netdev - Re: netlink: 16 bytes leftover after parsing attributes in process `ip'.

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Message-ID: <20180926160242.6fe00491@shemminger-XPS-13-9360>
Date:   Wed, 26 Sep 2018 16:02:42 +0100
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Jiri Benc <jbenc@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: netlink: 16 bytes leftover after parsing attributes in process
 `ip'.

On Wed, 26 Sep 2018 08:54:43 -0600
David Ahern <dsahern@...il.com> wrote:

> On 9/25/18 11:51 PM, Jiri Benc wrote:
> > On Tue, 25 Sep 2018 09:37:41 -0600, David Ahern wrote:  
> >> For ifaddrmsg ifa_flags aligns with ifi_type which is set by kernel side
> >> so this should be ok.  
> > 
> > Does the existing user space set ifi_type to anything? Does it zero out
> > the field?
> > 
> > Are we able to find a flag value that is not going to be set by unaware
> > user space? I.e., a bit that is unused by the current ARPHRD values on
> > both little and big endian? (ARPHRD_NONE might be a problem, though...)  
> 
> The goal is for userpsace to pass something to the kernel to
> definitively state which header is used.
> 

You can not safely assume anything about older code.
iproute2 is not the only thing using the API, others include Quagga, and other tools.

Sorry, this API is frozen.