Re: [PATCH v3 1/3] tracing/synthetic: use union instead of casts

[Steven Rostedt]

On Thu, 10 Aug 2023 08:05:36 +0200
Sven Schnelle <svens@xxxxxxxxxxxxx> wrote:

> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -59,6 +59,17 @@ int trace_raw_output_prep(struct trace_iterator *iter,
>  extern __printf(2, 3)
>  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;
> +#else
> +	u16	len;
> +	u16	offset;
> +#endif
> +};
> +

So this patch broke synthetic events. I found that it works with:

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 12f875e9e69a..4a98e23b0e77 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -65,10 +65,10 @@ struct trace_dynamic_info {
 	u16	offset;
 	u16	len;
 #else
-	u16	len;
 	u16	offset;
+	u16	len;
 #endif
-};
+} __packed;
 
 /*
  * The trace entry - the most basic unit of tracing. This is what

That is, I had to swap the order of len vs offset, and to also be safe, I
added "__packed".

I'm guessing that the BIG ENDIAN is broken too? So I'm going to swap that
as well.

It causes the following test to fail:

 ./ftracetest test.d/trigger/inter-event/trigger-synthetic-event-dynstring.tc

Which I didn't notice because I found that test to be broken, and needs this patch:

diff --git a/tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-event-dynstring.tc b/tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-event-dynstring.tc
index 213d890ed188..174376ddbc6c 100644
--- a/tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-event-dynstring.tc
+++ b/tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-event-dynstring.tc
@@ -1,7 +1,7 @@
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 # description: event trigger - test inter-event histogram trigger trace action with dynamic string param
-# requires: set_event synthetic_events events/sched/sched_process_exec/hist "char name[]' >> synthetic_events":README ping:program
+# requires: set_event synthetic_events events/sched/sched_process_exec/hist "' >> synthetic_events":README ping:program
 
 fail() { #msg
     echo $1

-- Steve