linux-kernel - Re: [RESEND][PATCH] tracing/synthetic: Fix order of struct trace_dynamic

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Date:   Mon, 11 Sep 2023 09:26:12 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [RESEND][PATCH] tracing/synthetic: Fix order of struct
 trace_dynamic_info

On Fri, 8 Sep 2023 16:39:29 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

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

Thank you,

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


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>