Re: [PATCH v2] perf trace: avoid garbage when not printing a trace event's arguments

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Fri, Nov 01, 2024 at 02:00:46PM -0700, Howard Chu wrote:
> Hello Benjamin,
> 
> Before your patch:
> 
> perf $ ./perf trace -e net:netif_rx_exit
>      0.000 irq/79-brcmf_p/1694977 net:netif_rx_exit(6n<)
>     28.153 irq/79-brcmf_p/1694977 net:netif_rx_exit(6n<)
>     36.429 irq/79-brcmf_p/1694977 net:netif_rx_exit(6n<)
>     36.461 irq/79-brcmf_p/1694977 net:netif_rx_exit(6n<)
> 
> After:
> 
> perf $ ./perf trace -e net:netif_rx_exit
>      0.000 irq/79-brcmf_p/1694977 net:netif_rx_exit()
>      7.352 irq/79-brcmf_p/1694977 net:netif_rx_exit()
>     30.232 irq/79-brcmf_p/1694977 net:netif_rx_exit()
>     37.529 irq/79-brcmf_p/1694977 net:netif_rx_exit()
> 
> It works beautifully, but I'm thinking can we simplify it by just doing:
> 
> +       char bf[2048] = { 0 };
>         size_t size = sizeof(bf);
> 
> That being said, trace__fprintf_tp_fields() should return void because
> currently its return value is not used by any other functions, plus
> this line:
> 
> return printed + fprintf(trace->output, "%s", bf);
> 
> in trace__fprintf_tp_fields() does not make much sense, since that
> will be 2 * printed. So I'm thinking maybe:
> 
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index 748b061f8678..8a628d1e80f3 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -3021,10 +3021,10 @@ static void bpf_output__fprintf(struct trace *trace,
>         ++trace->nr_events_printed;
>  }
> 
> -static size_t trace__fprintf_tp_fields(struct trace *trace, struct
> evsel *evsel, struct perf_sample *sample,
> +static void trace__fprintf_tp_fields(struct trace *trace, struct
> evsel *evsel, struct perf_sample *sample,
>                                        struct thread *thread, void
> *augmented_args, int augmented_args_size)
>  {
> -       char bf[2048];
> +       char bf[2048] = { 0 };
>         size_t size = sizeof(bf);
>         struct tep_format_field *field = evsel->tp_format->format.fields;
>         struct syscall_arg_fmt *arg = __evsel__syscall_arg_fmt(evsel);
> @@ -3087,7 +3087,7 @@ static size_t trace__fprintf_tp_fields(struct
> trace *trace, struct evsel *evsel,
>                 printed += syscall_arg_fmt__scnprintf_val(arg, bf +
> printed, size - printed, &syscall_arg, val);
>         }
> 
> -       return printed + fprintf(trace->output, "%s", bf);
> +       fprintf(trace->output, "%s", bf);
>  }
> 
>  static int trace__event_handler(struct trace *trace, struct evsel *evsel,
> 
> 
> Then this will be one more diff line than yours.
> 
> But that's just my opinion, yours works just fine, thank you :).
> Arnaldo, what do you think?
> 
> Tested-by: Howard Chu <howardchu95@gmail.com>

I haven't tested it yet, just in my mind :-)

The patch looks ok and seems to fix a real problem, my only concern, a
pet peeve, was that it, in addition to fixing a real problem, did an
unrelated change, the "Remove the return value...", that part looks like
a distraction, something that shouldn't be there.

- Arnaldo
 
> Thanks,
> Howard
> 
> On Fri, Nov 1, 2024 at 10:27 AM Benjamin Peterson <benjamin@engflow.com> wrote:
> >
> > trace__fprintf_tp_fields may not print any tracepoint arguments. E.g., if the
> > argument values are all zero. Previously, this would result in a totally
> > uninitialized buffer being passed to fprintf, which could lead to garbage on the
> > console. Fix the problem by passing the number of initialized bytes fprintf.
> >
> > Remove the return value of trace__fprintf_tp_fields, since it was meaningless
> > and ignored.
> >
> > Fixes: f11b2803bb88 ("perf trace: Allow choosing how to augment the tracepoint arguments")
> > Signed-off-by: Benjamin Peterson <benjamin@engflow.com>
> > ---
> >  tools/perf/builtin-trace.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> > index d3f11b90d025..4e785ea29df6 100644
> > --- a/tools/perf/builtin-trace.c
> > +++ b/tools/perf/builtin-trace.c
> > @@ -3021,7 +3021,7 @@ static void bpf_output__fprintf(struct trace *trace,
> >         ++trace->nr_events_printed;
> >  }
> >
> > -static size_t trace__fprintf_tp_fields(struct trace *trace, struct evsel *evsel, struct perf_sample *sample,
> > +static void trace__fprintf_tp_fields(struct trace *trace, struct evsel *evsel, struct perf_sample *sample,
> >                                        struct thread *thread, void *augmented_args, int augmented_args_size)
> >  {
> >         char bf[2048];
> > @@ -3087,7 +3087,7 @@ static size_t trace__fprintf_tp_fields(struct trace *trace, struct evsel *evsel,
> >                 printed += syscall_arg_fmt__scnprintf_val(arg, bf + printed, size - printed, &syscall_arg, val);
> >         }
> >
> > -       return printed + fprintf(trace->output, "%s", bf);
> > +       fprintf(trace->output, "%.*s", (int)printed, bf);
> >  }
> >
> >  static int trace__event_handler(struct trace *trace, struct evsel *evsel,
> > --
> > 2.39.5
> >