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

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Message-ID: <20100720141816.16f0a939@nehalam>
Date:	Tue, 20 Jul 2010 14:18:16 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	David Miller <davem@...emloft.net>
Cc:	bhutchings@...arflare.com, sassmann@...hat.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	gospo@...hat.com, gregory.v.rose@...el.com,
	alexander.h.duyck@...el.com, leedom@...lsio.com, harald@...hat.com
Subject: Re: [PATCH net-next] sysfs: add entry to indicate network
 interfaces with random MAC address

On Tue, 20 Jul 2010 13:17:48 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> From: Ben Hutchings <bhutchings@...arflare.com>
> Date: Tue, 20 Jul 2010 15:29:54 +0100
> 
> > On Tue, 2010-07-20 at 14:41 +0200, Stefan Assmann wrote:
> >> Btw, the driver itself could also alter the flag. Then we'd have a well
> >> defined way of setting a stable address.
> > 
> > The driver can't know whether an address assigned by the user is stable.
> 
> If userspace can somehow obtain a persistent address, it can kick
> udev too.
> 
> I really don't see any real value provided by letting userspace mess
> with this.  Because the permanence communicated in this value is from
> the perspective of the kernel driver, it's really therefore about the
> thing that's in ->perm_addr[] not what happens to be in ->addr[] right
> now.

No one mentioned that the first octet of an Ethernet address already
indicates "software generated" Ethernet address. Per the standard,
if bit 1 is set it means address is locally assigned.

static inline bool is_locally_assigned_ether(const u8 *addr)
{
	return (addr[0] & 0x2) != 0;
}
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