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 21:42:14 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...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

On Thu, Feb 10, 2022 at 1:12 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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.

Yeah, that's an idea. But some functions are more complex, such as
tcp_rcv_state_process() and tcp_rcv_state_process()->tcp_v4_conn_request().
The return value of tcp_rcv_state_process() can't be reused, and it's hard
to add a function param of type 'enum skb_drop_reason *' to
tcp_v4_conn_request().

There are some nice drop reasons in tcp_v4_conn_request(), it's a pity to
give up them.

How about introducing a field to 'struct sock' for drop reasons? As sk is
locked during the packet process in tcp_v4_do_rcv(), this seems to work.

Thanks!
Menglong Dong