netdev - Re: [BUG report] GSO cmsg always turns UDP into unconnected

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Message-ID: <661856692c8af_36e52529467@willemb.c.googlers.com.notmuch>
Date: Thu, 11 Apr 2024 17:30:17 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Yick Xie <yick.xie@...il.com>, 
 willemb@...gle.com, 
 davem@...emloft.net, 
 willemdebruijn.kernel@...il.com
Cc: netdev@...r.kernel.org
Subject: Re: [BUG report] GSO cmsg always turns UDP into unconnected

Yick Xie wrote:
> Greetings,
> 
> If "udp_cmsg_send()" returned 0 (i.e. only SOL_UDP cmsg),  "connected"
> would still be set to 0, later inevitably "ip_route_output_flow()".
> In other words, a connected UDP works as unconnected.
> 
> A potential fix like this:
> 
> ```
> https://github.com/torvalds/linux/blob/20cb38a7af88dc40095da7c2c9094da3873fea23/net/ipv4/udp.c#L1043
> @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>         if (msg->msg_controllen) {
>                 err = udp_cmsg_send(sk, msg, &ipc.gso_size);
> -               if (err > 0)
> +               if (err > 0) {
>                         err = ip_cmsg_send(sk, msg, &ipc,
>                                         sk->sk_family == AF_INET6);
> +                       connected = 0;
> +               }
> 
>                 if (unlikely(err < 0)) {
>                         kfree(ipc.opt);
>                         return err;
>                 }
>                 if (ipc.opt)
>                         free = 1;
> -
> -               connected = 0;
>         }
> ```

This is not actually a bug, is it?

It is not necessary to a do a route lookup if only udp_cmsg_send is
called. But neither is it for some cases of ip_cmsg_send and
__sock_cmsg_send (which it calls in turn), say SO_TXTIME.

The suggested patch is a nice optimization to use the cached route.
Do send it.