netdev - Re: [RFC 1/1] net: move IFF_LIVE_ADDR

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Date:   Thu, 04 Aug 2022 12:23:45 -0700
From:   James Prestwood <prestwoj@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org
Subject: Re: [RFC 1/1] net: move IFF_LIVE_ADDR_CHANGE to public flag

On Thu, 2022-08-04 at 11:43 -0700, Jakub Kicinski wrote:
> On Thu,  4 Aug 2022 10:43:07 -0700 James Prestwood wrote:
> > By exposing IFF_LIVE_ADDR_CHANGE to userspace it at least gives an
> > indication that we can successfully randomize the address and
> > connect. In the worst case address randomization can be avoided
> > ahead of time. A secondary win is also time, since userspace can
> > avoid a power down unless its required which saves some time.
> 
> It's not a generic thing tho, it's most of an implicit argument 
> to eth_mac_addr(). Not all netdevs are Ethernet.
> 
> The semantics in wireless are also a little stretched because
> normally
> if the flag is not set the netdev will _refuse_ (-EBUSY) to change
> the
> address while running, not do some crazy fw reset.

Sorry if I wasn't clear, but its not nl80211 doing the fw reset
automatically. The wireless subsystem actually completely disallows a
MAC change if the device is running, this flag isn't even checked. This
means userspace has to bring the device down itself, then change the
MAC.

I plan on also modifying mac80211 to first check this flag and allow a
live MAC change if possible. But ultimately userspace still needs to be
aware of the support.

> 
> Perhaps we should wait for Johannes to return form vacation but my
> immediate reaction would be to add a knob (in wireless?) that
> controls
> whether the reset dance is allowed.

Ok sounds good. Lets see what Johannes has to say.