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

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Date: Sat, 20 Apr 2024 11:22:32 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, dsahern@...nel.org, 
	matttbe@...nel.org, martineau@...nel.org, geliang@...nel.org, 
	pabeni@...hat.com, davem@...emloft.net, mhiramat@...nel.org, 
	mathieu.desnoyers@...icios.com, atenart@...nel.org, mptcp@...ts.linux.dev, 
	netdev@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v6 0/7] Implement reset reason mechanism to detect

Hello Steven,

On Sat, Apr 20, 2024 at 10:36 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri, 19 Apr 2024 16:00:20 +0800
> Jason Xing <kerneljasonxing@...il.com> wrote:
>
> > If other experts see this thread, please help me. I would appreciate
> > it. I have strong interests and feel strong responsibility to
> > implement something like this patch series. It can be very useful!!
>
> I'm not a networking expert, but as I'm Cc'd and this is about tracing,
> I'll jump in to see if I can help. Honestly, reading the thread, it
> appears that you and Eric are talking past each other.
>
> I believe Eric is concerned about losing the value of the enum. Enums
> are types, and if you typecast them to another type, they lose the
> previous type, and all the safety that goes with it.

Ah, I see. Possible lost value in another enum could cause a problem.

>
> Now, I do not really understand the problem trying to be solved here. I
> understand how TCP works but I never looked into the implementation of
> MPTCP.
>
> You added this:
>
> +static inline enum sk_rst_reason convert_mptcp_reason(u32 reason)
> +{
> +       return reason += RST_REASON_START;
> +}
>
> And used it for places like this:
>
> @@ -309,8 +309,13 @@ static struct dst_entry *subflow_v4_route_req(const struct sock *sk,
>                 return dst;
>
>         dst_release(dst);
> -       if (!req->syncookie)
> -               tcp_request_sock_ops.send_reset(sk, skb, SK_RST_REASON_NOT_SPECIFIED);
> +       if (!req->syncookie) {
> +               struct mptcp_ext *mpext = mptcp_get_ext(skb);
> +               enum sk_rst_reason reason;
> +
> +               reason = convert_mptcp_reason(mpext->reset_reason);
> +               tcp_request_sock_ops.send_reset(sk, skb, reason);
> +       }
>         return NULL;
>  }
>
> As I don't know this code or how MPTCP works, I do not understand the
> above. It use to pass to send_reset() SK_RST_REASON_NOT_SPECIFIED. But
> now it takes a "reset_reason" calls the "convert_mptcp_reason()" to get
> back a enum value.
>
> If you are mapping the reset_reason to enum sk_rst_reason, why not do
> it via a real conversion instead of this fragile arithmetic between the two
> values?
>
> static inline enum sk_rst_reason convert_mptcp_reason(u32 reason)
> {
>         switch(reason) {
>         case 0: return SK_RST_REASON_MPTCP_RST_EUNSPEC;
>         case 1: return SK_RST_REASON_MPTCP_RST_EMPTCP;
>         case 2: return SK_RST_REASON_MPTCP_RST_ERESOURCE;
>         [..]
>         default: return SK_RST_REASON_MAX; // or some other error value
>         ]
> }

This code snippet looks much better than mine.

>
> I'm not sure if this is any better, but it's not doing any casting and
> it's easier to understand. It's a simple mapping between the reason and
> the enum and there's no inherit dependency between the values. Could
> possibly create enums for the reason numbers and replace the hard coded
> values with them.

Right.

I also need to handle many drop reasons cases like what you do. Due to
too many of them, I will try the key-value map instead of switch-case
and then see if it works.

>
> That way that helper function is at least doing a real conversion of
> one type to another.
>
> But like I said from the beginning. I don't understand the details here
> and have not spent the time to dig deeper. I just read the thread and I
> agree with Eric that the arithmetic conversion of reason to an enum
> looks fragile at best and buggy at worst.

Thanks so much for your help, which I didn't even imagine.

Sure, after one night of investigation, I figured it out. I will drop
enum casts without any doubt as Eric and you suggested. But I believe
a new enum is needed, grouping various reasons together which help
ftrace print the valid string to userspace.

Thanks,
Jason

>
> -- Steve