Re: [PATCH net-next v6 0/7] Implement reset reason mechanism to detect

[Jason Xing <kerneljasonxing@gmail.com>]

On Fri, Apr 19, 2024 at 3:02 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Fri, Apr 19, 2024 at 4:31 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
> >
> > On Fri, Apr 19, 2024 at 7:26 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
> > >
> > > > When I said "If you feel the need to put them in a special group, this
> > > > is fine by me.",
> > > > this was really about partitioning the existing enum into groups, if
> > > > you prefer having a group of 'RES reasons'
> > >
> > > Are you suggesting copying what we need from enum skb_drop_reason{} to
> > > enum sk_rst_reason{}? Why not reusing them directly. I have no idea
> > > what the side effect of cast conversion itself is?
> >
> > Sorry that I'm writing this email. I'm worried my statement is not
> > that clear, so I write one simple snippet which can help me explain
> > well :)
> >
> > Allow me give NO_SOCKET as an example:
> > diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> > index e63a3bf99617..2c9f7364de45 100644
> > --- a/net/ipv4/icmp.c
> > +++ b/net/ipv4/icmp.c
> > @@ -767,6 +767,7 @@ void __icmp_send(struct sk_buff *skb_in, int type,
> > int code, __be32 info,
> >         if (!fl4.saddr)
> >                 fl4.saddr = htonl(INADDR_DUMMY);
> >
> > +       trace_icmp_send(skb_in, type, code);
> >         icmp_push_reply(sk, &icmp_param, &fl4, &ipc, &rt);
> >  ende:
> >         ip_rt_put(rt);
> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index 1e650ec71d2f..d5963831280f 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
> > @@ -2160,6 +2160,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
> >  {
> >         struct net *net = dev_net(skb->dev);
> >         enum skb_drop_reason drop_reason;
> > +       enum sk_rst_reason rst_reason;
> >         int sdif = inet_sdif(skb);
> >         int dif = inet_iif(skb);
> >         const struct iphdr *iph;
> > @@ -2355,7 +2356,8 @@ int tcp_v4_rcv(struct sk_buff *skb)
> >  bad_packet:
> >                 __TCP_INC_STATS(net, TCP_MIB_INERRS);
> >         } else {
> > -               tcp_v4_send_reset(NULL, skb);
> > +               rst_reason = RST_REASON_NO_SOCKET;
> > +               tcp_v4_send_reset(NULL, skb, rst_reason);
> >         }
> >
> >  discard_it:
> >
> > As you can see, we need to add a new 'rst_reason' variable which
> > actually is the same as drop reason. They are the same except for the
> > enum type... Such rst_reasons/drop_reasons are all over the place.
> >
> > Eric, if you have a strong preference, I can do it as you said.
> >
> > Well, how about explicitly casting them like this based on the current
> > series. It looks better and clearer and more helpful to people who is
> > reading codes to understand:
> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index 461b4d2b7cfe..eb125163d819 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
> > @@ -1936,7 +1936,7 @@ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
> >         return 0;
> >
> >  reset:
> > -       tcp_v4_send_reset(rsk, skb, (u32)reason);
> > +       tcp_v4_send_reset(rsk, skb, (enum sk_rst_reason)reason);
> >  discard:
> >         kfree_skb_reason(skb, reason);
> >         /* Be careful here. If this function gets more complicated and
>
> It makes no sense to declare an enum sk_rst_reason and then convert it
> to drop_reason
> or vice versa.
>
> Next thing you know, compiler guys will add a new -Woption that will
> forbid such conversions.
>
> Please add to tcp_v4_send_reset() an skb_drop_reason, not a new enum.

Ah... It looks like I didn't make myself clear again. Sorry...
Actually I wrote this part many times. My conclusion is that It's not
feasible to do this.

REASONS:
If we __only__ need to deal with this passive reset in TCP, it's fine.
We pass a skb_drop_reason which should also be converted to u32 type
in tcp_v4_send_reset() as you said, it can work. People who use the
trace will see the reason with the 'SKB_DROP_REASON' prefix stripped.

But we have to deal with other cases. A few questions are listed here:
1) What about tcp_send_active_reset() in TCP/MPTCP? Passing weird drop
reasons? There is no drop reason at all. I think people will get
confused. So I believe we should invent new definitions to cope with
it.
2) What about the .send_reset callback in the subflow_syn_recv_sock()
in MPTCP? The reasons in MPTCP are only definitions (such as
MPTCP_RST_EUNSPEC). I don't think we can convert them into the enum
skb_drop_reason type.

So where should we group those various reasons?

Introducing a new enum is for extension and compatibility for all
kinds of reset reasons.

What do you think?

Thanks,
Jason

>
> skb_drop_reason are simply values that are later converted to strings...
>
> So : Do not declare a new enum.