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

[Menglong Dong <menglong8.dong@gmail.com>]

Hello!

On Tue, Feb 1, 2022 at 1:14 AM David Ahern <dsahern@gmail.com> wrote:
>
> On 1/28/22 12:33 AM, menglong8.dong@gmail.com wrote:
> > From: Menglong Dong <imagedong@tencent.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@tencent.com>
> > ---
> >  include/linux/skbuff.h | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
>
> Reviewed-by: David Ahern <dsahern@kernel.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