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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Mon, 9 Nov 2020 09:36:12 -0500
Message-ID: <CA+FuTSfokZNJv2g2mCK284UTj7nN_-qXei42J4QWt7YniSrKog@mail.gmail.com>
Subject: Re: [PATCH net] net: udp: fix Fast/frag0 UDP GRO
To:     Alexander Lobakin <alobakin@pm.me>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Network Development <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Sat, Nov 7, 2020 at 8:11 PM Alexander Lobakin <alobakin@pm.me> wrote:
>
> While testing UDP GSO fraglists forwarding through driver that uses
> Fast GRO (via napi_gro_frags()), I was observing lots of out-of-order
> iperf packets:
>
> [ ID] Interval           Transfer     Bitrate         Jitter
> [SUM]  0.0-40.0 sec  12106 datagrams received out-of-order
>
> Simple switch to napi_gro_receive() any other method without frag0
> shortcut completely resolved them.
>
> I've found that UDP GRO uses udp_hdr(skb) in its .gro_receive()
> callback. While it's probably OK for non-frag0 paths (when all
> headers or even the entire frame are already in skb->data), this
> inline points to junk when using Fast GRO (napi_gro_frags() or
> napi_gro_receive() with only Ethernet header in skb->data and all
> the rest in shinfo->frags) and breaks GRO packet compilation and
> the packet flow itself.
> To support both modes, skb_gro_header_fast() + skb_gro_header_slow()
> are typically used. UDP even has an inline helper that makes use of
> them, udp_gro_udphdr(). Use that instead of troublemaking udp_hdr()
> to get rid of the out-of-order delivers.
>
> Present since the introduction of plain UDP GRO in 5.0-rc1.
>
> Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> ---
>  net/ipv4/udp_offload.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index e67a66fbf27b..13740e9fe6ec 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -366,7 +366,7 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
>  static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
>                                                struct sk_buff *skb)
>  {
> -       struct udphdr *uh = udp_hdr(skb);
> +       struct udphdr *uh = udp_gro_udphdr(skb);
>         struct sk_buff *pp = NULL;
>         struct udphdr *uh2;
>         struct sk_buff *p;

Good catch. skb_gro_header_slow may fail and return NULL. Need to
check that before dereferencing uh below in

        /* requires non zero csum, for symmetry with GSO */
        if (!uh->check) {
                NAPI_GRO_CB(skb)->flush = 1;
                return NULL;
        }