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

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Date:   Wed, 26 Sep 2018 08:54:43 -0600
From:   David Ahern <dsahern@...il.com>
To:     Jiri Benc <jbenc@...hat.com>
Cc:     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 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.

ifaddrmsg (proper header and one Christian's patch wants to use) is 8
bytes; ifinfomsg (legacy header from broken userspace) is 16. If you can
not trust that ifi_type is currently 0 on a dump request then you can
not trust ifi_flags to be correct or ifi_change to be correct and so you
can not move past the header and parse attributes. If that is the case
we are done - Christian's patches should be reverted as you can never
trust what is beyond the family entry.

But I do not believe that to be the case because of the route dump
analogy. As I mentioned route dumps have the same problem: sometimes
ifinfomsg is passed and sometimes rtmsg. Yet the kernel always looks at
rtm_flags.

In terms of which field to use the most logical to me is to pass in a
flag. Current address dumps have no reason to pass in a flag so it is
not like the field can be misinterpreted. ifa_flags is a single byte so
are there really endian issues to worry about?