From: "Nikita V. Shirokov" <tehnerd@tehnerd.com>
Date: Tue, 17 Apr 2018 21:42:14 -0700
Subject: bpf: make generic xdp compatible w/ bpf_xdp_adjust_tail
Patch-mainline: v4.18-rc1
Git-commit: 198d83bb3becf2e9c6c4fa744f35296c20da795a
References: bsc#1109837

w/ bpf_xdp_adjust_tail helper xdp's data_end pointer could be changed as
well (only "decrease" of pointer's location is going to be supported).
changing of this pointer will change packet's size.
for generic XDP we need to reflect this packet's length change by
adjusting skb's tail pointer

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/core/dev.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3954,9 +3954,9 @@ static u32 netif_receive_generic_xdp(str
 				     struct bpf_prog *xdp_prog)
 {
 	struct netdev_rx_queue *rxqueue;
+	void *orig_data, *orig_data_end;
 	u32 metalen, act = XDP_DROP;
 	struct xdp_buff xdp;
-	void *orig_data;
 	int hlen, off;
 	u32 mac_len;
 
@@ -3995,6 +3995,7 @@ static u32 netif_receive_generic_xdp(str
 	xdp.data_meta = xdp.data;
 	xdp.data_end = xdp.data + hlen;
 	xdp.data_hard_start = skb->data - skb_headroom(skb);
+	orig_data_end = xdp.data_end;
 	orig_data = xdp.data;
 
 	rxqueue = netif_get_rxqueue(skb);
@@ -4009,6 +4010,13 @@ static u32 netif_receive_generic_xdp(str
 		__skb_push(skb, -off);
 	skb->mac_header += off;
 
+	/* check if bpf_xdp_adjust_tail was used. it can only "shrink"
+	 * pckt.
+	 */
+	off = orig_data_end - xdp.data_end;
+	if (off != 0)
+		skb_set_tail_pointer(skb, xdp.data_end - xdp.data);
+
 	switch (act) {
 	case XDP_REDIRECT:
 	case XDP_TX: