Re: [PATCH net] net: dsa: fix flow dissection on Tx path

[Florian Fainelli <f.fainelli@gmail.com>]

On 12/5/2019 2:02 AM, Alexander Lobakin wrote:
> 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>
-- 
Florian