Re: TP_printk() bug with %c, and more?

[Luca Ceresoli]

Hello Steven,

On Fri, 15 Mar 2024 14:58:52 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Fri, 15 Mar 2024 19:03:12 +0100
> Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx> wrote:
> 
> > > > 
> > > > I've come across an unexpected behaviour in the kernel tracing
> > > > infrastructure that looks like a bug, or maybe two.
> > > > 
> > > > Cc-ing ASoC maintainers for as it appeared using ASoC traces, but it
> > > > does not look ASoC-specific.
> > > > 
> > > > It all started when using this trace-cmd sequence on an ARM64 board
> > > > running a mainline 6.8.0-rc7 kernel:
> > > > 
> > > >   trace-cmd record -e snd_soc_dapm_path ./my-play
> > > >   trace-cmd report
> > > > 
> > > > While this produces perfectly valid traces for other asoc events,
> > > > the snd_soc_dapm_path produces:
> > > > 
> > > >   snd_soc_dapm_path:    >c<* MIC1_EN <- (direct) <-
> > > > 
> > > > instead of the expected:
> > > > 
> > > >   snd_soc_dapm_path:    *MIC1 <- (direct) <- MIC1_EN
> > > > 
> > > > The originating macro is:
> > > > 
> > > > 	TP_printk("%c%s %s %s %s %s",
> > > > 		(int) __entry->path_node &&
> > > > 		(int) __entry->path_connect ? '*' : ' ',
> > > > 		__get_str(wname), DAPM_ARROW(__entry->path_dir),
> > > > 		__get_str(pname), DAPM_ARROW(__entry->path_dir),
> > > > 		__get_str(pnname))
> > > > 
> > > > It appears as if the %c placeholder always produces the three ">c<"
> > > > characters, the '*' or ' ' char is printed as the first %s, all the
> > > > other strings are shifted right by one position and the last string is
> > > > never printed.
> > > > 
> > > > On my x86_64 laptop running the default Ubuntu kernel (6.5) I'm able to
> > > > trace a few events having a '%c' in their TP_printk() macros and the
> > > > result is:
> > > > 
> > > >   intel_pipe_update_start: dev 0000:00:02.0, pipe >c<, frame=1,
> > > >   scanline=107856, min=2208, max=2154
> > > >       
> > > 
> > > What does /sys/kernel/tracing/trace show?    
> > 
> > It is correct:
> > 
> >    intel_pipe_update_start: dev 0000:00:02.0, pipe B, frame=377644, scanline=1466, min=2154, max=2159
> >   
> > > If that's fine, then the bug is in libtraceevent and not the kernel.
> > > 
> > > I'm testing it out now, and I see %c not being processed properly by
> > > libtraceevent. I'll take a deeper look.    
> > 
> > Thanks.
> >   
> > > > originating from:
> > > > 
> > > >   TP_printk("dev %s, pipe %c, frame=%u, scanline=%u, min=%u, max=%u",
> > > > 
> > > > Here it looks like the %c produced ">c<" again, but apparently without
> > > > any shifting.
> > > > 
> > > > Back on the ARM64 board I found a couple interesting clues.
> > > > 
> > > > First, using the <debugfs>/tracing/ interface instead of trace-cmd, I'm
> > > > getting correctly formatted strings:
> > > > 
> > > > trace-cmd: snd_soc_dapm_path: >c<* HPOUT_L -> (direct) ->
> > > > debugfs:   snd_soc_dapm_path: *HPOUT_L <- (direct) <- HPOUT_POP_SOUND_L
> > > > 
> > > > Notice the arrows pointing to the opposite direction though. The correct
> > > > arrow is the one in the debugfs run.    
> > 
> > This other issue appears a separate bug however.  
> 
> Can you make user you have the latest libtraceevent from:
> 
>    git://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git
> 
> And apply this patch.
> 
> Thanks,
> 
> -- Steve
> 
> diff --git a/src/event-parse.c b/src/event-parse.c
> index d607556..61b0966 100644
> --- a/src/event-parse.c
> +++ b/src/event-parse.c
> @@ -3732,8 +3732,19 @@ process_arg_token(struct tep_event *event, struct tep_print_arg *arg,
>  		arg->atom.atom = atom;
>  		break;
>  
> -	case TEP_EVENT_DQUOTE:
>  	case TEP_EVENT_SQUOTE:
> +		arg->type = TEP_PRINT_ATOM;
> +		/* Make characters into numbers */
> +		if (asprintf(&arg->atom.atom, "%d", token[0]) < 0) {
> +			free_token(token);
> +			*tok = NULL;
> +			arg->atom.atom = NULL;
> +			return TEP_EVENT_ERROR;
> +		}
> +		free_token(token);
> +		type = read_token_item(event->tep, &token);
> +		break;
> +	case TEP_EVENT_DQUOTE:
>  		arg->type = TEP_PRINT_ATOM;
>  		arg->atom.atom = token;
>  		type = read_token_item(event->tep, &token);

Indeed I was on an older version, apologies.

I upgraded both libtraceevent and trace-cmd to master and applied your
patch, now the %c is formatted correctly.

However the arrows are still reversed.

Is this what you were expecting?

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com