On Tue, May 11, 2021 at 2:51 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 increase 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 increase MSS when BPF_F_ADJ_ROOM_FIXED_GSO is not set.
>
> Fixes: 6578171a7ff0 (bpf: add bpf_skb_change_proto helper)
> Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
>
> ---
Thanks. Note that this feature does not preclude the alternatives
discussed, of converting the packet to non-TSO (by clearing gso_size)
or optionally modifying MSS (but that should get okay from TCP
experts).
I would target this for bpf-next and drop the Fixes. But that is
admittedly debatable.
> net/core/filter.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> v2:
> per Willem de Bruijn request,
> checked the flag instead of a generic approach.
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index cae56d0..a98b28d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3276,7 +3276,7 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
> return 0;
> }
>
> -static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
> +static int bpf_skb_proto_6_to_4(struct sk_buff *skb, u64 flags)
> {
> const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);
> u32 off = skb_mac_header_len(skb);
> @@ -3305,7 +3305,8 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
> }
>
> /* Due to IPv4 header, MSS can be upgraded. */
> - skb_increase_gso_size(shinfo, len_diff);
> + if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
> + skb_increase_gso_size(shinfo, len_diff);
> /* Header must be checked, and gso_segs recomputed. */
> shinfo->gso_type |= SKB_GSO_DODGY;
> shinfo->gso_segs = 0;
> @@ -3317,7 +3318,7 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
> return 0;
> }
>
> -static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto)
> +static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto, u64 flags)
> {
> __be16 from_proto = skb->protocol;
>
> @@ -3327,7 +3328,7 @@ static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto)
>
> if (from_proto == htons(ETH_P_IPV6) &&
> to_proto == htons(ETH_P_IP))
> - return bpf_skb_proto_6_to_4(skb);
> + return bpf_skb_proto_6_to_4(skb, flags);
>
> return -ENOTSUPP;
> }
> @@ -3337,7 +3338,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
> {
> int ret;
>
> - if (unlikely(flags))
> + if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO)))
> return -EINVAL;
Once allowing this flag, please immediately support it for both
bpf_skb_proto_6_to_4 and bpf_skb_4_to_6.
We cannot do that later if we ignore the second case now.
> /* General idea is that this helper does the basic groundwork
> @@ -3357,7 +3358,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
> * that. For offloads, we mark packet as dodgy, so that headers
> * need to be verified first.
> */
> - ret = bpf_skb_proto_xlat(skb, proto);
> + ret = bpf_skb_proto_xlat(skb, proto, flags);
> bpf_compute_data_pointers(skb);
> return ret;
> }
> --
> 2.7.4
>
