netdev - Re: [PATCH RFC 3/3] RDMA/siw: Require non-zero 6-byte MACs for soft iWARP

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Message-ID: <157EE22D-4A74-4C2B-B704-FAEE371B265C@oracle.com>
Date: Tue, 23 May 2023 22:50:36 +0000
From: Chuck Lever III <chuck.lever@...cle.com>
To: Tom Talpey <tom@...pey.com>
CC: Jason Gunthorpe <jgg@...pe.ca>, Chuck Lever <cel@...nel.org>,
        Netdev
	<netdev@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Bernard
 Metzler <BMT@...ich.ibm.com>
Subject: Re: [PATCH RFC 3/3] RDMA/siw: Require non-zero 6-byte MACs for soft
 iWARP



> On May 23, 2023, at 3:44 PM, Tom Talpey <tom@...pey.com> wrote:
> 
> On 5/23/2023 3:18 PM, Chuck Lever III wrote:
>>> On May 5, 2023, at 3:58 PM, Jason Gunthorpe <jgg@...pe.ca> wrote:
>>> 
>>> On Fri, May 05, 2023 at 11:43:11AM -0400, Chuck Lever wrote:
>>>> From: Chuck Lever <chuck.lever@...cle.com>
>>>> 
>>>> In the past, LOOPBACK and NONE (tunnel) devices had all-zero MAC
>>>> addresses. siw_device_create() would fall back to copying the
>>>> device's name in those cases, because an all-zero MAC address breaks
>>>> the RDMA core IP-to-device lookup mechanism.
>>> 
>>> Why not just make up a dummy address in SIW? It shouldn't need to leak
>>> out of it.. It is just some artifact of how the iWarp stuff has been
>>> designed
>> So that approach is already being done in siw_device_create(),
>> even though it is broken (the device name hasn't been initialized
>> when the phony MAC is created, so it is all zeroes). I've fixed
>> that and it still doesn't help.
>> siw cannot modify the underlying net_device to add a made-up
>> MAC address.
>> The core address resolution code wants to find an L2 address
>> for the egress device. The underlying ib_device, where a made-up
>> GID might be stored, is not involved with address resolution
>> AFAICT.
>> tun devices have no L2 address. Neither do loopback devices,
>> but address resolution makes an exception for LOOPBACK devices
>> by redirecting to a local physical Ethernet device.
>> Redirecting tun traffic to the local Ethernet device seems
>> dodgy at best.
>> I wasn't sure that an L2 address was required for siw before,
>> but now I'm pretty confident that it is required by our
>> implementation.
> 
> Does rxe work over tunnels?

(It's not tunnels per se, it's devices that don't have
L2 addresses... and tun happens to be one instance of
that class).

My (brief) reading of the source code is that the use of
devices that do not have L2 addresses is prohibited for
rxe.


> Seems like it would have the same issue.

Agreed, if rxe did not prohibit them, it would have the same
issue.

To be clear: siw itself and the family of iWARP protocols
shouldn't have any problem at all with such devices. The
issue seems to be with the Linux implementation of address
resolution.


> int rxe_register_device(struct rxe_dev *rxe, const char *ibdev_name)
> {
> ...
>        addrconf_addr_eui48((unsigned char *)&dev->node_guid,
>                            rxe->ndev->dev_addr);
> 
> static struct siw_device *siw_device_create(struct net_device *netdev)
> {
> ...
>        addrconf_addr_eui48((unsigned char *)&base_dev->node_guid,
>                                    netdev->dev_addr);
> 
> Tom.


--
Chuck Lever