netdev - Re: [PATCH v3 net-next 1/7] net: skb_drop_reason: add document for drop reasons

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Date:   Thu, 10 Feb 2022 11:19:49 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, mingo@...hat.com,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        pablo@...filter.org, kadlec@...filter.org,
        Florian Westphal <fw@...len.de>,
        Menglong Dong <imagedong@...cent.com>,
        Eric Dumazet <edumazet@...gle.com>, alobakin@...me,
        paulb@...dia.com, Kees Cook <keescook@...omium.org>,
        talalahmad@...gle.com, haokexin@...il.com, memxor@...il.com,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, Cong Wang <cong.wang@...edance.com>
Subject: Re: [PATCH v3 net-next 1/7] net: skb_drop_reason: add document for
 drop reasons

Hello!

On Tue, Feb 1, 2022 at 1:14 AM David Ahern <dsahern@...il.com> wrote:
>
> On 1/28/22 12:33 AM, menglong8.dong@...il.com wrote:
> > From: Menglong Dong <imagedong@...cent.com>
> >
> > Add document for following existing drop reasons:
> >
> > SKB_DROP_REASON_NOT_SPECIFIED
> > SKB_DROP_REASON_NO_SOCKET
> > SKB_DROP_REASON_PKT_TOO_SMALL
> > SKB_DROP_REASON_TCP_CSUM
> > SKB_DROP_REASON_SOCKET_FILTER
> > SKB_DROP_REASON_UDP_CSUM
> >
> > Signed-off-by: Menglong Dong <imagedong@...cent.com>
> > ---
> >  include/linux/skbuff.h | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
>
> Reviewed-by: David Ahern <dsahern@...nel.org>
>
>

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?

Thanks!
Menglong Dong