netdev - Re: [PATCH net-next 3/6] net: ipv4: use kfree_skb_reason() in ip_rcv_finish

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Date:   Tue, 25 Jan 2022 19:18:15 -0700
From:   David Ahern <dsahern@...il.com>
To:     menglong8.dong@...il.com, kuba@...nel.org
Cc:     rostedt@...dmis.org, mingo@...hat.com, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, pablo@...filter.org,
        kadlec@...filter.org, fw@...len.de, imagedong@...cent.com,
        edumazet@...gle.com, alobakin@...me, paulb@...dia.com,
        pabeni@...hat.com, talalahmad@...gle.com, haokexin@...il.com,
        keescook@...omium.org, memxor@...il.com,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        cong.wang@...edance.com
Subject: Re: [PATCH net-next 3/6] net: ipv4: use kfree_skb_reason() in
 ip_rcv_finish_core()

On 1/24/22 6:15 AM, menglong8.dong@...il.com wrote:
> diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
> index ab9bee4bbf0a..77bb9ddc441b 100644
> --- a/net/ipv4/ip_input.c
> +++ b/net/ipv4/ip_input.c
> @@ -318,8 +318,10 @@ static int ip_rcv_finish_core(struct net *net, struct sock *sk,
>  {
>  	const struct iphdr *iph = ip_hdr(skb);
>  	int (*edemux)(struct sk_buff *skb);
> +	int err, drop_reason;
>  	struct rtable *rt;
> -	int err;
> +
> +	drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
>  
>  	if (ip_can_use_hint(skb, iph, hint)) {
>  		err = ip_route_use_hint(skb, iph->daddr, iph->saddr, iph->tos,
> @@ -339,8 +341,10 @@ static int ip_rcv_finish_core(struct net *net, struct sock *sk,
>  		if (ipprot && (edemux = READ_ONCE(ipprot->early_demux))) {
>  			err = INDIRECT_CALL_2(edemux, tcp_v4_early_demux,
>  					      udp_v4_early_demux, skb);
> -			if (unlikely(err))
> +			if (unlikely(err)) {
> +				drop_reason = SKB_DROP_REASON_EARLY_DEMUX;

is there really value in this one? You ignore the error case from
ip_route_use_hint which is a similar, highly unlikely error path so why
care about this one? The only failure case is ip_mc_validate_source from
udp_v4_early_demux and 'early demux' drops really mean nothing to the user.


>  				goto drop_error;
> +			}
>  			/* must reload iph, skb->head might have changed */
>  			iph = ip_hdr(skb);
>  		}
> @@ -353,8 +357,10 @@ static int ip_rcv_finish_core(struct net *net, struct sock *sk,
>  	if (!skb_valid_dst(skb)) {
>  		err = ip_route_input_noref(skb, iph->daddr, iph->saddr,
>  					   iph->tos, dev);
> -		if (unlikely(err))
> +		if (unlikely(err)) {
> +			drop_reason = SKB_DROP_REASON_IP_ROUTE_INPUT;

The reason codes should be meaningful to users and not derived from a
code path. What does SKB_DROP_REASON_IP_ROUTE_INPUT mean as a failure?


>  			goto drop_error;
> +		}
>  	}
>  
>  #ifdef CONFIG_IP_ROUTE_CLASSID