netdev - Re: [PATCH v3 net-next RFC] Generic XDP

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Date:   Wed, 12 Apr 2017 22:08:28 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     dsa@...ulusnetworks.com
Cc:     netdev@...r.kernel.org, xdp-newbies@...r.kernel.org
Subject: Re: [PATCH v3 net-next RFC] Generic XDP

From: David Ahern <dsa@...ulusnetworks.com>
Date: Wed, 12 Apr 2017 13:54:20 -0600

> packet passed to xdp seems to be messed up

Ok, the problem is that skb->mac_len isn't set properly at this point.
That doesn't happen until __netif_receive_skb_core().

I'm also not setting xdp.data_hard_start properly.

It should work better with this small diff:

diff --git a/net/core/dev.c b/net/core/dev.c
index 9ed4569..d36ae8f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4289,6 +4289,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	u32 act = XDP_DROP;
 	void *orig_data;
 	int hlen, off;
+	u32 mac_len;
 
 	if (skb_linearize(skb))
 		goto do_drop;
@@ -4296,10 +4297,11 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 	/* The XDP program wants to see the packet starting at the MAC
 	 * header.
 	 */
-	hlen = skb_headlen(skb) + skb->mac_len;
-	xdp.data = skb->data - skb->mac_len;
+	mac_len = skb->data - skb_mac_header(skb);
+	hlen = skb_headlen(skb) + mac_len;
+	xdp.data = skb->data - mac_len;
 	xdp.data_end = xdp.data + hlen;
-	xdp.data_hard_start = xdp.data - skb_headroom(skb);
+	xdp.data_hard_start = skb->data - skb_headroom(skb);
 	orig_data = xdp.data;
 
 	act = bpf_prog_run_xdp(xdp_prog, &xdp);