linux-kernel - Re: [PATCH] perf stat-display: Check if snprintf()'s fmt argument is NULL

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Date:   Wed, 2 Aug 2023 10:45:47 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Kaige Ye <ye@...ge.org>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, namhyung@...nel.org, adrian.hunter@...el.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf stat-display: Check if snprintf()'s fmt argument is NULL

On Tue, Aug 1, 2023 at 9:04 PM Kaige Ye <ye@...ge.org> wrote:
>
> It is undefined behavior to pass NULL as snprintf()'s fmt argument.
> Here is an example to trigger the problem:
>
>   $ perf stat --metric-only -x, -e instructions -- sleep 1
>   insn per cycle,
>   Segmentation fault (core dumped)
>
> With this patch:
>
>   $ perf stat --metric-only -x, -e instructions -- sleep 1
>   insn per cycle,
>   ,
>
> Signed-off-by: Kaige Ye <ye@...ge.org>
> ---
>  tools/perf/util/stat-display.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
> index 7329b3340..e8936cffd 100644
> --- a/tools/perf/util/stat-display.c
> +++ b/tools/perf/util/stat-display.c
> @@ -438,7 +438,7 @@ static void print_metric_csv(struct perf_stat_config *config __maybe_unused,
>                 fprintf(out, "%s%s", config->csv_sep, config->csv_sep);
>                 return;
>         }
> -       snprintf(buf, sizeof(buf), fmt, val);
> +       snprintf(buf, sizeof(buf), fmt ?: "", val);

Hi,

I couldn't reproduce this error and the code immediately above here
does null check fmt:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/stat-display.c?h=perf-tools-next#n437
Perhaps this is an older version of the perf tool? You can run "perf
version" to determine this.

Thanks,
Ian


>         ends = vals = skip_spaces(buf);
>         while (isdigit(*ends) || *ends == '.')
>                 ends++;
> @@ -578,7 +578,7 @@ static void print_metric_only_csv(struct perf_stat_config *config __maybe_unused
>         if (!valid_only_metric(unit))
>                 return;
>         unit = fixunit(tbuf, os->evsel, unit);
> -       snprintf(buf, sizeof buf, fmt, val);
> +       snprintf(buf, sizeof(buf), fmt ?: "", val);
>         ends = vals = skip_spaces(buf);
>         while (isdigit(*ends) || *ends == '.')
>                 ends++;
> @@ -600,7 +600,7 @@ static void print_metric_only_json(struct perf_stat_config *config __maybe_unuse
>         if (!valid_only_metric(unit))
>                 return;
>         unit = fixunit(tbuf, os->evsel, unit);
> -       snprintf(buf, sizeof(buf), fmt, val);
> +       snprintf(buf, sizeof(buf), fmt ?: "", val);
>         ends = vals = skip_spaces(buf);
>         while (isdigit(*ends) || *ends == '.')
>                 ends++;
> --
> 2.41.0
>