quoted

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So I guess VIRTIO_NET_HDR_GSO_* without VIRTIO_NET_HDR_F_DATA_VALID
would be wrong on rx.

But the new check

        if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {

                [...]

                case SKB_GSO_TCPV4:
                case SKB_GSO_TCPV6:
                        if (skb->csum_offset != offsetof(struct tcphdr, check))
                                return -EINVAL;

should be limited to callers of virtio_net_hdr_to_skb on the tx/GSO path.

Looking what the cleanest/minimal patch is to accomplish that.

virtio_net_hdr_to_skb() translates virtio-net header to skb metadata,
so it's RX. For TX the helper should be virtio_net_hdr_from_skb()
which translates skb metadata to virtio hdr.

virtio_net_hdr_to_skb is used by PF_PACKET, tun and tap

Exactly.

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when injecting a packet into the egress path.

For tuntap it's still the RX path. For PF_PACEKT and macvtap, it's the tx.

Maybe a new parameter to virtio_net_hdr_to_skb()?

This is the most straightforward approach. But requires changse to all
callers, in a patch targeting all the stable branches.

I'd prefer if we can detect ingress vs egress directly.

Based on ip_summed, pkt_type, is_skb_wmem or so. But so far have not
found a suitable condition.

I noticed something else: as you point out TUN is ingress. Unlike
virtnet_receive, it does not set ip_summed to CHECKSUM_UNNECESSARY if
VIRTIO_NET_HDR_F_DATA_VALID. It probably should. GRO expects packets
to have had their integrity verified. CHECKSUM_NONE on ingress is not
correct for GRO.

And also related: no GRO should be generated by a device unless
VIRTIO_NET_HDR_F_DATA_VALID is also passed? I have to check the spec
if it says anything about this.