[PATCH v2] virtio: Work around frames incorrectly marked as gso

From: Anton Ivanov <anton.ivanov@cambridgegreys.com>

Some of the locally generated frames marked as GSO which
arrive at virtio_net_hdr_from_skb() have no GSO_TYPE, no
fragments (data_len = 0) and length significantly shorter
than the MTU (752 in my experiments).

This is observed on raw sockets reading off vEth interfaces
in all 4.x and 5.x kernels I tested.

These frames are reported as invalid while they are in fact
gso-less frames.

This patch marks the vnet header as no-GSO for them instead
of reporting it as invalid.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
---
 include/linux/virtio_net.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 0d1fe9297ac6..94fb78c3a2ab 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h

@@ -100,8 +100,8 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
 {
 	memset(hdr, 0, sizeof(*hdr));   /* no info leak */
 
-	if (skb_is_gso(skb)) {
-		struct skb_shared_info *sinfo = skb_shinfo(skb);
+	struct skb_shared_info *sinfo = skb_shinfo(skb);
+	if (skb_is_gso(skb) && sinfo->gso_type) {
 
 		/* This is a hint as to how much should be linear. */
 		hdr->hdr_len = __cpu_to_virtio16(little_endian,

-- 
2.20.1