On Tue, May 11, 2021 at 01:42:46PM -0400, Willem de Bruijn wrote:

On Tue, May 11, 2021 at 2:51 AM Dongseok Yi [off-list ref] wrote:

quoted

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 <redacted>

---

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.

No problem. We can make a better decision under bpf-next.

quoted

 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.

I will make v3 for both 6_to_4 and 4_to_6.

quoted

        /* 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