[PATCH v2] ftrace: avoid potential division by zero

[PATCH v2] ftrace: avoid potential division by zero

[Wen Yang]

The ftrace_profile->counter is unsigned long and
do_div truncates it to 32 bits, which means it can test
non-zero and be truncated to zero for division.
Fix this issue by using div64_ul() instead.

Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org
---
v2: get rid of the pre-assigning of avg.

 kernel/trace/ftrace.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index ac99a35..9bf1f2c 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -526,8 +526,7 @@ static int function_stat_show(struct seq_file *m, void *v)
 	}
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
-	avg = rec->time;
-	do_div(avg, rec->counter);
+	avg = div64_ul(rec->time, rec->counter);
 	if (tracing_thresh && (avg < tracing_thresh))
 		goto out;
 #endif
@@ -553,7 +552,8 @@ static int function_stat_show(struct seq_file *m, void *v)
 		 * Divide only 1000 for ns^2 -> us^2 conversion.
 		 * trace_print_graph_duration will divide 1000 again.
 		 */
-		do_div(stddev, rec->counter * (rec->counter - 1) * 1000);
+		stddev = div64_ul(stddev,
+				  rec->counter * (rec->counter - 1) * 1000);
 	}
 
 	trace_seq_init(&s);
-- 
1.8.3.1

Re: [PATCH v2] ftrace: avoid potential division by zero

[Justin Capella]

> -               do_div(stddev, rec->counter * (rec->counter - 1) * 1000);
>
> +               stddev = div64_ul(stddev,
> +                                 rec->counter * (rec->counter - 1) * 1000);


Is a rec->counter > 1 assertion needed here?

Re: [PATCH v2] ftrace: avoid potential division by zero

[Wen Yang]

On 2020/1/3 4:00 下午, Justin Capella wrote:
>> -               do_div(stddev, rec->counter * (rec->counter - 1) * 1000);
>>
>> +               stddev = div64_ul(stddev,
>> +                                 rec->counter * (rec->counter - 1) * 1000);
> 
> 
> Is a rec->counter > 1 assertion needed here?
> 

Hello, the above lines deal with this situation:

         if (rec->counter <= 1)
                 stddev = 0;
         else {


--
Regards,
Wen