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

[Google]

On Fri, 8 Sep 2023 16:01:57 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

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

Good catch! This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

> 
> Link: https://lore.kernel.org/all/20230908154417.5172e343@xxxxxxxxxxxxxxxxxx/
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: ddeea494a16f3 ("tracing/synthetic: Use union instead of casts")
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> ---
>  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
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>