netdev - [PATCH v3] virtio: Work around frames incorrectly marked as gso

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Message-Id: <20200224132550.2083-1-anton.ivanov@cambridgegreys.com>
Date:   Mon, 24 Feb 2020 13:25:50 +0000
From:   anton.ivanov@...bridgegreys.com
To:     netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-um@...ts.infradead.org
Cc:     mst@...hat.com, jasowang@...hat.com, eric.dumazet@...il.com,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>
Subject: [PATCH v3] virtio: Work around frames incorrectly marked as gso

From: Anton Ivanov <anton.ivanov@...bridgegreys.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. The frames are reported as
invalid, while they are in fact gso-less frames.

The easiest way to reproduce is to connect a User Mode
Linux instance to the host using the vector raw transport
and a vEth interface. Vector raw uses recvmmsg/sendmmsg
with virtio headers on af_packet sockets. When running iperf
between the UML and the host, UML regularly complains about
EINVAL return from recvmmsg.

This patch marks the vnet header as non-GSO instead of
reporting it as invalid.

Signed-off-by: Anton Ivanov <anton.ivanov@...bridgegreys.com>
---
 include/linux/virtio_net.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 0d1fe9297ac6..2c99c752cb20 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -98,10 +98,11 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
 					  bool has_data_valid,
 					  int vlan_hlen)
 {
+	struct skb_shared_info *sinfo = skb_shinfo(skb);
+
 	memset(hdr, 0, sizeof(*hdr));   /* no info leak */

-	if (skb_is_gso(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