Re: [PATCH v2 net 2/2] net: tap: fix wrong transport_header when sending VLAN-tagged frame

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

wei.fang@ wrote:
> From: Wei Fang <wei.fang@nxp.com>
> 
> In tap_get_user_xdp(), when processing a VLAN-tagged frame (e.g.
> ETH_P_8021Q), skb_set_network_header() is called first to advance
> network_header past the VLAN tag to the inner protocol header.
> skb_probe_transport_header() is then called with skb->protocol still
> set to ETH_P_8021Q, while nhoff (derived from skb_network_offset())
> already points past the VLAN tag to the inner protocol header.
> 
> In __skb_flow_dissect(), proto is initialized to ETH_P_8021Q and nhoff
> points past the VLAN tag. When the dissector hits case ETH_P_8021Q, it
> reads a struct vlan_hdr at the current nhoff via __skb_header_pointer(),
> but that offset contains the inner protocol header (e.g. an IP header).
> The bytes are misinterpreted as a VLAN header, yielding a garbage
> encapsulated EtherType that matches no known protocol. The dissector
> returns false, so skb_probe_transport_header() never calls
> skb_set_transport_header(), leaving transport_header at its uninitialized
> sentinel value (~0U).
> 
> Move skb_set_network_header() to after skb_probe_transport_header(). At
> the time skb_probe_transport_header() is called, network_header still
> points to the VLAN header (offset ETH_HLEN), so nhoff is correct and the
> flow dissector can parse the VLAN header, extract the inner EtherType,
> and advance nhoff to the inner protocol header, allowing transport_header
> to be set correctly.
> 
> Fixes: 8c76e77f9069 ("tap: call skb_probe_transport_header after setting skb->dev")
> Assisted-by: WChat:claude-opus-4-8
> Signed-off-by: Wei Fang <wei.fang@nxp.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

Only if respinning: include the explanation why tap_get_user does not
need this, only tap_get_user_xdp.