Re: [PATCH] tracing: Fix synth event printk format for str fields

[Douglas Raillard]

Hi,

On 19-03-2025 12:18, Masami Hiramatsu (Google) wrote:
> Hi,
>
> On Tue, 18 Mar 2025 18:09:38 +0000
> Douglas RAILLARD <douglas.raillard@xxxxxxx> wrote:
>
> > From: Douglas Raillard <douglas.raillard@xxxxxxx>
> >
> > The printk format for synth event uses "%.*s" to print string fields,
> > but then only passes the pointer part as var arg.
> >
> > Add the missing precision var arg.
>
> I'm not sure what you want to. Would you mean showing the string length too?
> But I think actual output(*) shows only string, right?
>
> (*) the output data which can be read from `trace` file.

The "%.*s" printf/printk format specifier has 2 components:
1. the "*" part that in printf documentation called the "precision"
2. the "s" part that is called the "specifier"

For a "s" specifier, the precision is interpreted as the number of chars to display from that string.
In any case, using "*" as precision means the value is passed dynamically, as a parameter to printf/printk,
before the actual char *. Both those args are consumed by printk to display "%.*s", the precision is never
displayed on its own.

In the current state, synthetic event use "%.*s" for string fields, but then only a single `__get_str(field)`
is passed to printk, so it's missing the precision arg. Both trace-cmd and our parser report an error because of that.

The alternative would be to just use "%s" instead of "%.*s", but I assume whoever wrote the code initially had a reason
to use the precision (maybe to print non-null-terminated strings ?), so I simply fixed the arguments.

> Thank you,
>
> > Signed-off-by: Douglas Raillard <douglas.raillard@xxxxxxx>
> > ---
> >   kernel/trace/trace_events_synth.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
> > index e3f7d09e5512..274b9b1d9f7d 100644
> > --- a/kernel/trace/trace_events_synth.c
> > +++ b/kernel/trace/trace_events_synth.c
> > @@ -620,7 +620,8 @@ static int __set_synth_event_print_fmt(struct synth_event *event,
> >   		if (event->fields[i]->is_string &&
> >   		    event->fields[i]->is_dynamic)
> >   			pos += snprintf(buf + pos, LEN_OR_ZERO,
> > -				", __get_str(%s)", event->fields[i]->name);
> > +				", (int)__get_dynamic_array_len(%s), __get_str(%s)",
> > +				event->fields[i]->name, event->fields[i]->name);
> >   		else if (event->fields[i]->is_stack)
> >   			pos += snprintf(buf + pos, LEN_OR_ZERO,
> >   				", __get_stacktrace(%s)", event->fields[i]->name);
> > --
> > 2.43.0