linux-kernel - Re: [PATCH] netfilter: amanda: fix RCU pointer typing for nf_nat_amanda

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Message-ID: <aYH9FwwOTD28Gn04@strlen.de>
Date: Tue, 3 Feb 2026 14:50:15 +0100
From: Florian Westphal <fw@...len.de>
To: Sun Jian <sun.jian.kdev@...il.com>
Cc: Pablo Neira Ayuso <pablo@...filter.org>, Phil Sutter <phil@....cc>,
	Simon Horman <horms@...nel.org>, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netfilter: amanda: fix RCU pointer typing for
 nf_nat_amanda_hook

Sun Jian <sun.jian.kdev@...il.com> wrote:
> The nf_nat_amanda_hook pointer is accessed via rcu_dereference(), but
> it lacks the __rcu annotation in its declaration and definition. Sparse
> reports "incompatible types in comparison expression (different
> address spaces)" errors in nf_conntrack_amanda.c.
> 
> Fix this by:
> 1. Adding __rcu and __read_mostly to the global nf_nat_amanda_hook
>    declaration.
> 2. Adding __rcu to the global nf_nat_amanda_hook definition.
> 3. Explicitly declaring the local nf_nat_amanda function pointer
>    without __rcu to store the dereferenced pointer.
> 4. Using rcu_dereference_raw() to fetch the hook address, which
>    satisfies sparse's type checking for function pointers.

This doesn't look right, esp. step 4.  Why not:

diff --git a/include/linux/netfilter/nf_conntrack_amanda.h b/include/linux/netfilter/nf_conntrack_amanda.h
--- a/include/linux/netfilter/nf_conntrack_amanda.h
+++ b/include/linux/netfilter/nf_conntrack_amanda.h
@@ -7,7 +7,7 @@
 #include <linux/skbuff.h>
 #include <net/netfilter/nf_conntrack_expect.h>

-extern unsigned int (*nf_nat_amanda_hook)(struct sk_buff *skb,
+extern unsigned int (__rcu *nf_nat_amanda_hook)(struct sk_buff *skb,
                                          enum ip_conntrack_info ctinfo,
                                          unsigned int protoff,
                                          unsigned int matchoff,
diff --git a/net/netfilter/nf_conntrack_amanda.c b/net/netfilter/nf_conntrack_amanda.c
--- a/net/netfilter/nf_conntrack_amanda.c
+++ b/net/netfilter/nf_conntrack_amanda.c
@@ -37,7 +37,7 @@ MODULE_PARM_DESC(master_timeout, "timeout for the master connection");
 module_param(ts_algo, charp, 0400);
 MODULE_PARM_DESC(ts_algo, "textsearch algorithm to use (default kmp)");

-unsigned int (*nf_nat_amanda_hook)(struct sk_buff *skb,
+unsigned int (__rcu *nf_nat_amanda_hook)(struct sk_buff *skb,
                                   enum ip_conntrack_info ctinfo,
                                   unsigned int protoff,
                                   unsigned int matchoff,
?