Subject: [PATCH] perf: Avoid -Warray-bounds warning for __rel_loc macro

From: Kees Cook <[email-masked]>
Date: Tue, 25 Jan 2022 14:00:37 -0800

As done for trace_events.h, also fix the __rel_loc macro in perf.h,
which silences the -Warray-bounds warning:

In file included from ./include/linux/string.h:253,
                 from ./include/linux/bitmap.h:11,
                 from ./include/linux/cpumask.h:12,
                 from ./include/linux/mm_types_task.h:14,
                 from ./include/linux/mm_types.h:5,
                 from ./include/linux/buildid.h:5,
                 from ./include/linux/module.h:14,
                 from samples/trace_events/trace-events-sample.c:2:
In function '__fortify_strcpy',
    inlined from 'perf_trace_foo_rel_loc' at samples/trace_events/./trace-events-sample.h:519:1:
./include/linux/fortify-string.h:47:33: warning: '__builtin_strcpy' offset 12 is out of the bounds [
0, 4] [-Warray-bounds]
   47 | #define __underlying_strcpy     __builtin_strcpy
      |                                 ^
./include/linux/fortify-string.h:445:24: note: in expansion of macro '__underlying_strcpy'
  445 |                 return __underlying_strcpy(p, q);
      |                        ^~~~~~~~~~~~~~~~~~~

Also make __data struct member a proper flexible array to avoid future
problems.

Cc: Steven Rostedt <[email-masked]>
Cc: Masami Hiramatsu <[email-masked]>
Reported-by: Stephen Rothwell <[email-masked]>
Signed-off-by: Kees Cook <[email-masked]>
---
 include/trace/perf.h         | 5 +++--
 include/trace/trace_events.h | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/trace/perf.h b/include/trace/perf.h
index b77d09c70a93..5800d13146c3 100644
--- a/include/trace/perf.h
+++ b/include/trace/perf.h
@@ -26,8 +26,9 @@
 
 #undef __get_rel_dynamic_array
 #define __get_rel_dynamic_array(field)	\
-		((void *)(&__entry->__rel_loc_##field) +	\
-		 sizeof(__entry->__rel_loc_##field) +		\
+		((void *)__entry +					\
+		 offsetof(typeof(*__entry), __rel_loc_##field) +	\
+		 sizeof(__entry->__rel_loc_##field) +			\
 		 (__entry->__rel_loc_##field & 0xffff))
 
 #undef __get_rel_dynamic_array_len
diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index cefefed18e85..7c86cc541c7a 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -134,7 +134,7 @@ TRACE_MAKE_SYSTEM_STR();
 	struct trace_event_raw_##name {					\
 		struct trace_entry	ent;				\
 		tstruct							\
-		char			__data[0];			\
+		char			__data[];			\
 	};								\
 									\
 	static struct trace_event_class event_class_##name;
-- 
2.30.2