net: packet: fix wrong transport_header when sending VLAN-tagged frame

net: packet: fix wrong transport_header when sending VLAN-tagged frame

In packet_parse_headers(), when processing a VLAN-tagged frame,
skb_set_network_header() is called 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 the outer VLAN EtherType
(e.g. 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 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_probe_transport_header() to before skb_set_network_header(). At
the time skb_probe_transport_header() is called, network_header still
points to the VLAN header, so nhoff correctly points to the VLAN header.
The flow dissector can then parse the VLAN header, extract the inner
EtherType, and advance nhoff to the inner protocol header, allowing
transport_header to be set correctly.

Fixes: dfed913e8b55 ("net/af_packet: add VLAN support for AF_PACKET SOCK_RAW GSO")
Assisted-by: WChat:claude-opus-4-8
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260807063405.688780-2-wei.fang@oss.nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>