netdev - Re: [RFC PATCH net-next 0/3] make nla_nest_start() add NLA_F

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Message-ID: <20190425090846.GC3951@nanopsycho>
Date:   Thu, 25 Apr 2019 11:08:46 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Michal Kubecek <mkubecek@...e.cz>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        David Ahern <dsahern@...il.com>,
        Johannes Berg <johannes.berg@...el.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        netfilter-devel@...r.kernel.org
Subject: Re: [RFC PATCH net-next 0/3] make nla_nest_start() add NLA_F_NESTED
 flag

Wed, Apr 24, 2019 at 11:34:21PM CEST, mkubecek@...e.cz wrote:
>One of the comments in recent review of the ethtool netlink series pointed
>out that proposed ethnl_nest_start() helper which adds NLA_F_NESTED to
>second argument of nla_nest_start() is not really specific to ethtool
>netlink code. That is hard to argue with as closer inspection revealed that
>exactly the same helper already exists in ipset code (except it's a macro
>rather than an inline function).
>
>Another observation was that even if NLA_F_NESTED flag was introduced in
>2007, only few netlink based interfaces set it in kernel generated messages
>and even many recently added APIs omit it. That is unfortunate as without
>the flag, message parsers not familiar with attribute semantics cannot
>recognize nested attributes and do not see message structure; this affects
>e.g. wireshark dissector or mnl_nlmsg_fprintf() from libmnl.
>
>This is why I'm suggesting to rename existing nla_nest_start() to different
>name (nla_nest_start_noflag) and reintroduce nla_nest_start() as a wrapper
>adding NLA_F_NESTED flag. This is implemented in first patch which is
>mostly generated by spatch. Second patch drops ipset helper macros which
>lose their purpose. Third patch cleans up minor coding style issues found
>by checkpatch.pl in first patch.
>
>If this approach is considered too intrusive, we can leave nla_nest_start()
>untouched and simply add a wrapper adding NLA_F_NESTED but that would
>probably preserve the state when even most new code doesn't set the flag.

I like this. Long overdue...