linux-kernel - [PATCH 1/2] softirq: fix integer overflow in function show

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Date:   Mon, 24 Jul 2023 21:22:23 +0800
From:   thunder.leizhen@...weicloud.com
To:     "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:     Zhen Lei <thunder.leizhen@...wei.com>
Subject: [PATCH 1/2] softirq: fix integer overflow in function show_stat()

From: Zhen Lei <thunder.leizhen@...wei.com>

The statistics function of softirq is supported by commit aa0ce5bbc2db
("softirq: introduce statistics for softirq") in 2009. At that time,
64-bit processors should not have many cores and would not face
significant count overflow problems. Now it's common for a processor to
have hundreds of cores. Assume that there are 100 cores and 10
TIMER_SOFTIRQ are generated per second, then the 32-bit sum will be
overflowed after 50 days.

For example:
seq_put_decimal_ull(p, "softirq ", (unsigned long long)sum_softirq);
for (i = 0; i < NR_SOFTIRQS; i++)
	seq_put_decimal_ull(p, " ", per_softirq_sums[i]);

$ cat /proc/stat | tail -n 1
softirq 22929066124 9 2963267579 4128150 2618598635 546358555 0 \
629391610 1326100278 74637 1956244783

Here, the sum of per_softirq_sums[] is 10044164236 and is not equal to
22929066124. Because integers overflowed.

Therefore, change the type of local variable per_softirq_sums[] to u64.

Fixes: d3d64df21d3d ("proc: export statistics for softirq to /proc")
Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
---
 fs/proc/stat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index da60956b2915645..84aac577a50cabb 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -86,7 +86,7 @@ static int show_stat(struct seq_file *p, void *v)
 	u64 guest, guest_nice;
 	u64 sum = 0;
 	u64 sum_softirq = 0;
-	unsigned int per_softirq_sums[NR_SOFTIRQS] = {0};
+	u64 per_softirq_sums[NR_SOFTIRQS] = {0};
 	struct timespec64 boottime;
 
 	user = nice = system = idle = iowait =
-- 
2.25.1