linux-kernel - [PATCH] ftrace: Avoid potential division by zero in function_stat

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Message-Id: <20250131155346.1313580-1-kniv@yandex-team.ru>
Date: Fri, 31 Jan 2025 18:53:46 +0300
From: Nikolay Kuratov <kniv@...dex-team.ru>
To: linux-kernel@...r.kernel.org
Cc: linux-trace-kernel@...r.kernel.org,
	Wen Yang <wenyang@...ux.alibaba.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Nikolay Kuratov <kniv@...dex-team.ru>,
	stable@...r.kernel.org
Subject: [PATCH] ftrace: Avoid potential division by zero in function_stat_show()

Cases rec->counter == {0, 1} are checked already.

While x * (x - 1) * 1000 = 0 have many solutions greater than 1 for both
modulo 2^32 and 2^64, that is not the case for x * (x - 1) = 0, so split
division into two.

It is not scary in practice because mod 2^64 solutions are huge and
minimal mod 2^32 solution is 30-bit number.

Cc: stable@...r.kernel.org
Fixes: e31f7939c1c27 ("ftrace: Avoid potential division by zero in function profiler")
Signed-off-by: Nikolay Kuratov <kniv@...dex-team.ru>
---
 kernel/trace/ftrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 728ecda6e8d4..e1c05c4c29c2 100644
--- 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);
 	}
 
 	trace_seq_init(&s);
-- 
2.34.1