linux-kernel - Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

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Date:   Wed, 12 Aug 2020 11:52:43 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, oleg@...hat.com, acme@...nel.org,
        jolsa@...nel.org, Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com,
        alexander.shishkin@...ux.intel.com, mark.rutland@....com,
        like.xu@...ux.intel.com
Subject: Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

Hi Peter,

On 8/11/2020 4:45 PM, Peter Zijlstra wrote:
> On Tue, Aug 11, 2020 at 04:31:10PM +0800, Jin, Yao wrote:
>> Hi Peter,
>>
>> On 8/11/2020 3:59 PM, Peter Zijlstra wrote:
>>> On Tue, Aug 11, 2020 at 03:50:43PM +0800, Jin, Yao wrote:
>>>> Could I post v2 which basically refers to your patch but removes some
>>>> conditions since I see some issues in test if we use these conditions.
>>>>
>>>>    1. Remove '!event->attr.exclude_hv || !event->attr.exclude_host ||
>>>>       !event->attr.exclude_guest' at the entry of sanitize_sample_regs().
>>>>
>>>>    2. Remove '!attr.exclude_hv || !attr.exclude_host || !attr.exclude_guest'
>>>>       at the perf_event_open syscall entry.
>>>
>>> exclude_host, maybe -- due to the dodgy semantics of it, but the others
>>> should definitely be there.
>>>
>>
>> exclude_guest and exclude_hv are tricky too.
>>
>> If we do 'perf record -e cycles:u' in both host and guest, we can see:
>>
>> event->attr.exclude_guest = 0
>>
>> thus sanitize_sample_regs() returns regs directly even if exclude_kernel = 1.
>>
>> And in guest, exclude_hv = 0, it's out of my expectation too.
> 
> I'm confused, how can 'perf record -e cycles:u' _ever_ have
> exclude_guest=0, exclude_hv=0 ? That simply makes no sense and is utterly
> broken.
> 
> You explicitly ask for userspace-only, reporting hypervisor or guest
> events is a straight up bug.
> 

If we run 'perf record -e cycles:u',

1. On host, exclude_guest = 0 and exclude_hv = 1

perf tool doesn't specially set 'exclude_guest' when it parses the 'u' modifier. I agree that can be 
improved. I will post a perf tool patch to fix that.

2. On guest, exclude_guest = 0 and exclude_hv = 0.

For exclude_hv = 0, it looks like a bug but x86 doesn't use exclude_hv. But yes, we should fix that.

CC Like Xu <like.xu@...ux.intel.com>.

Thanks
Jin Yao