Re: [PATCH v2 02/12] wifi: mwifiex: fix MAC address handling

[Brian Norris <briannorris@chromium.org>]

Hi Sascha,

Sorry for being a bit slow here. And even now, I probably don't have
enough time to review this whole series today.

But I'll still share some initial thoughts, in case you can help address
them before I next look at this:

On Wed, Sep 18, 2024 at 01:10:27PM +0200, Sascha Hauer wrote:
> The mwifiex driver tries to derive the MAC addresses of the virtual
> interfaces from the permanent address by adding the bss_num of the
> particular interface used. It does so each time the virtual interface
> is changed from AP to station or the other way round. This means that
> the devices MAC address changes during a change_virtual_intf call which
> is pretty unexpected by userspace.

Ack, the "change_virtual_intf" part looks wrong.

> Furthermore the driver doesn't use the permanent address to add the
> bss_num to, but instead the current MAC address increases each time
> we do a change_virtual_intf.
> 
> Fix this by initializing the MAC address once from the permanent MAC
> address during creation of the virtual interface and never touch it
> again. This also means that userspace can set a different MAC address
> which then stays like this forever and is not unexpectedly changed
> by the driver.
> 
> It is not clear how many (if any) MAC addresses after the permanent MAC
> address are reserved for a device, so set the locally admistered
> bit for all MAC addresses derived from the permanent address.

I think I'm generally supportive of the direction this changes things,
but I'm a bit hesitant about two things:
1. the potential user-visible changes and
2. the linux-stable backport (Cc stable below)

For 1: MAC addresses are important in some contexts, and this might
significantly change the addresses that devices get in practice. Such
users might not really care about the weird details of when the address
incremented; but they *probably* care that a certain sequence of "boot
device; run hostapd with <foo> config file" produces the same address.

Also, I'm not sure I know enough of the implications of potential
over-use of the locally administered bit. Are there significant
downsides to it (aside from the simple fact that it's a different
address)?

And I see that you rightly don't know how many addresses are actually
reserved, but I have an educated guess that it's not just 1. For one,
this driver used to default-create 3 interfaces:
1211c961170c mwifiex: do not create AP and P2P interfaces upon driver loading

and when we stopped doing that, we still kept support for a module
parameter for the old way:
0013c7cebed6 mwifiex: module load parameter for interface creation

Perhaps these "initial" interfaces should at least be allowed permanent
addresses?

So anyway, I don't really know for sure the right answer, but I want to
log my concerns, in case you had more thoughts on backward
compatibility.

And given all the uncertainty above, I'm extra hesitant about
backporting likely-user-visible changes to stable (#2).

> With this patch MWIFIEX_BSS_TYPE_ANY becomes unused, so it's removed.
> 
> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> Cc: stable@vger.kernel.org

Regards,
Brian