netdev - Re: [PATCH net 1/2] net/af_packet: adjust network header position for VLAN tagged packets

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Date:   Mon, 18 Apr 2022 11:38:14 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     netdev@...r.kernel.org, "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        virtualization@...ts.linux-foundation.org,
        Balazs Nemeth <bnemeth@...hat.com>,
        Mike Pattrick <mailmpattric@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Flavio Leitner <fbl@...hat.com>
Subject: Re: [PATCH net 1/2] net/af_packet: adjust network header position for
 VLAN tagged packets

On Mon, Apr 18, 2022 at 12:44 AM Hangbin Liu <liuhangbin@...il.com> wrote:
>
> Flavio reported that 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.
>
> Fix it by adjusting network header position in packet_parse_headers()
> and move the function before calling virtio_net_hdr_* functions.
>
> I also moved skb->no_fcs setting upper to make all skb setting together
> and keep consistence of function packet_sendmsg_spkt().
>
> No need to update tpacket_snd() as it calls packet_parse_headers() in
> tpacket_fill_skb() before calling virtio_net_hdr_* functions.
>
> Fixes: 75c65772c3d1 ("net/packet: Ask driver for protocol if not provided by user")
> Reported-by: Flavio Leitner <fbl@...hat.com>
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>

Strictly speaking VLAN tagged GSO packets have never been supported.
The only defined types are TCP and UDP over IPv4 and IPv6:

  define VIRTIO_NET_HDR_GSO_TCPV4        1       /* GSO frame, IPv4 TCP (TSO) */
  define VIRTIO_NET_HDR_GSO_UDP          3       /* GSO frame, IPv4 UDP (UFO) */
  define VIRTIO_NET_HDR_GSO_TCPV6        4       /* GSO frame, IPv6 TCP */

I don't think this is a bug, more a stretching of the definition of those flags.

> ---
>  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;
> -

Moving packet_parse_headers before or after virtio_net_hdr_to_skb may
have additional subtle effects on protocol detection.

I think it's probably okay, as tpacket_snd also calls in the inverse
order. But there have been many issues in this codepath.

We should also maintain feature consistency between packet_snd,
tpacket_snd and to the limitations of its feature set to
packet_sendmsg_spkt. The no_fcs is already lacking in tpacket_snd as
far as I can tell. But packet_sendmsg_spkt also sets it and calls
packet_parse_headers.

Because this patch touches many other packets besides the ones
intended, I am a bit concerned about unintended consequences. Perhaps
stretching the definition of the flags to include VLAN is acceptable
(unlike outright tunnels), but even then I would suggest for net-next.

>         err = po->xmit(skb);
>         if (unlikely(err != 0)) {
>                 if (err > 0)