linux-kernel - Re: [PATCH 1/5] tracing: Don't show dynamic string internals in synthetic event description

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Date:   Mon, 12 Oct 2020 10:37:19 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     rostedt@...dmis.org, axelrasmussen@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] tracing: Don't show dynamic string internals in
 synthetic event description

Hi Masami,

On Sun, 2020-10-11 at 00:03 +0900, Masami Hiramatsu wrote:
> Hi Tom,
> 
> On Fri,  9 Oct 2020 10:17:07 -0500
> Tom Zanussi <zanussi@...nel.org> wrote:
> 
> > For synthetic event dynamic fields, the type contains "__data_loc",
> > which is basically an internal part of the type which is only meant
> > to
> > be displayed in the format, not in the event description itself,
> > which
> > is confusing to users since they can't use __data_loc on the
> > command-line to define an event field, which printing it would lead
> > them to believe.
> > 
> > So filter it out from the description, while leaving it in the
> > type.
> > 
> 
> OK, I confirmed this removes __data_loc from synth_events interface.
> However, I also found another issue.
> 
>   /sys/kernel/debug/tracing # echo "myevent char str[]; int v" >>
> synthetic_events  
>   /sys/kernel/debug/tracing # cat synthetic_events 
>   myevent	char[]; str; int v
> 
> It seems that the type "char[]" includes ";" as a type, this results
> 

Yeah, this isn't a result of this patchset - it's a different bug which
I'll submit a new fix for.  Basically in the array case it doesn't
effectively strip off trailing characters when creating the array type.

Thanks,

Tom

> 
>   /sys/kernel/debug/tracing # cat events/synthetic/myevent/format 
>   name: myevent
>   ID: 1220
>   format:
> 	field:unsigned short common_type;	offset:0;	size:2;	signe
> d:0;
> 	field:unsigned char common_flags;	offset:2;	size:1;	signe
> d:0;
> 	field:unsigned char common_preempt_count;	offset:3;	size:
> 1;	signed:0;
> 	field:int common_pid;	offset:4;	size:4;	signed:1;
> 
> 	field:__data_loc char[]; str;	offset:8;	size:8;	signe
> d:1;
> 	field:int v;	offset:16;	size:4;	signed:1;
> 
>   print fmt: "str=%.*s, v=%d", __get_str(str), REC->v
> 
> 
> As you can see, the field type has ";" in format file too. This will
> prevent
> parsing event information correctly.
> I also try to remove ";" as below, it seems to work correctly.
> 
>   /sys/kernel/debug/tracing # echo "myevent char[] str; int v" >>
> synthetic_events 
>   /sys/kernel/debug/tracing # cat events/synthetic/myevent/format 
>   name: myevent
>   ID: 1221
>   format:
> 	field:unsigned short common_type;	offset:0;	size:2;	signe
> d:0;
> 	field:unsigned char common_flags;	offset:2;	size:1;	signe
> d:0;
> 	field:unsigned char common_preempt_count;	offset:3;	size:
> 1;	signed:0;
> 	field:int common_pid;	offset:4;	size:4;	signed:1;
> 
> 	field:__data_loc char[] str;	offset:8;	size:8;	signe
> d:1;
> 	field:int v;	offset:16;	size:4;	signed:1;
> 
>   print fmt: "str=%.*s, v=%d", __get_str(str), REC->v
> 
> 
> Thank you,
> 
> > Reported-by: Masami Hiramatsu <mhiramat@...nel.org>
> > Signed-off-by: Tom Zanussi <zanussi@...nel.org>
> > ---
> >  kernel/trace/trace_events_synth.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/trace_events_synth.c
> > b/kernel/trace/trace_events_synth.c
> > index 3b2dcc42b8ee..b19e2f4159ab 100644
> > --- a/kernel/trace/trace_events_synth.c
> > +++ b/kernel/trace/trace_events_synth.c
> > @@ -1867,14 +1867,22 @@ static int __synth_event_show(struct
> > seq_file *m, struct synth_event *event)
> >  {
> >  	struct synth_field *field;
> >  	unsigned int i;
> > +	char *type, *t;
> >  
> >  	seq_printf(m, "%s\t", event->name);
> >  
> >  	for (i = 0; i < event->n_fields; i++) {
> >  		field = event->fields[i];
> >  
> > +		type = field->type;
> > +		t = strstr(type, "__data_loc");
> > +		if (t) { /* __data_loc belongs in format but not event
> > desc */
> > +			t += sizeof("__data_loc");
> > +			type = t;
> > +		}
> > +
> >  		/* parameter values */
> > -		seq_printf(m, "%s %s%s", field->type, field->name,
> > +		seq_printf(m, "%s %s%s", type, field->name,
> >  			   i == event->n_fields - 1 ? "" : "; ");
> >  	}
> >  
> > -- 
> > 2.17.1
> > 
> 
>