On Wed, Sep 30, 2020 at 08:06:23PM -0700, Florian Fainelli wrote:
> The intent of 412a1526d067 ("net: dsa: untag the bridge pvid from rx
> skbs") is to transparently untag the bridge's default_pvid when the
> Ethernet switch can only support egress tagged of that default_pvid
> towards the CPU port.
>
> Prior to this commit, users would have to configure an 802.1Q upper on
> the bridge master device when the bridge is configured with
> vlan_filtering=0 in order to pop the VLAN tag:
>
> ip link add name br0 type bridge vlan_filtering 0
> ip link add link br0 name br0.1 type vlan id 1
>
> After this commit we added support for managing a switch port 802.1Q
> upper but those are not usually added as bridge members, and if they do,
> they do not actually require any special management, the data path would
> pop the desired VLAN tag accordingly.
>
> What we want to preserve is that use case and to manage when the user
> creates that 802.1Q upper for the bridge port.
>
> While we are it, call __vlan_find_dev_deep_rcu() which makes use the
> VLAN group array which is faster.
>
> As soon as we return the VLAN tagged SKB though it will be used by the
> following call path:
>
> netif_receive_skb_list_internal
> -> __netif_receive_skb_list_core
> -> __netif_receive_skb_core
> -> vlan_do_receive()
>
> which uses skb->vlan_proto, if we do not set it to the appropriate VLAN
> protocol, we will leave it set to what the DSA master has set
> (ETH_P_XDSA).
>
The explanation is super confusing, although I think the placement of
the "skb->vlan_proto = vlan_dev_vlan_proto(upper_dev)" is correct.
Here's what I think is going on. It has to do with what's upwards of the
code you're changing:
/* Move VLAN tag from data to hwaccel */
if (!skb_vlan_tag_present(skb) && hdr->h_vlan_proto == htons(proto)) {
skb = skb_vlan_untag(skb);
if (!skb)
return NULL;
}
So skb->vlan_proto should already be equal to the protocol of the 8021q
upper, see the call path below.
this is the problem
|
skb_vlan_untag() v
-> __vlan_hwaccel_put_tag(skb, skb->protocol, vlan_tci);
-> skb->vlan_proto = vlan_proto;
But the problem is that skb_vlan_untag() calls __vlan_hwaccel_put_tag
with the wrong vlan_proto, it calls it with the skb->protocol which is
still ETH_P_XDSA because we haven't re-run eth_type_trans() yet.
It looks like this function wants pretty badly to be called after
eth_type_trans(), and it's getting pretty messy because of that, but we
don't have any other driver-specific hook afterwards..
I don't have a lot of experience, the alternatives are either to:
- move dsa_untag_bridge_pvid() after eth_type_trans(), similar to what
you did in your initial patch - maybe this is the cleanest
- make dsa_untag_bridge_pvid() call eth_type_trans() and this gets rid
of the extra step you need to do in tag_brcm.c
- document this very well
> Fixes: 412a1526d067 ("net: dsa: untag the bridge pvid from rx skbs")
> Signed-off-by: Florian Fainelli <f.faine...@gmail.com>
> ---
> Changes in v2:
>
> - removed unused list_head iter argument
>
> net/dsa/dsa_priv.h | 11 ++++-------
> 1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
> index 0348dbab4131..b4aafb2e90fa 100644
> --- a/net/dsa/dsa_priv.h
> +++ b/net/dsa/dsa_priv.h
> @@ -205,7 +205,6 @@ static inline struct sk_buff
> *dsa_untag_bridge_pvid(struct sk_buff *skb)
> struct net_device *br = dp->bridge_dev;
> struct net_device *dev = skb->dev;
> struct net_device *upper_dev;
> - struct list_head *iter;
> u16 vid, pvid, proto;
> int err;
>
> @@ -247,12 +246,10 @@ static inline struct sk_buff
> *dsa_untag_bridge_pvid(struct sk_buff *skb)
> * supports because vlan_filtering is 0. In that case, we should
> * definitely keep the tag, to make sure it keeps working.
> */
> - netdev_for_each_upper_dev_rcu(dev, upper_dev, iter) {
> - if (!is_vlan_dev(upper_dev))
> - continue;
> -
> - if (vid == vlan_dev_vlan_id(upper_dev))
> - return skb;
> + upper_dev = __vlan_find_dev_deep_rcu(br, htons(proto), vid);
> + if (upper_dev) {
> + skb->vlan_proto = vlan_dev_vlan_proto(upper_dev);
> + return skb;
> }
>
> __vlan_hwaccel_clear_tag(skb);
> --
> 2.25.1
>
