veth: Allow jumbo frames in xdp mode

Allow increasing the MTU over page boundaries on veth devices
if the attached xdp program declares to support xdp fragments.
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/d5dc039c3d4123426e7023a488c449181a7bc57f.1646989407.git.lorenzo@kernel.org

drivers/net/veth.c

@@ -1528,9 +1528,14 @@ static int veth_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 			goto err;
 		}
 
-		max_mtu = PAGE_SIZE - VETH_XDP_HEADROOM -
-			  peer->hard_header_len -
-			  SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+		max_mtu = SKB_WITH_OVERHEAD(PAGE_SIZE - VETH_XDP_HEADROOM) -
+			  peer->hard_header_len;
+		/* Allow increasing the max_mtu if the program supports
+		 * XDP fragments.
+		 */
+		if (prog->aux->xdp_has_frags)
+			max_mtu += PAGE_SIZE * MAX_SKB_FRAGS;
+
 		if (peer->mtu > max_mtu) {
 			NL_SET_ERR_MSG_MOD(extack, "Peer MTU is too large to set XDP");
 			err = -ERANGE;