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

[Hangbin Liu <liuhangbin@gmail.com>]

On Wed, Apr 20, 2022 at 09:45:15AM -0400, Willem de Bruijn wrote:
> 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.

Before we set network position, we need to check the skb->protocol to make
sure it's a VLAN packet.

If we set skb->protocol and adjust network header here, like
packet_parse_headers() does. How should we do with later

        skb->protocol = proto;
        skb->dev = dev;

settings?

If you are afraid that skb_probe_transport_header(skb) would affect the
virtio_net_hdr operation. How about split the skb_probe_transport_header()
from packet_parse_headers()? Something like

--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1924,13 +1924,19 @@ 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);
        }

-       skb_probe_transport_header(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);
 }

 /*
@@ -3047,6 +3055,8 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
        skb->mark = sockc.mark;
        skb->tstamp = sockc.transmit_time;

+       packet_parse_headers(skb, sock);
+
        if (has_vnet_hdr) {
                err = virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le());
                if (err)
@@ -3055,7 +3065,7 @@ 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);
+       skb_probe_transport_header(skb);

        if (unlikely(extra_len == 4))
                skb->no_fcs = 1;


> 
> 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?

I think adjust the network position should be safe, as tap device also did that.

Thanks
Hangbin