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

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Message-ID: <20200826162901.4js4u5u2whusp4l4@vega>
Date:   Wed, 26 Aug 2020 16:29:01 +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 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?

-- 
Regards,
Mira