netdev - Re: [PATCH 1/2] veth: Initialize dev->perm

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Message-ID: <20200827010403.r3zic7s66shcjcrb@vega>
Date:   Thu, 27 Aug 2020 01:04:03 +0000
From:   Mira Ressel <aranea@...ah.de>
To:     David Miller <davem@...emloft.net>
Cc:     kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] veth: Initialize dev->perm_addr

On Wed, Aug 26, 2020 at 09:33:29AM -0700, David Miller wrote:
> From: Mira Ressel <aranea@...ah.de>
> Date: Wed, 26 Aug 2020 16:29:01 +0000
> 
> > On Wed, Aug 26, 2020 at 08:28:57AM -0700, David Miller wrote:
> >> From: Mira Ressel <aranea@...ah.de>
> >> Date: Wed, 26 Aug 2020 15:20:00 +0000
> >> 
> >> > I'm setting the peer->perm_addr, which would otherwise be zero, to its
> >> > dev_addr, which has been either generated randomly by the kernel or
> >> > provided by userland in a netlink attribute.
> >> 
> >> Which by definition makes it not necessarily a "permanent address" and
> >> therefore is subject to being different across boots, which is exactly
> >> what you don't want to happen for automatic address generation.
> > 
> > That's true, but since veth devices aren't backed by any hardware, I
> > unfortunately don't have a good source for a permanent address. The only
> > inherently permanent thing about them is their name.
> > 
> > People who use the default eui64-based address generation don't get
> > persistent link-local addresses for their veth devices out of the box
> > either -- the EUI64 is derived from the device's dev_addr, which is
> > randomized by default.
> > 
> > If that presents a problem for anyone, they can configure their userland
> > to set the dev_addr to a static value, which handily fixes this problem
> > for both address generation algorithms.
> > 
> > I'm admittedly glancing over one problem here -- I'm only setting the
> > perm_addr during device creation, whereas userland can change the
> > dev_addr at any time. I'm not sure if it'd make sense here to update the
> > perm_addr if the dev_addr is changed later on?
> 
> We are talking about which parent device address to inherit from, you
> have choosen to use dev_addr and I am saying you should use perm_addr.
> 
> Can you explain why this isn't clear?

Which parent device? This is the veth patch, not the vlan patch. I'm
setting the perm_addrs of both sides of the veth pair to their
respective dev_addrs that they were assigned by userland or through
random generation. If I were to give both of them the same perm_addr,
we'd again have the problem that they'd get conflicting link-local
addresses and trigger DAD.

My apologies if I'm misunderstanding something here!

Regards,
Mira