[RESEND][PATCH] tracing/synthetic: Fix order of struct trace_dynamic_info

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

To make handling BIG and LITTLE endian better the offset/len of dynamic
fields of the synthetic events was changed into a structure of:

 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
	u16	offset;
	u16	len;
 #else
	u16	len;
	u16	offset;
 #endif
 };

to replace the manual changes of:

 data_offset = offset & 0xffff;
 data_offest = len << 16;

But if you look closely, the above is:

  <len> << 16 | offset

Which in little endian would be in memory:

 offset_lo offset_hi len_lo len_hi

and in big endian:

 len_hi len_lo offset_hi offset_lo

Which if broken into a structure would be:

 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
	u16	len;
	u16	offset;
 #else
	u16	offset;
	u16	len;
 #endif
 };

Which is the opposite of what was defined.

Fix this and just to be safe also add "__packed".

Link: https://lore.kernel.org/all/20230908154417.5172e343@gandalf.local.home/

Cc: stable@vger.kernel.org
Fixes: ddeea494a16f3 ("tracing/synthetic: Use union instead of casts")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---

 [ Resending to the correct mailing list this time :-p ]

 include/linux/trace_events.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 12f875e9e69a..21ae37e49319 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h

@@ -62,13 +62,13 @@ void trace_event_printf(struct trace_iterator *iter, const char *fmt, ...);
 /* Used to find the offset and length of dynamic fields in trace events */
 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
-	u16	offset;
 	u16	len;
+	u16	offset;
 #else
-	u16	len;
 	u16	offset;
+	u16	len;
 #endif
-};
+} __packed;
 
 /*
  * The trace entry - the most basic unit of tracing. This is what

-- 
2.40.1

On Fri, 8 Sep 2023 16:39:29 -0400
Steven Rostedt [off-list ref] wrote:

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

To make handling BIG and LITTLE endian better the offset/len of dynamic
fields of the synthetic events was changed into a structure of:

 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
	u16	offset;
	u16	len;
 #else
	u16	len;
	u16	offset;
 #endif
 };

to replace the manual changes of:

 data_offset = offset & 0xffff;
 data_offest = len << 16;

But if you look closely, the above is:

  <len> << 16 | offset

Which in little endian would be in memory:

 offset_lo offset_hi len_lo len_hi

and in big endian:

 len_hi len_lo offset_hi offset_lo

Which if broken into a structure would be:

 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
	u16	len;
	u16	offset;
 #else
	u16	offset;
	u16	len;
 #endif
 };

Which is the opposite of what was defined.

Fix this and just to be safe also add "__packed".

Link: https://lore.kernel.org/all/20230908154417.5172e343@gandalf.local.home/

Good catch! I'm not sure why this worked. Maybe we don't have any testcase
for this feature?

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

-- 
Masami Hiramatsu (Google) [off-list ref]

On Mon, 11 Sep 2023 09:26:12 +0900
Masami Hiramatsu (Google) [off-list ref] wrote:

quoted

Fix this and just to be safe also add "__packed".

Link: https://lore.kernel.org/all/20230908154417.5172e343@gandalf.local.home/  

Good catch! I'm not sure why this worked. Maybe we don't have any testcase
for this feature?

We have a test case, it was broken by a change in the README :-p

I noticed issues with it, and then looked at the tests, fixed the test and
it failed. Before that, it was just reporting "unsupported", which is why I
included that fix with this queue.

  https://lore.kernel.org/linux-trace-kernel/20230614091046.2178539-1-naveen@kernel.org/

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks!

-- Steve