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Date:   Wed, 9 Feb 2022 21:12:02 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Menglong Dong <menglong8.dong@gmail.com>
Cc:     David Ahern <dsahern@gmail.com>, David Ahern <dsahern@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>, mingo@redhat.com,
        David Miller <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        pablo@netfilter.org, kadlec@netfilter.org,
        Florian Westphal <fw@strlen.de>,
        Menglong Dong <imagedong@tencent.com>,
        Eric Dumazet <edumazet@google.com>, alobakin@pm.me,
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        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>, netfilter-devel@vger.kernel.org,
        coreteam@netfilter.org, Cong Wang <cong.wang@bytedance.com>
Subject: Re: [PATCH v3 net-next 1/7] net: skb_drop_reason: add document for
 drop reasons
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On Thu, 10 Feb 2022 11:19:49 +0800 Menglong Dong wrote:
> I'm doing the job of using kfree_skb_reason() for the TCP layer,
> and I have some puzzles.
> 
> When collecting drop reason for tcp_v4_inbound_md5_hash() in
> tcp_v4_rcv(), I come up with 2 ways:
> 
> First way: pass the address of reason to tcp_v4_inbound_md5_hash()
> like this:
> 
>  static bool tcp_v4_inbound_md5_hash(const struct sock *sk,
>                       const struct sk_buff *skb,
> -                    int dif, int sdif)
> +                    int dif, int sdif,
> +                    enum skb_drop_reason *reason)
> 
> This can work, but many functions like tcp_v4_inbound_md5_hash()
> need to do such a change.
> 
> Second way: introduce a 'drop_reason' field to 'struct sk_buff'. Therefore,
> drop reason can be set by 'skb->drop_reason = SKB_DROP_REASON_XXX'
> anywhere.
> 
> For TCP, there are many cases where you can't get a drop reason in
> the place where skb is freed, so I think there needs to be a way to
> deeply collect drop reasons. The second can resolve this problem
> easily, but extra fields may have performance problems.
> 
> Do you have some better ideas?

On a quick look tcp_v4_inbound_md5_hash() returns a drop / no drop
decision, so you could just change the return type to enum
skb_drop_reason. SKB_DROP_REASON_NOT_SPECIFIED is 0 is false, 
so if (reason) goto drop; logic will hold up.