netdev - Re: [PATCH net-next 1/3] net: gre_demux: add skb drop reasons to gre

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Date:   Tue, 15 Mar 2022 21:49:01 -0600
From:   David Ahern <dsahern@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>, menglong8.dong@...il.com
Cc:     rostedt@...dmis.org, mingo@...hat.com, xeb@...l.ru,
        davem@...emloft.net, yoshfuji@...ux-ipv6.org,
        imagedong@...cent.com, edumazet@...gle.com, kafai@...com,
        talalahmad@...gle.com, keescook@...omium.org, alobakin@...me,
        flyingpeng@...cent.com, mengensun@...cent.com,
        dongli.zhang@...cle.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Biao Jiang <benbjiang@...cent.com>
Subject: Re: [PATCH net-next 1/3] net: gre_demux: add skb drop reasons to
 gre_rcv()

On 3/15/22 9:08 PM, Jakub Kicinski wrote:
> On Mon, 14 Mar 2022 21:33:10 +0800 menglong8.dong@...il.com wrote:
>> +	reason = SKB_DROP_REASON_NOT_SPECIFIED;
>>  	if (!pskb_may_pull(skb, 12))
>>  		goto drop;
> 
> REASON_HDR_TRUNC ?
> 
>>  	ver = skb->data[1]&0x7f;
>> -	if (ver >= GREPROTO_MAX)
>> +	if (ver >= GREPROTO_MAX) {
>> +		reason = SKB_DROP_REASON_GRE_VERSION;
> 
> TBH I'm still not sure what level of granularity we should be shooting
> for with the reasons. I'd throw all unexpected header values into one 
> bucket, not go for a reason per field, per protocol. But as I'm said
> I'm not sure myself, so we can keep what you have..

I have stated before I do not believe every single drop point in the
kernel needs a unique reason code. This is overkill. The reason augments
information we already have -- the IP from kfree_skb tracepoint.

> 
>>  		goto drop;
>> +	}
>>  
>>  	rcu_read_lock();
>>  	proto = rcu_dereference(gre_proto[ver]);
>> -	if (!proto || !proto->handler)
>> +	if (!proto || !proto->handler) {
>> +		reason = SKB_DROP_REASON_GRE_NOHANDLER;
> 
> I think the ->handler check is defensive programming, there's no
> protocol upstream which would leave handler NULL.
> 
> This is akin to SKB_DROP_REASON_PTYPE_ABSENT, we can reuse that or add
> a new reason, but I'd think the phrasing should be kept similar.
> 
>>  		goto drop_unlock;
>> +	}
>>  	ret = proto->handler(skb);