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net: remove IPX raw 802.3 detection from eth_type_trans()

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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>
This commit is contained in:
Eric Dumazet 2026-10-02 20:55:09 +02:00 committed by Jakub Kicinski
parent 94085db4a1
commit 6451576b53
3 changed files with 5 additions and 34 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
include/linux/if_vlan.h
View File

@ -698,7 +698,6 @@ static inline void vlan_set_encap_proto(struct sk_buff *skb,
struct vlan_hdr *vhdr)
{
__be16 proto;
unsigned short *rawp;
/*
* Was a VLAN packet, grab the encapsulated protocol, which the layer
@ -711,21 +710,8 @@ static inline void vlan_set_encap_proto(struct sk_buff *skb,
return;
}
rawp = (unsigned short *)(vhdr + 1);
if (*rawp == 0xFFFF)
/*
* This is a magic hack to spot IPX packets. Older Novell
* breaks the protocol design and runs IPX over 802.3 without
* an 802.2 LLC layer. We look for FFFF which isn't a used
* 802.2 SSAP/DSAP. This won't work for fault tolerant netware
* but does for the rest.
*/
skb->protocol = htons(ETH_P_802_3);
else
/*
* Real 802.2 LLC
*/
skb->protocol = htons(ETH_P_802_2);
/* No ethertype: this is an 802.2 LLC frame (length field). */
skb->protocol = htons(ETH_P_802_2);
}
/**

3
net/core/skbuff.c
View File

@ -6355,8 +6355,7 @@ struct sk_buff *skb_vlan_untag(struct sk_buff *skb)
skb = skb_share_check(skb, GFP_ATOMIC);
if (unlikely(!skb))
goto err_free;
/* We may access the two bytes after vlan_hdr in vlan_set_encap_proto(). */
if (unlikely(!pskb_may_pull(skb, VLAN_HLEN + sizeof(unsigned short))))
if (unlikely(!pskb_may_pull(skb, VLAN_HLEN)))
goto err_free;
vhdr = (struct vlan_hdr *)skb->data;

18
net/ethernet/eth.c
View File

@ -154,9 +154,7 @@ EXPORT_SYMBOL(eth_get_headlen);
*/
__be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
{
const unsigned short *sap;
const struct ethhdr *eth;
__be16 res;
skb->dev = dev;
skb_reset_mac_header(skb);
@ -176,20 +174,8 @@ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
if (likely(eth_proto_is_802_3(eth->h_proto)))
return eth->h_proto;
/*
* This is a magic hack to spot IPX packets. Older Novell breaks
* the protocol design and runs IPX over 802.3 without an 802.2 LLC
* layer. We look for FFFF which isn't a used 802.2 SSAP/DSAP. This
* won't work for fault tolerant netware but does for the rest.
* We use skb->dev as temporary storage to not hit
* CONFIG_STACKPROTECTOR_STRONG=y costs on some platforms.
*/
sap = skb_header_pointer(skb, 0, sizeof(*sap), &skb->dev);
res = (sap && *sap == 0xFFFF) ? htons(ETH_P_802_3) : htons(ETH_P_802_2);
/* restore skb->dev in case it was mangled by skb_header_pointer(). */
skb->dev = dev;
return res;
/* No ethertype: this is an 802.2 LLC frame (length field). */
return htons(ETH_P_802_2);
}
EXPORT_SYMBOL(eth_type_trans);