[PATCH RFC v2] net: xdp: allow for layer 3 packets in generic skb handler

["Jason A. Donenfeld" <Jason@zx2c4.com>]

A user reported a few days ago that packets from wireguard were possibly
ignored by XDP [1]. We haven't heard back from the original reporter to
receive more info, so this here is mostly speculative. Successfully nerd
sniped, Toke and I started poking around. Toke noticed that the generic
skb xdp handler path seems to assume that packets will always have an
ethernet header, which really isn't always the case for layer 3 packets,
which are produced by multiple drivers. This patch is untested, but I
wanted to gauge interest in this approach: if the mac_len is 0, then we
assume that it's a layer 3 packet, and in that case prepend a pseudo
ethhdr to the packet whose h_proto is copied from skb->protocol, which
will have the appropriate v4 or v6 ethertype. This allows us to keep XDP
programs' assumption correct about packets always having that ethernet
header, so that existing code doesn't break, while still allowing layer
3 devices to use the generic XDP handler.

[1] https://lore.kernel.org/wireguard/M5WzVK5--3-2@tuta.io/

Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 net/core/dev.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 522288177bbd..845a7d17abb9 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4505,12 +4505,12 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 				     struct xdp_buff *xdp,
 				     struct bpf_prog *xdp_prog)
 {
+	bool orig_bcast, add_eth_hdr = false;
 	struct netdev_rx_queue *rxqueue;
 	void *orig_data, *orig_data_end;
 	u32 metalen, act = XDP_DROP;
 	__be16 orig_eth_type;
 	struct ethhdr *eth;
-	bool orig_bcast;
 	int hlen, off;
 	u32 mac_len;
 
@@ -4544,6 +4544,13 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	 * header.
 	 */
 	mac_len = skb->data - skb_mac_header(skb);
+	if (!mac_len) {
+		add_eth_hdr = true;
+		mac_len = sizeof(struct ethhdr);
+		*((struct ethhdr *)skb_push(skb, mac_len)) = (struct ethhdr) {
+			.h_proto = skb->protocol
+		};
+	}
 	hlen = skb_headlen(skb) + mac_len;
 	xdp->data = skb->data - mac_len;
 	xdp->data_meta = xdp->data;
@@ -4611,6 +4618,8 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 		kfree_skb(skb);
 		break;
 	}
+	if (add_eth_hdr)
+		skb_pull(skb, sizeof(struct ethhdr));
 
 	return act;
 }
-- 
2.26.2