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Message-ID: <cfb812c9-173b-dd22-1f22-335f126d6389@linux.intel.com>
Date:   Thu, 29 Jun 2017 08:27:15 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Kyle Huey <me@...ehuey.com>, Mark Rutland <mark.rutland@....com>
Cc:     Vince Weaver <vincent.weaver@...ne.edu>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        stable@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, acme@...nel.org,
        jolsa@...nel.org, kan.liang@...el.com,
        Will Deacon <will.deacon@....com>, gregkh@...uxfoundation.org,
        Robert O'Callahan <robert@...llahan.org>,
        open list <linux-kernel@...r.kernel.org>, yao.jin@...el.com
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)



On 6/29/2017 6:55 AM, Kyle Huey wrote:
> On Wed, Jun 28, 2017 at 10:49 AM, Mark Rutland <mark.rutland@....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@....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