[PATCH bpf-next] bpf: fix virtio-net's length calc for XDP_PASS

["Nikita V. Shirokov" <tehnerd@tehnerd.com>]

In commit 6870de435b90 ("bpf: make virtio compatible w/ 
bpf_xdp_adjust_tail") i didn't account for vi->hdr_len during new 
packet's length calculation after bpf_prog_run in receive_mergeable. 
because of this all packets, if they were passed to the kernel, 
were truncated by 12 bytes.

Fixes:6870de435b90 ("bpf: make virtio compatible w/ bpf_xdp_adjust_tail")
Reported-by: David Ahern <dsahern@gmail.com>

Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
---

Notes:
    unfortunately it looks like that xdp_tx is still broken because
    fix by Jason (introduced in "XDP_TX for virtio_net not working in recent kernel?
    " thread) haven't landed yet)

 drivers/net/virtio_net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 779a4f798522..08ac2cc986aa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -761,7 +761,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			/* recalculate len if xdp.data or xdp.data_end were
 			 * adjusted
 			 */
-			len = xdp.data_end - xdp.data;
+			len = xdp.data_end - xdp.data + vi->hdr_len;
 			/* We can only create skb based on xdp_page. */
 			if (unlikely(xdp_page != page)) {
 				rcu_read_unlock();
-- 
2.15.1