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

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Date:   Fri, 26 Apr 2019 13:56:29 +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 01:23:46PM +0200, Johannes Berg wrote:
> On Fri, 2019-04-26 at 13:19 +0200, Michal Kubecek wrote:
> > 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.
> 
> I guess anyway we can only do it for *new* things, not really for all
> existing attributes.

Right... but what I wanted to say is that if there is already (at least)
one attribute which may or may not be a nest, depending on a context, we
should expect there may be also new attributes like that in the future.

Michal