netdev - Re: [PATCH v2] netfilter: amanda: fix RCU pointer typing for nf_nat_amanda

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Message-ID: <CABFUUZELXbEKyjMxOBfoL246dmtBSS_oe0zeWwnkmrXXpyv3Yg@mail.gmail.com>
Date: Wed, 4 Feb 2026 09:56:59 +0800
From: sun jian <sun.jian.kdev@...il.com>
To: Florian Westphal <fw@...len.de>
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 v2] netfilter: amanda: fix RCU pointer typing for nf_nat_amanda_hook

On Wed, Feb 4, 2026 at 12:59 AM Florian Westphal <fw@...len.de> wrote:
>
> sun jian <sun.jian.kdev@...il.com> wrote:
> > > Sun Jian <sun.jian.kdev@...il.com> wrote:
> > > >  enum amanda_strings {
> > > > @@ -98,7 +98,12 @@ static int amanda_help(struct sk_buff *skb,
> > > >       u_int16_t len;
> > > >       __be16 port;
> > > >       int ret = NF_ACCEPT;
> > > > -     typeof(nf_nat_amanda_hook) nf_nat_amanda;
> > > > +     unsigned int (*nf_nat_amanda)(struct sk_buff *skb,
> > > > +                                   enum ip_conntrack_info ctinfo,
> > > > +                                   unsigned int protoff,
> > > > +                                   unsigned int matchoff,
> > > > +                                   unsigned int matchlen,
> > > > +                                   struct nf_conntrack_expect *exp);
> > >
> > > Why is that needed?
> > Correct. Manual declaration is indeed verbose.
> >
> > The reason I used it was that typeof(nf_nat_amanda_hook) carries over
> > the __rcu attribute to the local variable, which triggers a Sparse
> > warning when assigning the result of rcu_dereference().
>
> sparse doesn't generate such a warning for me.

I re-verified this with GCC 13.3.0 and Sparse v0.6.4-73-gfbdde312.
Even without LLVM=1, Sparse still reports the "different address spaces"
error for amanda on my machine:

net/netfilter/nf_conntrack_amanda.c:158:33: error: incompatible types
in comparison expression (different address spaces):
net/netfilter/nf_conntrack_amanda.c:158:33: unsigned int ( [noderef]
__rcu * )( ... ) net/netfilter/nf_conntrack_amanda.c:158:33: unsigned
int ( * )( ... )

It seems newer Sparse versions are more strict about RCU attributes on
function pointers.
To avoid manual declaration while stripping the __rcu attribute, I
will switch to:
typeof(*nf_nat_amanda_hook) *nf_nat_amanda;

>
> Also, this pattern you are changing here isn't specific to amanda, it
> exists elsewhere as well:
>
> net/netfilter/nf_conntrack_snmp.c:42:23: error: incompatible types in comparison expression (different address spaces):
> net/netfilter/nf_conntrack_tftp.c:78:31: error: incompatible types in comparison expression (different address spaces):
> net/netfilter/nf_conntrack_irc.c:242:38: error: incompatible types in comparison expression (different address spaces):
> net/netfilter/nf_conntrack_ftp.c:521:22: error: incompatible types in comparison expression (different address spaces):
>
> so why only fix this annotation for amanda?
Ack, I will prepare a V4 patch series to fix it for  amanda, snmp,
tftp, irc, and ftp together.

Regards,

Sun