netdev - Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow path

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Date:   Fri, 26 Mar 2021 14:30:24 -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 1/8] udp: fixup csum for GSO receive slow path

On Thu, Mar 25, 2021 at 1:24 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> When UDP packets generated locally by a socket with UDP_SEGMENT
> traverse the following path:
>
> UDP tunnel(xmit) -> veth (segmentation) -> veth (gro) ->
>         UDP tunnel (rx) -> UDP socket (no UDP_GRO)
>
> they are segmented as part of the rx socket receive operation, and
> present a CHECKSUM_NONE after segmentation.

would be good to capture how this happens, as it was not immediately obvious.

>
> Additionally the segmented packets UDP CB still refers to the original
> GSO packet len. Overall that causes unexpected/wrong csum validation
> errors later in the UDP receive path.
>
> We could possibly address the issue with some additional checks and
> csum mangling in the UDP tunnel code. Since the issue affects only
> this UDP receive slow path, let's set a suitable csum status there.
>
> v1 -> v2:
>  - restrict the csum update to the packets strictly needing them
>  - hopefully clarify the commit message and code comments
>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>

> +       if (skb->ip_summed == CHECKSUM_NONE && !skb->csum_valid)
> +               skb->csum_valid = 1;

Not entirely obvious is that UDP packets arriving on a device with rx
checksum offload off, i.e., with CHECKSUM_NONE, are not matched by
this test.

I assume that such packets are not coalesced by the GRO layer in the
first place. But I can't immediately spot the reason for it..