Subject: Re: [PATCH net 1/2] ipv6: Dump route exceptions too in rt6_dump_route()

From: Stefano Brivio <[email-masked]>
Date: Fri, 7 Jun 2019 00:58:52 +0200

On Thu, 6 Jun 2019 22:37:11 +0000
Martin Lau <[email-masked]> wrote:

> On Fri, Jun 07, 2019 at 12:17:47AM +0200, Stefano Brivio wrote:
> > On Thu, 6 Jun 2019 21:44:58 +0000
> > Martin Lau <[email-masked]> wrote:
> >   
> > > > +	if (!(filter->flags & RTM_F_CLONED)) {
> > > > +		err = rt6_fill_node(net, arg->skb, rt, NULL, NULL, NULL, 0,
> > > > +				    RTM_NEWROUTE,
> > > > +				    NETLINK_CB(arg->cb->skb).portid,
> > > > +				    arg->cb->nlh->nlmsg_seq, flags);
> > > > +		if (err)
> > > > +			return err;
> > > > +	} else {
> > > > +		flags |= NLM_F_DUMP_FILTERED;
> > > > +	}
> > > > +
> > > > +	bucket = rcu_dereference(rt->rt6i_exception_bucket);
> > > > +	if (!bucket)
> > > > +		return 0;
> > > > +
> > > > +	for (i = 0; i < FIB6_EXCEPTION_BUCKET_SIZE; i++) {
> > > > +		hlist_for_each_entry(rt6_ex, &bucket->chain, hlist) {
> > > > +			if (rt6_check_expired(rt6_ex->rt6i))
> > > > +				continue;
> > > > +
> > > > +			err = rt6_fill_node(net, arg->skb, rt,
> > > > +					    &rt6_ex->rt6i->dst,
> > > > +					    NULL, NULL, 0, RTM_NEWROUTE,
> > > > +					    NETLINK_CB(arg->cb->skb).portid,
> > > > +					    arg->cb->nlh->nlmsg_seq, flags);    
> > > Thanks for the patch.
> > > 
> > > A question on when rt6_fill_node() returns -EMSGSIZE while dumping the
> > > exception bucket here.  Where will the next inet6_dump_fib() start?  
> > 
> > And thanks for reviewing.
> > 
> > It starts again from the same node, see fib6_dump_node(): w->leaf = rt;
> > where rt is the fib6_info where we failed dumping, so we won't skip
> > dumping any node.  
> If the same node will be dumped, does it mean that it will go through this
> loop and iterate all exceptions again?

Yes (well, all the exceptions for that node).

> > This also means that to avoid sending duplicates in the case where at
> > least one rt6_fill_node() call goes through and one fails, we would
> > need to track the last bucket and entry sent, or, alternatively, to
> > make sure we can fit the whole node before dumping.  
> My another concern is the dump may never finish.

That's not a guarantee in general, even without this, because in theory
the skb passed might be small enough that we can't even fit a single
node without exceptions.

We could add a guard on w->leaf not being the same before and after the
walk in inet6_dump_fib() and, if it is, terminate the dump. I just
wonder if we have to do this at all -- I can't find this being done
anywhere else (at a quick look at least).

By the way, we can also trigger a never-ending dump by touching the
tree frequently enough during a dump: it would always start again from
the root, see fib6_dump_table().

-- 
Stefano