Re: [PATCH] ftrace: Avoid potential division by zero in function_stat_show()

[Steven Rostedt]

On Fri, 31 Jan 2025 18:53:46 +0300
Nikolay Kuratov <k...@yandex-team.ru> wrote:

> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -570,12 +570,12 @@ static int function_stat_show(struct seq_file *m, void 
> *v)
>               stddev = rec->counter * rec->time_squared -
>                        rec->time * rec->time;
>  
> +             stddev = div64_ul(stddev, rec->counter * (rec->counter - 1));
>               /*
>                * Divide only 1000 for ns^2 -> us^2 conversion.
>                * trace_print_graph_duration will divide 1000 again.
>                */
> -             stddev = div64_ul(stddev,
> -                               rec->counter * (rec->counter - 1) * 1000);
> +             stddev = div64_ul(stddev, 1000);
>       }
>  

Why make it more complex than it needs to be? We can simply have:

        if (rec->counter <= 1) {
                stddev = 0;
        } else {
                unsigned long counter = rec->counter * (rec->counter - 1) * 
1000;

                stddev = rec->counter * rec->time_squared -
                         rec->time * rec->time;

                /* Check for overflow */
                stddev = counter > rec->counter ? div64_ul(stddev, counter) : 0;
        }

And be done with it.

I'll make the change and give you a "Reported-by".

Thanks,

-- Steve