Re: [PATCH bpf-next v3] bpf: check for BPF_F_ADJ_ROOM_FIXED_GSO when bpf_skb_change_proto

[Willem de Bruijn]

On Wed, May 12, 2021 at 3:41 AM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote:
>
> In the forwarding path GRO -> BPF 6 to 4 -> GSO for TCP traffic, the
> coalesced packet payload can be > MSS, but < MSS + 20.
> bpf_skb_proto_6_to_4 will upgrade the MSS and it can be > the payload
> length. After then tcp_gso_segment checks for the payload length if it
> is <= MSS. The condition is causing the packet to be dropped.
>
> tcp_gso_segment():
>         [...]
>         mss = skb_shinfo(skb)->gso_size;
>         if (unlikely(skb->len <= mss))
>                 goto out;
>         [...]
>
> Allow to upgrade/downgrade MSS only when BPF_F_ADJ_ROOM_FIXED_GSO is
> not set.
>
> Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>

Acked-by: Willem de Bruijn <willemb@xxxxxxxxxx>