Re: [net-next PATCH v2 1/8] udp: Record gso_segs when supporting UDP segmentation offload

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Fri, May 4, 2018 at 8:28 PM, Alexander Duyck
<alexander.duyck@gmail.com> wrote:
> From: Alexander Duyck <alexander.h.duyck@intel.com>
>
> We need to record the number of segments that will be generated when this
> frame is segmented. The expectation is that if gso_size is set then
> gso_segs is set as well. Without this some drivers such as ixgbe get
> confused if they attempt to offload this as they record 0 segments for the
> entire packet instead of the correct value.
>
> Reviewed-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> ---
>  net/ipv4/udp.c |    2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index dd3102a37ef9..e07db83b311e 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -793,6 +793,8 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
>
>                 skb_shinfo(skb)->gso_size = cork->gso_size;
>                 skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> +               skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(len - sizeof(uh),
> +                                                        cork->gso_size);
>                 goto csum_partial;
>         }

Acked-by: Willem de Bruijn <willemb@google.com>

But we have to be careful that both UDP GSO and UFO packets can traverse
qdisc_pkt_len_init. This does seem to fix the UDP GSO case, as it adds the
header size to each segment. But that is odd, as the code precedes UDP
GSO, so must have been intended to estimate UFO size on the wire. Only
external sources can generate UFO. It seems like we need to not add
sizeof(struct udphdr) to hdrlen in the SKB_GSO_DODGY branch.