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

[Liang, Kan]

On 1/26/2021 9:42 AM, Peter Zijlstra wrote:
On Tue, Jan 19, 2021 at 12:38:20PM -0800, kan.li...@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