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Subject[PATCH 5.4 04/37] net: dsa: fix flow dissection on Tx path
Date
From: Alexander Lobakin <alobakin@dlink.ru>

[ Upstream commit 8bef0af09a5415df761b04fa487a6c34acae74bc ]

Commit 43e665287f93 ("net-next: dsa: fix flow dissection") added an
ability to override protocol and network offset during flow dissection
for DSA-enabled devices (i.e. controllers shipped as switch CPU ports)
in order to fix skb hashing for RPS on Rx path.

However, skb_hash() and added part of code can be invoked not only on
Rx, but also on Tx path if we have a multi-queued device and:
- kernel is running on UP system or
- XPS is not configured.

The call stack in this two cases will be like: dev_queue_xmit() ->
__dev_queue_xmit() -> netdev_core_pick_tx() -> netdev_pick_tx() ->
skb_tx_hash() -> skb_get_hash().

The problem is that skbs queued for Tx have both network offset and
correct protocol already set up even after inserting a CPU tag by DSA
tagger, so calling tag_ops->flow_dissect() on this path actually only
breaks flow dissection and hashing.

This can be observed by adding debug prints just before and right after
tag_ops->flow_dissect() call to the related block of code:

Before the patch:

Rx path (RPS):

[ 19.240001] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
[ 19.244271] tag_ops->flow_dissect()
[ 19.247811] Rx: proto: 0x0800, nhoff: 8 /* ETH_P_IP */

[ 19.215435] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
[ 19.219746] tag_ops->flow_dissect()
[ 19.223241] Rx: proto: 0x0806, nhoff: 8 /* ETH_P_ARP */

[ 18.654057] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
[ 18.658332] tag_ops->flow_dissect()
[ 18.661826] Rx: proto: 0x8100, nhoff: 8 /* ETH_P_8021Q */

Tx path (UP system):

[ 18.759560] Tx: proto: 0x0800, nhoff: 26 /* ETH_P_IP */
[ 18.763933] tag_ops->flow_dissect()
[ 18.767485] Tx: proto: 0x920b, nhoff: 34 /* junk */

[ 22.800020] Tx: proto: 0x0806, nhoff: 26 /* ETH_P_ARP */
[ 22.804392] tag_ops->flow_dissect()
[ 22.807921] Tx: proto: 0x920b, nhoff: 34 /* junk */

[ 16.898342] Tx: proto: 0x86dd, nhoff: 26 /* ETH_P_IPV6 */
[ 16.902705] tag_ops->flow_dissect()
[ 16.906227] Tx: proto: 0x920b, nhoff: 34 /* junk */

After:

Rx path (RPS):

[ 16.520993] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
[ 16.525260] tag_ops->flow_dissect()
[ 16.528808] Rx: proto: 0x0800, nhoff: 8 /* ETH_P_IP */

[ 15.484807] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
[ 15.490417] tag_ops->flow_dissect()
[ 15.495223] Rx: proto: 0x0806, nhoff: 8 /* ETH_P_ARP */

[ 17.134621] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
[ 17.138895] tag_ops->flow_dissect()
[ 17.142388] Rx: proto: 0x8100, nhoff: 8 /* ETH_P_8021Q */

Tx path (UP system):

[ 15.499558] Tx: proto: 0x0800, nhoff: 26 /* ETH_P_IP */

[ 20.664689] Tx: proto: 0x0806, nhoff: 26 /* ETH_P_ARP */

[ 18.565782] Tx: proto: 0x86dd, nhoff: 26 /* ETH_P_IPV6 */

In order to fix that we can add the check 'proto == htons(ETH_P_XDSA)'
to prevent code from calling tag_ops->flow_dissect() on Tx.
I also decided to initialize 'offset' variable so tagger callbacks can
now safely leave it untouched without provoking a chaos.

Fixes: 43e665287f93 ("net-next: dsa: fix flow dissection")
Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/core/flow_dissector.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -870,9 +870,10 @@ bool __skb_flow_dissect(const struct net
nhoff = skb_network_offset(skb);
hlen = skb_headlen(skb);
#if IS_ENABLED(CONFIG_NET_DSA)
- if (unlikely(skb->dev && netdev_uses_dsa(skb->dev))) {
+ if (unlikely(skb->dev && netdev_uses_dsa(skb->dev) &&
+ proto == htons(ETH_P_XDSA))) {
const struct dsa_device_ops *ops;
- int offset;
+ int offset = 0;

ops = skb->dev->dsa_ptr->tag_ops;
if (ops->flow_dissect &&

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