Re: [PATCH v3 6/9] perf, x86: handle multiple records in PEBS buffer

["Yan, Zheng" <zheng.z.yan@intel.com>]

On 07/25/2014 04:10 PM, Peter Zijlstra wrote:
> On Tue, Jul 22, 2014 at 04:09:59PM +0800, Yan, Zheng wrote:
>> One corner case needs to mention is that the PEBS hardware doesn't
>> deal well with collisions, when PEBS events happen near to each
>> other. The records for the events can be collapsed into a single
>> one. However in practice collisions are extremely rare, as long as
>> different events are used. The periods are typically very large,
>> so any collision is unlikely. When collision happens, we can either
>> drop the PEBS record or use the record to serve multiple events.
>> This patch chooses the later approach.
> 
> You can't.. the events might have different security context.
> 
> Remember, the overflow bit is set from the overflow until the PEBS
> event is generated, this is quite a long time. So if another PEBS event
> gets generated while the other is still pending it will have both bits
> set. Even though the second bit is for another (unrelated) counter.
> 
> The unrelated counter might not have privilege to observe the data of
> the generated event.
> 
> I think you can unwind and fully correct this trainwreck.

could you give more information how to do this.

Regards
Yan, Zheng

> But simply
> delivering an even with multiple bits set to all relevant events is
> wrong and might leak sensitive information.
>