linux-kernel - [PATCH] netfilter: nft_counter: Fix reset of counters on 32bit archs

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Message-ID: <20251215121258.843823-1-anders.grahn@westermo.com>
Date: Mon, 15 Dec 2025 13:12:57 +0100
From: Anders Grahn <anders.grahn@...il.com>
To: Pablo Neira Ayuso <pablo@...filter.org>,
	Jozsef Kadlecsik <kadlec@...filter.org>,
	Florian Westphal <fw@...len.de>,
	Phil Sutter <phil@....cc>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Anders Grahn <anders.grahn@...termo.com>,
	linux-kernel@...r.kernel.org,
	netfilter-devel@...r.kernel.org,
	coreteam@...filter.org,
	netdev@...r.kernel.org
Subject: [PATCH] netfilter: nft_counter: Fix reset of counters on 32bit archs

nft_counter_reset() calls u64_stats_add() with a negative value to reset
the counter. This will work on 64bit archs, hence the negative value
added will wrap as a 64bit value which then can wrap the stat counter as
well.

On 32bit archs, the added negative value will wrap as a 32bit value and
_not_ wrapping the stat counter properly. In most cases, this would just
lead to a very large 32bit value being added to the stat counter.

Fix by introducing u64_stats_sub().

Fixes: 4a1d3acd6ea8 ("netfilter: nft_counter: Use u64_stats_t for statistic")
Signed-off-by: Anders Grahn <anders.grahn@...termo.com>
---
 include/linux/u64_stats_sync.h | 10 ++++++++++
 net/netfilter/nft_counter.c    |  4 ++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
index 457879938fc1..9942d29b17e5 100644
--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -89,6 +89,11 @@ static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
 	local64_add(val, &p->v);
 }
 
+static inline void u64_stats_sub(u64_stats_t *p, unsigned long val)
+{
+	local64_sub(val, &p->v);
+}
+
 static inline void u64_stats_inc(u64_stats_t *p)
 {
 	local64_inc(&p->v);
@@ -130,6 +135,11 @@ static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
 	p->v += val;
 }
 
+static inline void u64_stats_sub(u64_stats_t *p, unsigned long val)
+{
+	p->v -= val;
+}
+
 static inline void u64_stats_inc(u64_stats_t *p)
 {
 	p->v++;
diff --git a/net/netfilter/nft_counter.c b/net/netfilter/nft_counter.c
index cc7325329496..0d70325280cc 100644
--- a/net/netfilter/nft_counter.c
+++ b/net/netfilter/nft_counter.c
@@ -117,8 +117,8 @@ static void nft_counter_reset(struct nft_counter_percpu_priv *priv,
 	nft_sync = this_cpu_ptr(&nft_counter_sync);
 
 	u64_stats_update_begin(nft_sync);
-	u64_stats_add(&this_cpu->packets, -total->packets);
-	u64_stats_add(&this_cpu->bytes, -total->bytes);
+	u64_stats_sub(&this_cpu->packets, total->packets);
+	u64_stats_sub(&this_cpu->bytes, total->bytes);
 	u64_stats_update_end(nft_sync);
 
 	local_bh_enable();
-- 
2.43.0