Re: [PATCH net-next] net/af_packet: add VLAN support for AF_PACKET SOCK_RAW GSO

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

On Wed, Apr 20, 2022 at 4:28 AM Hangbin Liu <liuhangbin@gmail.com> wrote:
>
> Currently, the kernel drops GSO VLAN tagged packet if it's created with
> socket(AF_PACKET, SOCK_RAW, 0) plus virtio_net_hdr.
>
> The reason is AF_PACKET doesn't adjust the skb network header if there is
> a VLAN tag. Then after virtio_net_hdr_set_proto() called, the skb->protocol
> will be set to ETH_P_IP/IPv6. And in later inet/ipv6_gso_segment() the skb
> is dropped as network header position is invalid.
>
> Let's handle VLAN packets by adjusting network header position in
> packet_parse_headers(), and move the function in packet_snd() before
> calling virtio_net_hdr_set_proto().

The network header is set in

        skb_reset_network_header(skb);

        err = -EINVAL;
        if (sock->type == SOCK_DGRAM) {
                offset = dev_hard_header(skb, dev, ntohs(proto), addr,
NULL, len);
                if (unlikely(offset < 0))
                        goto out_free;
        } else if (reserve) {
                skb_reserve(skb, -reserve);
                if (len < reserve + sizeof(struct ipv6hdr) &&
                    dev->min_header_len != dev->hard_header_len)
                        skb_reset_network_header(skb);
        }

If all that is needed is to move the network header beyond an optional
VLAN tag in the case of SOCK_RAW, then this can be done in the else
for Ethernet packets.

It is not safe to increase reserve, as that would eat into the
reserved hlen LL_RESERVED_SPACE(dev), which does not account for
optional VLAN headers.

Instead of setting here first, then patching up again later in
packet_parse_headers.

This change affects all packets with VLAN headers, not just those with
GSO. I imagine that moving the network header is safe for all, but
don't know that code well enough to verify that it does not have
unintended side effects. Does dev_queue_xmit expect the network header
to point to the start of the VLAN headers or after, for instance?

> No need to update tpacket_snd() as it calls packet_parse_headers() in
> tpacket_fill_skb(), which is already before calling virtio_net_hdr_*
> functions.
>
> skb->no_fcs setting is also moved upper to make all skb settings together
> and keep consistence with function packet_sendmsg_spkt().
>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
> ---
>  net/packet/af_packet.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 002d2b9c69dd..cfdbda28ef82 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -1924,12 +1924,20 @@ static int packet_rcv_spkt(struct sk_buff *skb, struct net_device *dev,
>
>  static void packet_parse_headers(struct sk_buff *skb, struct socket *sock)
>  {
> +       int depth;
> +
>         if ((!skb->protocol || skb->protocol == htons(ETH_P_ALL)) &&
>             sock->type == SOCK_RAW) {
>                 skb_reset_mac_header(skb);
>                 skb->protocol = dev_parse_header_protocol(skb);
>         }
>
> +       /* Move network header to the right position for VLAN tagged packets */
> +       if (likely(skb->dev->type == ARPHRD_ETHER) &&
> +           eth_type_vlan(skb->protocol) &&
> +           __vlan_get_protocol(skb, skb->protocol, &depth) != 0)
> +               skb_set_network_header(skb, depth);
> +
>         skb_probe_transport_header(skb);
>  }
>
> @@ -3047,6 +3055,11 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>         skb->mark = sockc.mark;
>         skb->tstamp = sockc.transmit_time;
>
> +       if (unlikely(extra_len == 4))
> +               skb->no_fcs = 1;
> +
> +       packet_parse_headers(skb, sock);
> +
>         if (has_vnet_hdr) {
>                 err = virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le());
>                 if (err)
> @@ -3055,11 +3068,6 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>                 virtio_net_hdr_set_proto(skb, &vnet_hdr);
>         }
>
> -       packet_parse_headers(skb, sock);
> -
> -       if (unlikely(extra_len == 4))
> -               skb->no_fcs = 1;
> -
>         err = po->xmit(skb);
>         if (unlikely(err != 0)) {
>                 if (err > 0)
> --
> 2.35.1
>