[PATCH 3.16 154/245] ftrace: Avoid potential division by zero in function profiler

[Ben Hutchings]

3.16.83-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Wen Yang <wenyang@xxxxxxxxxxxxxxxxx>

commit e31f7939c1c27faa5d0e3f14519eaf7c89e8a69d upstream.

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.

Link: http://lkml.kernel.org/r/20200103030248.14516-1-wenyang@xxxxxxxxxxxxxxxxx

Fixes: e330b3bcd8319 ("tracing: Show sample std dev in function profiling")
Fixes: 34886c8bc590f ("tracing: add average time in function to function profiler")
Signed-off-by: Wen Yang <wenyang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/trace/ftrace.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -595,8 +595,7 @@ static int function_stat_show(struct seq
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	seq_printf(m, "    ");
-	avg = rec->time;
-	do_div(avg, rec->counter);
+	avg = div64_ul(rec->time, rec->counter);
 
 	/* Sample standard deviation (s^2) */
 	if (rec->counter <= 1)
@@ -613,7 +612,8 @@ static int function_stat_show(struct seq
 		 * 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);