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

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Message-ID: <aUAAuyGGhDjyfNoM@strlen.de>
Date: Mon, 15 Dec 2025 13:36:11 +0100
From: Florian Westphal <fw@...len.de>
To: Anders Grahn <anders.grahn@...il.com>
Cc: Pablo Neira Ayuso <pablo@...filter.org>,
	Jozsef Kadlecsik <kadlec@...filter.org>, 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>,
	Anders Grahn <anders.grahn@...termo.com>,
	linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
	coreteam@...filter.org, netdev@...r.kernel.org
Subject: Re: [PATCH] netfilter: nft_counter: Fix reset of counters on 32bit
 archs

Anders Grahn <anders.grahn@...il.com> wrote:
> 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);
> +}

That still truncates val on 32bit.  Maybe use "s64 val"?