Re: [PATCH 01/12] perf/core: Add PERF_SAMPLE_WEIGHT_EXT

["Liang, Kan" <kan.liang@linux.intel.com>]

On 1/26/2021 9:42 AM, Peter Zijlstra wrote:
> On Tue, Jan 19, 2021 at 12:38:20PM -0800, kan.liang@linux.intel.com wrote:
> 
>> @@ -900,6 +901,13 @@ enum perf_event_type {
>>   	 *	  char			data[size]; } && PERF_SAMPLE_AUX
>>   	 *	{ u64			data_page_size;} && PERF_SAMPLE_DATA_PAGE_SIZE
>>   	 *	{ u64			code_page_size;} && PERF_SAMPLE_CODE_PAGE_SIZE
>> +	 *	{ union {
>> +	 *		u64		weight_ext;
>> +	 *		struct {
>> +	 *			u64	instr_latency:16,
>> +	 *				reserved:48;
>> +	 *		};
>> +	 *	} && PERF_SAMPLE_WEIGHT_EXT
>>   	 * };
>>   	 */
>>   	PERF_RECORD_SAMPLE			= 9,
>> @@ -1248,4 +1256,12 @@ struct perf_branch_entry {
>>   		reserved:40;
>>   };
>>   
>> +union perf_weight_ext {
>> +	__u64		val;
>> +	struct {
>> +		__u64	instr_latency:16,
>> +			reserved:48;
>> +	};
>> +};
>> +
>>   #endif /* _UAPI_LINUX_PERF_EVENT_H */
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 55d1879..9363d12 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -1903,6 +1903,9 @@ static void __perf_event_header_size(struct perf_event *event, u64 sample_type)
>>   	if (sample_type & PERF_SAMPLE_CODE_PAGE_SIZE)
>>   		size += sizeof(data->code_page_size);
>>   
>> +	if (sample_type & PERF_SAMPLE_WEIGHT_EXT)
>> +		size += sizeof(data->weight_ext);
>> +
>>   	event->header_size = size;
>>   }
>>   
>> @@ -6952,6 +6955,9 @@ void perf_output_sample(struct perf_output_handle *handle,
>>   			perf_aux_sample_output(event, handle, data);
>>   	}
>>   
>> +	if (sample_type & PERF_SAMPLE_WEIGHT_EXT)
>> +		perf_output_put(handle, data->weight_ext);
>> +
>>   	if (!event->attr.watermark) {
>>   		int wakeup_events = event->attr.wakeup_events;
>>   
> 
> This patch is broken and will expose uninitialized kernel stack.
> 

Could we initialize the 'weight_ext' in perf_sample_data_init()?

I understand that we prefer not to set the field in 
perf_sample_data_init() to minimize the cachelines touched.
However, the perf_sample_data_init() should be the most proper place to 
do the initialization. Also, the 'weight' is already initialized in it. 
As an extension, I think the 'weight_ext' should be initialized in it as 
well.

In the perf_prepare_sample(), I think we can only clear the unused 
fields. The [0:15] bits may still leak the data.

Thanks,
Kan