netdev - Re: [PATCH net-next] net: make dev_set_mtu() honor notification return code

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Message-Id: <20140113.151855.1046745397621207165.davem@davemloft.net>
Date:	Mon, 13 Jan 2014 15:18:55 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	vfalico@...hat.com
Cc:	netdev@...r.kernel.org, jiri@...nulli.us, edumazet@...gle.com,
	alexander.h.duyck@...el.com, nicolas.dichtel@...nd.com
Subject: Re: [PATCH net-next] net: make dev_set_mtu() honor notification
 return code

From: Veaceslav Falico <vfalico@...hat.com>
Date: Fri, 10 Jan 2014 13:48:17 +0100

> Currently, after changing the MTU for a device, dev_set_mtu() calls
> NETDEV_CHANGEMTU notification, however doesn't verify it's return code -
> which can be NOTIFY_BAD - i.e. some of the net notifier blocks refused this
> change, and continues nevertheless.
> 
> To fix this, verify the return code, and if it's an error - then revert the
> MTU to the original one, and pass the error code.
> 
> Signed-off-by: Veaceslav Falico <vfalico@...hat.com>

This is really a precariously designed code path.

If one of the notifiers says NOTIFY_BAD, well we've already changed
dev->mtu, therefore what if a packet got sent during this time?

Whoever the NOTIFY_BAD signaller is, obviously cannot handle an MTU
setting which we've already set in the netdev.  So allowing it's a
terribly idea to allow visibility of the new MTU value until we can be
sure everyone can handle it.

The problem is that we really need a transaction of some sort to fix
this properly.  First, we'd need to ask all the notifiers if they
can handle the new MTU, then we somehow atomically set netdev->mtu
and have the notifiers commit their own state changes.

Then we'll have the stick issue of what to do if a notifier is
unregistered between the check and the commit. :-)

Your patch is an improvement so I will apply it, this stuff really
is full of holes still.

Thanks.

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