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

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Date:   Mon, 29 Mar 2021 08:31:23 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.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 Mon, Mar 29, 2021 at 4:14 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> 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.

Sounds good.

> 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.

UDP sockets can segment, but it is expensive. In this case I think the
simplification is not worth the possible regression.