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

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Date:   Fri, 26 Apr 2019 13:19:54 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     netdev@...r.kernel.org
Cc:     Johannes Berg <johannes@...solutions.net>,
        "David S. Miller" <davem@...emloft.net>,
        David Ahern <dsahern@...il.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" <netfilter-devel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 0/3] make nla_nest_start() add NLA_F_NESTED flag

On Fri, Apr 26, 2019 at 11:24:15AM +0200, Johannes Berg wrote:
> On Fri, 2019-04-26 at 09:13 +0000, Michal Kubecek wrote:
> 
> > 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.
> 
> Indeed.
> 
> I wonder if we should also (start) enforcing that the userspace sender
> side sets this, if the policy is strict?

I suppose we should, at least the part that attribute with NLA_NESTED
policy has NLA_F_NESTED flag. I'm not so sure about the opposite (i.e.
that attributes with other policies do not have the flag) as when I was
checking where kernel accesses nlattr::nla_type directly rather than
with nla_type(), I stumbled upon an attribute NL80211_ATTR_VENDOR_DATA
which has policy NLA_BINARY but is sometimes a nest, AFAICS.

Michal