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

["Jin, Yao" <yao.jin@linux.intel.com>]

On 6/16/2018 8:56 AM, Kyle Huey wrote:
> On Fri, Jun 15, 2018 at 5:50 PM, Jin, Yao <yao.jin@linux.intel.com> 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@kylehuey.com> 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