Re: net/netfilter/nft_synproxy.c:51:40: sparse: sparse: cast to non-scalar

[Fernando Fernandez Mancera <fmancera@suse.de>]

On 3/8/26 1:21 PM, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   c23719abc3308df7ed3ad35650ad211fb2d2003d
> commit: 36a3200575642846a96436d503d46544533bb943 netfilter: nft_synproxy: avoid possible data-race on update operation
> date:   9 weeks ago
> config: alpha-randconfig-r132-20260308 (https://download.01.org/0day-ci/archive/20260308/202603082002.ACHbGTsI-lkp@intel.com/config)
> compiler: alpha-linux-gcc (GCC) 8.5.0
> sparse: v0.6.5-rc1
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260308/202603082002.ACHbGTsI-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202603082002.ACHbGTsI-lkp@intel.com/
> 
> sparse warnings: (new ones prefixed by >>)
>>> net/netfilter/nft_synproxy.c:51:40: sparse: sparse: cast to non-scalar
>>> net/netfilter/nft_synproxy.c:51:40: sparse: sparse: cast from non-scalar
>     net/netfilter/nft_synproxy.c:82:40: sparse: sparse: cast to non-scalar
>     net/netfilter/nft_synproxy.c:82:40: sparse: sparse: cast from non-scalar
> 

Hi,

Is this a false positive? nf_synproxy_info struct is smaller than the 
word size and therefore READ_ONCE() should be fine. I guess I can use 
READ_ONCE() for every struct field.

What do you think Florian?

Thanks,
Fernando.

> vim +51 net/netfilter/nft_synproxy.c
> 
>      43	
>      44	static void nft_synproxy_eval_v4(const struct nft_synproxy *priv,
>      45					 struct nft_regs *regs,
>      46					 const struct nft_pktinfo *pkt,
>      47					 const struct tcphdr *tcp,
>      48					 struct tcphdr *_tcph,
>      49					 struct synproxy_options *opts)
>      50	{
>    > 51		struct nf_synproxy_info info = READ_ONCE(priv->info);
>      52		struct net *net = nft_net(pkt);
>      53		struct synproxy_net *snet = synproxy_pernet(net);
>      54		struct sk_buff *skb = pkt->skb;
>      55	
>      56		if (tcp->syn) {
>      57			/* Initial SYN from client */
>      58			nft_synproxy_tcp_options(opts, tcp, snet, &info, priv);
>      59			synproxy_send_client_synack(net, skb, tcp, opts);
>      60			consume_skb(skb);
>      61			regs->verdict.code = NF_STOLEN;
>      62		} else if (tcp->ack) {
>      63			/* ACK from client */
>      64			if (synproxy_recv_client_ack(net, skb, tcp, opts,
>      65						     ntohl(tcp->seq))) {
>      66				consume_skb(skb);
>      67				regs->verdict.code = NF_STOLEN;
>      68			} else {
>      69				regs->verdict.code = NF_DROP;
>      70			}
>      71		}
>      72	}
>      73	
>