[PATCH 2/2] tracing: Do not calculate strlen() twice for __string() fields

[Steven Rostedt <rostedt@goodmis.org>]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The TRACE_EVENT() macro handles dynamic strings by having:

  TP_PROTO(struct some_struct *s),
  TP_ARGS(s),
  TP_STRUCT__entry(
        __string(my_string, s->string)
 ),
 TP_fast_assign(
        __assign_str(my_string, s->string);
 )
 TP_printk("%s", __get_str(my_string))

There's even some code that may call a function helper to find the
s->string value. The problem with the above is that the work to get the
length of s->string is done twice. Once at the __string() and again in the
__assign_str().

The length of the string is calculated via a strlen(), not once, but twice
(via strcpy() in __assign_str()). Once during the __string() macro and again
in __assign_str(). But the length is actually already recorded in the data
location and there's no reason to call strlen() again.

Just use the saved length that was saved in the __string() code for the
__assign_str() code.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 include/trace/stages/stage6_event_callback.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/trace/stages/stage6_event_callback.h b/include/trace/stages/stage6_event_callback.h
index e09e3d019dcc..d389725937e9 100644
--- a/include/trace/stages/stage6_event_callback.h
+++ b/include/trace/stages/stage6_event_callback.h
@@ -32,8 +32,9 @@
 
 #undef __assign_str
 #define __assign_str(dst, src)						\
-	strcpy(__get_str(dst), __data_offsets.dst##_ptr_ ?		\
-	       __data_offsets.dst##_ptr_ : "(null)")
+	memcpy(__get_str(dst), __data_offsets.dst##_ptr_ ?		\
+	       __data_offsets.dst##_ptr_ : "(null)",			\
+	       __get_dynamic_array_len(dst))
 
 #undef __assign_str_len
 #define __assign_str_len(dst, src, len)					\
@@ -94,8 +95,9 @@
 
 #undef __assign_rel_str
 #define __assign_rel_str(dst, src)					\
-	strcpy(__get_rel_str(dst), __data_offsets.dst##_ptr_ ?		\
-	       __data_offsets.dst##_ptr_ : "(null)")
+	memcpy(__get_rel_str(dst), __data_offsets.dst##_ptr_ ?		\
+	       __data_offsets.dst##_ptr_ : "(null)",			\
+	       __get_rel_dynamic_array_len(dst))
 
 #undef __assign_rel_str_len
 #define __assign_rel_str_len(dst, src, len)				\
-- 
2.43.0