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

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Date:   Mon, 29 Mar 2021 08:28:10 -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 Mon, Mar 29, 2021 at 7:26 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Fri, 2021-03-26 at 14:30 -0400, Willem de Bruijn wrote:
> > 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.
>
> The CHECKSUM_PARTIAL is propagated up to the UDP tunnel processing,
> where we have:
>
>         __iptunnel_pull_header() -> skb_pull_rcsum() ->
> skb_postpull_rcsum() -> __skb_postpull_rcsum() and the latter do the
> conversion.

Please capture this in the commit message.

> > > 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..
>
> Packets with CHECKSUM_NONE are actually aggregated by the GRO engine.
>
> Their checksum is validated by:
>
> udp4_gro_receive -> skb_gro_checksum_validate_zero_check()
>         -> __skb_gro_checksum_validate -> __skb_gro_checksum_validate_complete()
>
> and skb->ip_summed is changed to CHECKSUM_UNNECESSARY by:
>
> __skb_gro_checksum_validate -> skb_gro_incr_csum_unnecessary
>         -> __skb_incr_checksum_unnecessary()
>
> and finally to CHECKSUM_PARTIAL by:
>
> udp4_gro_complete() -> udp_gro_complete() -> udp_gro_complete_segment()
>
> Do you prefer I resubmit with some more comments, either in the commit
> message or in the code?

That breaks the checksum-and-copy optimization when delivering to
local sockets. I wonder if that is a regression.