netdev - Re: [PATCH net-next v2 4/8] udp: never accept GSO

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Date:   Mon, 29 Mar 2021 10:11:29 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alexander Lobakin <alobakin@...me>
Subject: Re: [PATCH net-next v2 4/8] udp: never accept GSO_FRAGLIST packets

On Fri, 2021-03-26 at 14:15 -0400, Willem de Bruijn wrote:
> On Thu, Mar 25, 2021 at 1:24 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > Currently the UDP protocol delivers GSO_FRAGLIST packets to
> > the sockets without the expected segmentation.
> > 
> > This change addresses the issue introducing and maintaining
> > a couple of new fields to explicitly accept SKB_GSO_UDP_L4
> > or GSO_FRAGLIST packets. Additionally updates  udp_unexpected_gso()
> > accordingly.
> > 
> > UDP sockets enabling UDP_GRO stil keep accept_udp_fraglist
> > zeroed.
> > 
> > v1 -> v2:
> >  - use 2 bits instead of a whole GSO bitmask (Willem)
> > 
> > Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> 
> This looks good to me in principle, thanks for the revision.
> 
> I hadn't fully appreciated that gro_enabled implies accept_udp_l4, but
> not necessarily vice versa.
> 
> It is equivalent to (accept_udp_l4 && !up->gro_receive), right?

In this series, yes. 

> Could the extra bit be avoided with
> 
> "
> +      /* Prefer fraglist GRO unless target is a socket with UDP_GRO,
> +       * which requires all but last segments to be of same gso_size,
> passed in cmsg */
>         if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
> -                NAPI_GRO_CB(skb)->is_flist = sk ? !udp_sk(sk)->gro_enabled: 1;
> +               NAPI_GRO_CB(skb)->is_flist = sk ?
> (!udp_sk(sk)->gro_enabled || udp_sk(sk)->accept_udp_fraglist) : 1;

This is not ovious at all to me.

> +     /* Apply transport layer GRO if forwarding is enabled or the
> flow lands at a local socket */
>        if ((!sk && (skb->dev->features & NETIF_F_GRO_UDP_FWD)) ||
>             (sk && udp_sk(sk)->gro_enabled && !up->encap_rcv) ||
> NAPI_GRO_CB(skb)->is_flist) {
>                 pp = call_gro_receive(udp_gro_receive_segment, head, skb);
>                 return pp;
>         }
> 
> +      /* Continue with tunnel GRO */
> "
> 
> .. not that the extra bit matters a lot. And these two conditions with
> gro_enabled are not very obvious.
> 
> Just a thought.

Overall looks more complex to me. I would keep the extra bit, unless
you have strong opinion.

Side note: I was wondering about a follow-up to simplify the condition:

	if ((!sk && (skb->dev->features & NETIF_F_GRO_UDP_FWD)) ||
             (sk && udp_sk(sk)->gro_enabled && !up->encap_rcv) || NAPI_GRO_CB(skb)->is_flist) {

Since UDP sockets could process (segmenting as needed) unexpected GSO
packets, we could always do 'NETIF_F_GRO_UDP_FWD', when enabled on the
device level. The above becomes:

	if (skb->dev->features & NETIF_F_GRO_UDP_FWD) ||
            (sk && udp_sk(sk)->gro_enabled && !up->encap_rcv) ||
            NAPI_GRO_CB(skb)->is_flist) {

which is hopefully more clear (and simpler). As said, non for this
series anyhow.

Thanks,

Paolo