Re: [PATCH net] net: lan78xx: Disable hardware vlan filtering in promiscuous mode

["Markus Blöchl" <Markus.Bloechl@ipetronik.com>]

On Wed, Nov 11, 2020 at 06:47:27PM +0200, Vladimir Oltean wrote:
> On Wed, Nov 11, 2020 at 07:56:58AM -0800, Florian Fainelli wrote:
> > The semantics of promiscuous are pretty clear though, and if you have a
> > NIC with VLAN filtering capability which could prevent the stack from
> > seeing *all* packets, that would be considered a bug. I suppose that you
> > could not disable VLAN filtering but instead install all 4096 - N VLANs
> > (N being currently used) into the filter to guarantee receiving those
> > VLAN tagged frames?

Adding all VLAN ids to the filter is certainly a possibility, I just don't
see why that redundant extra lookup per frame should be done in the NIC.

I guess, we could also put that feature on WISH while promisc ist active.
That, at least, makes it clear what happened.

> 
> Are they?
> 
> IEEE 802.3 clause 30.3.1.1.16 aPromiscuousStatus says:
> 
> APPROPRIATE SYNTAX:
> BOOLEAN
> 
> BEHAVIOUR DEFINED AS:
> A GET operation returns the value “true” for promiscuous mode enabled, and “false” otherwise.
> 
> Frames without errors received solely because this attribute has the value “true” are counted as
> frames received correctly; frames received in this mode that do contain errors update the
> appropriate error counters.
> 
> A SET operation to the value “true” provides a means to cause the LayerMgmtRecognizeAddress
> function to accept frames regardless of their destination address.
> 
> A SET operation to the value “false” causes the MAC sublayer to return to the normal operation
> of carrying out address recognition procedures for station, broadcast, and multicast group
> addresses (LayerMgmtRecognizeAddress function).;
> 
> 
> As for IEEE 802.1Q, there's nothing about promiscuity in the context of
> VLAN there.
> 
> Sadly, I think promiscuity refers only to address recognition for the
> purpose of packet termination. I cannot find any reference to VLAN in
> the context of promiscuity, or, for that matter, I cannot find any
> reference coming from a standards body that promiscuity would mean
> "accept all packets".

From what I can see, most other drivers use a special hardware register
flag to enable promiscuous mode, which overrules all other filters.
e.g. from the ASIX AX88178 datasheet:

  PRO:  PACKET_TYPE_PROMISCUOUS.
    1: All frames received by the ASIC are forwarded up toward the host.
    0: Disabled (default).

It is just so that the lan78xx controllers don't have this explicit flag.


But since my change is more controversial than I anticipated, I would like
to take a step back and ask some general questions first:

We used to connect something akin to a port mirror to a lan7800 NIC
(and a few others) in order to record and debug some VLAN heavy traffic.
On older kernel versions putting the interface into promiscuous mode
and opening and binding an AF_PACKET socket (or just throwing tcpdump
or libpcap at it) was sufficient.
After a kernel upgrade the same setup missed most of the traffic.
Does this qualify as a regression? Why not?

Should there be a documented and future proof way to receive *all*
valid ethernet frames from an interface? This could be something like:

a) - Bring up the interface
   - Put the interface into promiscuous mode
   - Open, bind and read a raw AF_PACKET socket with ETH_P_ALL
   - Patch up the 801.1Q headers if required.

b) - The same as a)
   - Additionally enumerate and disable all available offloading features

c) - Use libpcap / Do whatever libpcap does (like with TPACKET)
   In this case you need to help me convince the tcpdump folks that this
   is a bug on their side... ;-)

d) - Read the controller datasheet
   - Read the kernel documentation
   - Read your kernels and drivers sources
   - Do whatever might be necessary

e) - No, there is no guaranteed way to to this

Any opinions on these questions?
After those are answered, I am open to suggestions on how to fix this
differently (if still needed).
I'd rather not get involved into discussions on flow filters as I am
absolutely clueless in this regard.


PS: Sorry for sending from my companies mail server. It does some nasty
transformations on the body to outgoing mails I just complained about.
If this isn't resolved soon, I might follow up using another
address which should preserve patches.