linux-kernel - Re: [PATCH net-next] net: avoid unneeded UDP L4 and fraglist GSO resegmentation

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Date:   Mon, 2 Nov 2020 10:14:02 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alexander Lobakin <alobakin@...me>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Antoine Tenart <atenart@...nel.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: avoid unneeded UDP L4 and fraglist GSO resegmentation

On Sat, Oct 31, 2020 at 11:56 AM Alexander Lobakin <alobakin@...me> wrote:
>
> From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Date: Sat, 31 Oct 2020 11:26:24 -0400
>
> >>>> I think it is fine to reenable this again, now that UDP sockets will
> >>>> segment unexpected UDP GSO packets that may have looped. We previously
> >>>> added general software support in commit 83aa025f535f ("udp: add gso
> >>>> support to virtual devices"). Then reduced its scope to egress only in
> >>>> 8eea1ca82be9 ("gso: limit udp gso to egress-only virtual devices") to
> >>>> handle that edge case.
> >>
> >> Regarding bonding and teaming: I think they should also use
> >> NETIF_F_GSO_SOFTWARE mask, not NETIF_F_ALL_TSO, as SCTP also has
> >> a software fallback. This way we could also remove a separate
> >> advertising of NETIF_F_GSO_UDP_L4, as it will be included in the first.
> >>
> >> So, if this one:
> >> 1. Add NETIF_F_GSO_UDP_L4 and NETIF_F_GSO_FRAGLIST to
> >>    NETIF_F_GSO_SOFTWARE;
> >> 2. Change bonding and teaming features mask from NETIF_F_ALL_TSO |
> >>    NETIF_F_GSO_UDP_L4 to NETIF_F_GSO_SOFTWARE;
> >> 3. Check that every virtual netdev has NETIF_F_GSO_SOFTWARE _or_
> >>    NETIF_F_GSO_MASK in its advertising.
> >>
> >> is fine for everyone, I'll publish more appropriate and polished v2 soon.
> >
> > I think we can revert 8eea1ca82be9. Except for the part where it
> > defines the feature in NETIF_F_GSO_ENCAP_ALL instead of
> > NETIF_F_GSO_SOFTWARE. That appears to have been a peculiar choice. I
> > can't recall exactly why I chose that. Most likely because that was
> > (at the time) the only macro that covered all the devices I wanted to
> > capture.
> >
> > As for SCTP: that has the same concern that prompted that commit for
> > UDP: is it safe to forward those packets to the ingress path today?
>
> Oh well. I just looked up into net/sctp/offload.c and see no GRO
> receiving callbacks, only GSO ones. On the other hand,
> NETIF_F_GSO_SOFTWARE includes GSO_SCTP and is used in almost every
> virtual netdev driver, including macvlan and veth mentioned earlier,
> so that seems to be fine.

To follow up: SCTP sockets can handle such packets. So both local
reception and forwarding are fine. This was expressly added to the
second revision of the SCTP GSO commit.