netdev - Re: [Question] Should NLMSG_DONE has flag NLM_F

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Message-ID: <Ywb4nCoi24S5iAtx@Laptop-X1>
Date:   Thu, 25 Aug 2022 12:20:44 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Dmitry Safonov <0x7f454c46@...il.com>,
        Florian Westphal <fw@...len.de>,
        Arkadi Sharshevsky <arkadis@...lanox.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [Question] Should NLMSG_DONE has flag NLM_F_MULTI?

On Wed, Aug 24, 2022 at 06:39:45PM -0700, Jakub Kicinski wrote:
> On Wed, 24 Aug 2022 11:20:18 +0800 Hangbin Liu wrote:
> > When checking the NLMSG_DONE message in kernel, I saw lot of functions would
> > set NLM_F_MULTI flag. e.g. netlink_dump_done(),
> > devlink_dpipe_{tables, entries, headers}_fill().
> > 
> > But from rfc3549[1]:
> > 
> >    [...] For multipart
> >    messages, the first and all following headers have the NLM_F_MULTI
> >    Netlink header flag set, except for the last header which has the
> >    Netlink header type NLMSG_DONE.
> > 
> > What I understand is the last nlmsghdr(NLMSG_DONE message) doesn't need to
> > have NLM_F_MULTI flag. Am I missing something?
> > 
> > [1] https://www.rfc-editor.org/rfc/rfc3549.html#section-2.3.2
> 
> Looks like you're right, we seem to fairly consistently set it.
> Yet another thing in Netlink we defined and then used differently?
> In practice it likely does not matter, I'd think.

Yes, thanks for the confirmation. I have no plan to change the current
kernel behavior. But for my later patch, I will not add NLM_F_MULTI for
NLMSG_DONE message.

Thanks
Hangbin