netdev - Re: [PATCH RFC v2 net-next 03/25] netlink: introduce NLM_F_DUMP_PROPER

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Date:   Tue, 2 Oct 2018 13:18:32 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Jiri Benc <jbenc@...hat.com>
Cc:     David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        davem@...emloft.net, stephen@...workplumber.org,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH RFC v2 net-next 03/25] netlink: introduce
 NLM_F_DUMP_PROPER_HDR flag

On Tue, Oct 02, 2018 at 01:06:14PM +0200, Jiri Benc wrote:
> On Mon,  1 Oct 2018 17:28:29 -0700, David Ahern wrote:
> > Add a new flag, NLM_F_DUMP_PROPER_HDR, for userspace to indicate to the
> > kernel that it believes it is sending the right header struct for the
> > dump message type (ifinfomsg, ifaddrmsg, rtmsg, fib_rule_hdr, ...).
> 
> Why is this limited to dumps? Other kind of netlink messages contain
> the common struct, too. When introducing such mechanism, please make it
> generic.
> 
> Last time when we were discussing strict checking in netlink, it was
> suggested to add a socket option instead of adding NLM flags[1].

I didn't find this in the linked thread. What I find interesting and
convincing is one of Dave's points:

"I'm beginning to wonder if we can just change this unilaterally to
not ignore unrecognized attributes.

I am increasingly certain that things that would "break" we wouldn't
want to succeed anyways." [1]

:)

But a socket option or this header flag both sound acceptable to me. Was
there any more detail on how a socket option would look like, i.e. an
api proposal or something?

[1]: https://marc.info/?l=linux-netdev&m=144522081220166&w=2

> It makes a lot of sense: the number of flags is very limited and we'd
> run out of them pretty fast. It's not just the header structure that
> is currently checked sloppily. It's also attributes, flags in
> attributes, etc. We can't assign a flag to all of them.
> 
> You should also consider a different name for the flag: it should
> reflect what the effect of the flag is. "Proper header" is not an
> effect, it's a requirement for the message to pass. The effect is
> enforced strict checking of the header.
> 
>  Jiri
> 
> [1] https://marc.info/?l=linux-netdev&m=144492718118955