linux-kernel - [PATCH] ftrace: avoid potential division by zero

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Date:   Wed,  1 Jan 2020 17:32:19 +0800
From:   Wen Yang <wenyang@...ux.alibaba.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     xlpang@...ux.alibaba.com, Wen Yang <wenyang@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] ftrace: avoid potential division by zero

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@...ux.alibaba.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: linux-kernel@...r.kernel.org
---
 kernel/trace/ftrace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index ac99a35..a490ba5 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -527,7 +527,7 @@ 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(avg, rec->counter);
 	if (tracing_thresh && (avg < tracing_thresh))
 		goto out;
 #endif
@@ -553,7 +553,8 @@ 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