Re: [PATCH v2 2/3] perf trace: avoid garbage when not printing a syscall's arguments

[Howard Chu]

Hello,

This patch solves the garbage output problem I got, and your test can be run.

Thanks,
Howard

On Thu, Nov 7, 2024 at 3:21 PM Benjamin Peterson <benjamin@xxxxxxxxxxx> wrote:
>
> syscall__scnprintf_args may not place anything in the output buffer
> (e.g., because the arguments are all zero). If that happened in
> trace__fprintf_sys_enter, its fprintf would receive an unitialized
> buffer leading to garbage output.
>
> Fix the problem by passing the (possibly zero) bounds of the argument
> buffer to the output fprintf.
>
> Fixes: a98392bb1e169 ("perf trace: Use beautifiers on syscalls:sys_enter_ handlers")
> Signed-off-by: Benjamin Peterson <benjamin@xxxxxxxxxxx>
> ---
>  tools/perf/builtin-trace.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index f6179b13b8b4..28f61d10a2f8 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -2702,6 +2702,7 @@ static int trace__fprintf_sys_enter(struct trace *trace, struct evsel *evsel,
>         char msg[1024];
>         void *args, *augmented_args = NULL;
>         int augmented_args_size;
> +       size_t printed = 0;
>
>         if (sc == NULL)
>                 return -1;
> @@ -2717,8 +2718,8 @@ static int trace__fprintf_sys_enter(struct trace *trace, struct evsel *evsel,
>
>         args = perf_evsel__sc_tp_ptr(evsel, args, sample);
>         augmented_args = syscall__augmented_args(sc, sample, &augmented_args_size, trace->raw_augmented_syscalls_args_size);
> -       syscall__scnprintf_args(sc, msg, sizeof(msg), args, augmented_args, augmented_args_size, trace, thread);
> -       fprintf(trace->output, "%s", msg);
> +       printed += syscall__scnprintf_args(sc, msg, sizeof(msg), args, augmented_args, augmented_args_size, trace, thread);
> +       fprintf(trace->output, "%.*s", (int)printed, msg);
>         err = 0;
>  out_put:
>         thread__put(thread);
> --
> 2.39.5
>
>