linux-kernel - Re: [PATCH net-next] sysfs: add entry to indicate network interfaces with random MAC address

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Message-Id: <1279627656.2110.13.camel@achroite.uk.solarflarecom.com>
Date:	Tue, 20 Jul 2010 13:07:36 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Stefan Assmann <sassmann@...hat.com>
Cc:	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	davem@...emloft.net, Andy Gospodarek <gospo@...hat.com>,
	"Rose, Gregory V" <gregory.v.rose@...el.com>,
	"Duyck, Alexander H" <alexander.h.duyck@...el.com>,
	Casey Leedom <leedom@...lsio.com>,
	Harald Hoyer <harald@...hat.com>
Subject: Re: [PATCH net-next] sysfs: add entry to indicate network
 interfaces with random MAC address

On Tue, 2010-07-20 at 13:47 +0200, Stefan Assmann wrote:
> On 20.07.2010 13:20, Ben Hutchings wrote:
> > On Tue, 2010-07-20 at 12:50 +0200, Stefan Assmann wrote:
> >> From: Stefan Assmann <sassmann@...hat.com>
> >>
> >> Reserve a bit in struct net_device to indicate whether an interface
> >> generates its MAC address randomly, and expose the information via
> >> sysfs.
> >> May look like this:
> >> /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/net/eth0/ifrndmac
> >>
> >> By default the value of ifrndmac is 0. Any driver that generates the MAC
> >> address randomly should return a value to 1.
> > 
> > The name should incorporate 'address', not 'mac', for consistency with
> > the generic 'address' attribute.
> 
> We can call it "ifrndhwaddr" if that's more consistent.
> 
> > 
> > What about devices that 'steal' MAC addresses from slave devices?
> > Currently I believe udev has special cases for them but ideally these
> > drivers would indicate explicitly that their addresses are not stable
> > identifiers (even though they aren't random).
> 
> It's really up to the driver to decide whether it makes sense to set the
> flag or not. The question is what should udev do with these MAC address
> stealing devices apart from ignoring them? Sorry I have no higher
> insight into it.
> This flag has the purpose to allow udev to explicitly handle devices
> that generate their MAC address randomly and generate a persistent
> rule based on the device path instead of the MAC address.
> I'm open for suggestions but I'm not sure we can handle both cases with
> a single flag.

OK, then call it something like 'address_temporary'.

> JFYI this is an alternative approach to changing the kernel name of VFs
> to vfeth. The advantage of this way should be that we don't break any
> user-space applications that rely on network interfaces being names
> "ethX". Actually this goes in the direction of "fixing udev" which was
> what you asked for in your comment on my first patch concering vfeth. :)
> 
> > 
> > [...]
> >> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> >> index b626289..2ea0298 100644
> >> --- a/include/linux/netdevice.h
> >> +++ b/include/linux/netdevice.h
> >> @@ -845,6 +845,7 @@ struct net_device {
> >>  #define NETIF_F_FCOE_MTU	(1 << 26) /* Supports max FCoE MTU, 2158 bytes*/
> >>  #define NETIF_F_NTUPLE		(1 << 27) /* N-tuple filters supported */
> >>  #define NETIF_F_RXHASH		(1 << 28) /* Receive hashing offload */
> >> +#define NETIF_F_RNDMAC		(1 << 29) /* Interface with random MAC address */
> > [...]
> > 
> > This is not really a feature, and we are running out of real feature
> > bits.  Can you find somewhere else to put this flag?
> 
> Actually Dave Miller suggested to put it there. What other place is
> there to put it?

If Dave said that then I'm sure it's OK.

However, if you define this as an interface flag (net_device::flags;
<linux/if.h>) and add it to the set of changeable flags in
__dev_change_flags(), user-space will be able to clear the flag if it
later sets a stable address.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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