Re: [PATCH] genetlink: fix usage of NLM_F_EXCL or NLM_F_REPLACE

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi David!

On Tue, Jul 30, 2013 at 04:44:23PM -0700, David Miller wrote:
> From: Pablo Neira Ayuso <pablo@netfilter.org>
> Date: Mon, 29 Jul 2013 12:30:04 +0200
> 
> > Currently, it is not possible to use neither NLM_F_EXCL nor
> > NLM_F_REPLACE from genetlink. This is due to this checking in
> > genl_family_rcv_msg:
> > 
> > 	if (nlh->nlmsg_flags & NLM_F_DUMP)
> > 
> > NLM_F_DUMP is NLM_F_MATCH|NLM_F_ROOT. Thus, if NLM_F_EXCL or
> > NLM_F_REPLACE flag is set, genetlink believes that you're
> > requesting a dump and it calls the .dumpit callback.
> > 
> > The solution that I propose is to refine this checking to
> > make it stricter:
> > 
> > 	if ((nlh->nlmsg_flags & NLM_F_DUMP) == NLM_F_DUMP)
> > 
> > And given the combination NLM_F_REPLACE and NLM_F_EXCL does
> > not make sense to me, it removes the ambiguity.
> > 
> > There was a patch that tried to fix this some time ago (0ab03c2
> > netlink: test for all flags of the NLM_F_DUMP composite) but it
> > tried to resolve this ambiguity in *all* existing netlink subsystems,
> > not only genetlink. That patch was reverted since it broke iproute2,
> > which is using NLM_F_ROOT to request the dump of the routing cache.
> > 
> > Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> 
> Yes, I remember that old attempt to fix this.
> 
> Ok, let's see what happens when we limit the scope of this change
> to just genetlink users.
>
> I honestly can't believe that NLM_F_EXCL and NLM_F_REPLACE are
> completely unusable in normal rtnetlink interfaces.

I guess you mean 'genetlink' instead of 'rtnetlink'.

This repo provides one genetlink example kernel module and userspace
utility using libmnl:

git clone git://1984.lsi.us.es/netlink-example

make
make userspace
insmod genlexample.ko

This registers 'nlex', an example genetlink family that allows you to
update the value of a variable in kernel-space.

To use it, we have to resolve the genetlink family-id first (sorry,
these examples are a bit rudimentary):

./genl-resolve nlex
family-id: 24
multicast -> id
"example" -> 7

OK, so it's family-id 24, let's update that variable in kernel-space
without NLM_F_EXCL:

./genl-change 24        # uses time(NULL) to update that variable
done

./genl-get 24
myvar=1375268456
done

Now if we try to make it with NLM_F_EXCL:

./genl-change 24 excl
mnl_cb_run: Operation not supported

My examples have no .dumpit callback. thus, genetlink was
misiterpreting this and it returned EOPNOTSUPP.

So nobody using genetlink could use these two flags so far.