[RFC PATCH v4 1/3] tracing/events: Fix wrong sample output by storing array length instead of size

[He Kuang]

The output result of trace_foo_bar event in traceevent samples is
wrong. This problem can be reproduced as following:

  (Build kernel with SAMPLE_TRACE_EVENTS=m)

  $ insmod trace-events-sample.ko

  $ echo 1 > /sys/kernel/debug/tracing/events/sample-trace/foo_bar/enable

  $ cat /sys/kernel/debug/tracing/trace

  event-sample-980 [000] ....  43.649559: foo_bar: foo hello 21 0x15
  BIT1|BIT3|0x10 {0x1,0x6f6f6e53,0xff007970,0xffffffff} Snoopy
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                 The array length is not right, should be {0x1}.
  (ffffffff,ffffffff)

  event-sample-980 [000] ....  44.653827: foo_bar: foo hello 22 0x16
  BIT2|BIT3|0x10
  {0x1,0x2,0x646e6147,0x666c61,0xffffffff,0xffffffff,0x750aeffe,0x7}
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                 The array length is not right, should be {0x1,0x2}.
  Gandalf (ffffffff,ffffffff)

The event defined in samples/trace_events/trace-events-sample.h uses
this helper function to output dynamic list:

   __print_array(__get_dynamic_array(list),
                 __get_dynamic_array_len(list),
                 sizeof(int))

Currently, __get_dynamic_array_len() returns the total size of the
array instead of the number of items by referencing the high 16 bits
of entry->data_loc_##item. The element size for calculating the number
of items can not be fetched by referencing fields from __entry, so
macro __get_dynamic_array_len can not return the expected value.

This patch stores array item number instead of the total size in
entry->__data_loc_##item, and makes __get_dynamic_array_len get the
right value directly. Because the function __get_bitmask() is affected
by this change, __bitmask_size is assigned to the array len by
multiplied bitmask type size.

After this patch:

  event-sample-993 [000] ....  692.348562: foo_bar: foo hello 201
  0xc9 BIT1|BIT4|0xc0 {0x1} Snoopy (ffffffff,ffffffff)
                      ^^^^^
                      Array length fixed.

  event-sample-993 [000] ....  693.349276: foo_bar: foo hello 202
  0xca BIT2|BIT4|0xc0 {0x1,0x2} Gandalf (ffffffff,ffffffff)
                      ^^^^^^^^^
                      Array length fixed.

Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
---
 include/trace/trace_events.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index 43be3b0..5abe027 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -257,7 +257,8 @@ TRACE_MAKE_SYSTEM_STR();
 	({								\
 		void *__bitmask = __get_dynamic_array(field);		\
 		unsigned int __bitmask_size;				\
-		__bitmask_size = __get_dynamic_array_len(field);	\
+		__bitmask_size = (__get_dynamic_array_len(field) *	\
+				  sizeof(unsigned long));		\
 		trace_print_bitmask_seq(p, __bitmask, __bitmask_size);	\
 	})
 
@@ -453,7 +454,7 @@ trace_event_define_fields_##call(struct trace_event_call *event_call)	\
 	__item_length = (len) * sizeof(type);				\
 	__data_offsets->item = __data_size +				\
 			       offsetof(typeof(*entry), __data);	\
-	__data_offsets->item |= __item_length << 16;			\
+	__data_offsets->item |= (len) << 16;				\
 	__data_size += __item_length;
 
 #undef __string
-- 
1.8.5.2

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