Subject: Re: [PATCH net] macsec: Support VLAN-filtering lower devices

From: Sabrina Dubroca <[email-masked]>
Date: Fri, 9 Jan 2026 13:06:39 +0100

2026-01-09, 11:38:59 +0000, Cosmin Ratiu wrote:
> On Fri, 2026-01-09 at 11:26 +0100, Sabrina Dubroca wrote:
> > 2026-01-07, 12:47:23 +0200, Cosmin Ratiu wrote:
> > > VLAN-filtering is done through two netdev features
> > > (NETIF_F_HW_VLAN_CTAG_FILTER and NETIF_F_HW_VLAN_STAG_FILTER) and
> > > two
> > > netdev ops (ndo_vlan_rx_add_vid and ndo_vlan_rx_kill_vid).
> > > 
> > > Implement these and advertise the features if the lower device
> > > supports
> > > them. This allows proper VLAN filtering to work on top of macsec
> > > devices, when the lower device is capable of VLAN filtering.
> > > As a concrete example, having this chain of interfaces now works:
> > > vlan_filtering_capable_dev(1) -> macsec_dev(2) ->
> > > macsec_vlan_dev(3)
> > > 
> > > Before the "Fixes" commit this used to accidentally work because
> > > the
> > > macsec device (and thus the lower device) was put in promiscuous
> > > mode
> > > and the VLAN filter was not used. But after that commit correctly
> > > made
> > > the macsec driver expose the IFF_UNICAST_FLT flag, promiscuous mode
> > > was
> > > no longer used and VLAN filters on dev 1 kicked in. Without support
> > > in
> > > dev 2 for propagating VLAN filters down, the register_vlan_dev ->
> > > vlan_vid_add -> __vlan_vid_add -> vlan_add_rx_filter_info call from
> > > dev
> > > 3 is silently eaten (because vlan_hw_filter_capable returns false
> > > and
> > > vlan_add_rx_filter_info silently succeeds).
> > 
> > We only want to propagate VLAN filters when macsec offload is used,
> > no? If offload isn't used, the lower device should be unaware of
> > whatever is happening on top of macsec, so I don't think non-
> > offloaded
> > setups are affected by this?
> 
> VLAN filters are not related to macsec offload, right? It's about
> informing the lower netdevice which VLANs should be allowed. Without
> this patch, the VLAN-tagged packets intended for the macsec vlan device
> are discarded by the lower device VLAN filter.

Why does the lower device need to know in the non-offload case? It has
no idea whether it's VLAN traffic or anything else once it's stuffed
into macsec.

The packet will look like

ETH | MACSEC | [some opaque data that may or may not start with a VLAN header ]

> > Even when offload is used, the lower device should probably handle
> > "ETH + VLAN 5" differently from "ETH + MACSEC + VLAN 5", but that may
> > not be possible with just the existing device ops.
> 
> I don't see how macsec plays a role into how the lower device handles
> VLANs. From the protocol diagrams, I see that it's ETH + VLAN 5 +
> MACSEC, the VLAN isn't encrypted if present.

Wait, if we're talking about ETH + VLAN 5 + MACSEC, macsec shouldn't
even be involved in VLAN id 5.

ip link add link eth0 type vlan id 5

should never go through any macsec code at all.

-- 
Sabrina