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

[Kyle Huey]

On Fri, Jun 15, 2018 at 5:50 PM, Jin, Yao <yao.jin@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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