eth_type_trans() and vlan_set_encap_proto() still carry a "magic hack"
to spot Novell raw 802.3 frames (IPX directly over 802.3, without
an 802.2 LLC header) by looking for 0xFFFF in the first two bytes
of the payload, and report them as ETH_P_802_3.
IPX was removed in commit 7a2e838d28 ("staging: ipx: delete it
from the tree") and there is no in-kernel packet handler for
ETH_P_802_3 on the Ethernet receive path anymore.
Simply classify all frames using a length field as ETH_P_802_2,
which is what the remaining LLC users (STP, GARP, SNAP) register for.
This removes a skb_header_pointer() call (and the skb->dev scratch
trick used to avoid a stack canary) from eth_type_trans(), and lets
skb_vlan_untag() only pull VLAN_HLEN bytes, as the extra two bytes
were only needed for this check since commit 55eff0eb74
("net: Fix potential wrong skb->protocol in skb_vlan_untag()").
Note that such frames are now reported to AF_PACKET, tc, BPF and
nftables with skb->protocol == ETH_P_802_2 instead of ETH_P_802_3.
$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 2/5 up/down: 33/-187 (-154)
Function old new delta
skb_shift 1736 1768 +32
eth_type_trans.cold 45 46 +1
skb_vlan_untag 690 678 -12
__skb_vlan_pop 489 477 -12
nf_flow_encap_pop 462 449 -13
eth_type_trans 354 220 -134
Total: Before=31212910, After=31212756, chg -0.00%
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20261002185509.17226-1-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
6451576b53