Re: [PATCH v2 4/7] netlink, pasta: Disable DAD for link-local addresses on namespace interface

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Aug 15, 2024 at 12:59:32PM +0200, Stefano Brivio wrote:
> On Thu, 15 Aug 2024 20:38:17 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Thu, Aug 15, 2024 at 10:36:46AM +0200, Stefano Brivio wrote:
> > > It makes no sense for a container or a guest to try and perform
> > > duplicate address detection for their link-local address, as we'll
> > > anyway not relay neighbour solicitations with an unspecified source
> > > address.
> > > 
> > > While they perform duplicate address detection, the link-local address
> > > is not usable, which prevents us from bringing up especially
> > > containers and communicate with them right away via IPv6.
> > > 
> > > This is not enough to prevent DAD and reach the container right away:
> > > we'll need a couple more patches.
> > > 
> > > As we send NLM_F_REPLACE requests right away, while we still have to
> > > read out other addresses on the same socket, we can't use nl_do():
> > > keep a count of messages we send (addresses we change) and deal with
> > > the answer to those NLM_F_REPLACE requests in a separate loop, later.
> > > 
> > > Link: https://github.com/containers/podman/pull/23561#discussion_r1711639663
> > > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > > ---
> > >  netlink.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  netlink.h |  1 +
> > >  pasta.c   |  6 ++++++
> > >  3 files changed, 62 insertions(+)
> > > 
> > > diff --git a/netlink.c b/netlink.c
> > > index 873e6c7..59f2fd9 100644
> > > --- a/netlink.c
> > > +++ b/netlink.c
> > > @@ -673,6 +673,61 @@ int nl_route_dup(int s_src, unsigned int ifi_src,
> > >  	return 0;
> > >  }
> > >  
> > > +/**
> > > + * nl_addr_set_ll_nodad() - Set IFA_F_NODAD on IPv6 link-local addresses
> > > + * @s:		Netlink socket
> > > + * @ifi:	Interface index in target namespace
> > > + *
> > > + * Return: 0 on success, negative error code on failure
> > > + */
> > > +int nl_addr_set_ll_nodad(int s, unsigned int ifi)
> > > +{
> > > +	struct req_t {
> > > +		struct nlmsghdr nlh;
> > > +		struct ifaddrmsg ifa;
> > > +	} req = {
> > > +		.ifa.ifa_family    = AF_INET6,
> > > +		.ifa.ifa_index     = ifi,
> > > +	};
> > > +	unsigned ll_addrs = 0;
> > > +	struct nlmsghdr *nh;
> > > +	char buf[NLBUFSIZ];
> > > +	ssize_t status;
> > > +	uint32_t seq;
> > > +
> > > +	seq = nl_send(s, &req, RTM_GETADDR, NLM_F_DUMP, sizeof(req));
> > > +	nl_foreach_oftype(nh, status, s, buf, seq, RTM_NEWADDR) {
> > > +		struct ifaddrmsg *ifa = (struct ifaddrmsg *)NLMSG_DATA(nh);
> > > +		struct rtattr *rta;
> > > +		size_t na;
> > > +
> > > +		if (ifa->ifa_index != ifi || ifa->ifa_scope != RT_SCOPE_LINK)
> > > +			continue;
> > > +
> > > +		ifa->ifa_flags |= IFA_F_NODAD;
> > > +
> > > +		for (rta = IFA_RTA(ifa), na = IFA_PAYLOAD(nh); RTA_OK(rta, na);
> > > +		     rta = RTA_NEXT(rta, na)) {
> > > +			/* If 32-bit flags are used, add IFA_F_NODAD there */
> > > +			if (rta->rta_type == IFA_FLAGS)
> > > +				*(uint32_t *)RTA_DATA(rta) |= IFA_F_NODAD;
> > > +		}
> > > +
> > > +		nl_send(s, nh, RTM_NEWADDR, NLM_F_REPLACE, nh->nlmsg_len);
> > > +		ll_addrs++;
> > > +	}
> > > +
> > > +	if (status < 0)
> > > +		return status;  
> > 
> > Ah... one gotcha with the nl_send() in the loop.  We should make sure
> > we get the responses from any of those we sent, even if the original
> > request failed.  Otherwise we'll be out of sync on the netlink socket again.
> 
> I'm ignoring the return code of nl_send(), so, minus the issue you're
> raising about nl_foreach() below, that should already be sorted, right?

No.  The return code from nl_send() is mostly irrelevant - it's just
the sequence number (other errors die()).  But the point is you've
queued requests, so the kernel will queue responses and if you exit
the function here, nothing will consume them.

> > > +	seq += ll_addrs;
> > > +
> > > +	nl_foreach(nh, status, s, buf, seq)
> > > +		warn("netlink: Unexpected response message");  
> > 
> > I don't think this will work right if there's > 1 address.  It will be
> > looking for the last sequence number on the first iteration and will
> > die in nl_status() when it mismatches.
> 
> Ah, oops, right.
> 
> > Maybe just loop on nl_next() until you get the last seq number, then
> > call nl_status()?
> 
> How do I check for errors on the answers before the next one? I mean,
> nl_foreach() should fit here, it's just that I need to start from the
> right sequence number.
> 
> > That also means you could just save the seq each
> > time you nl_send(), overwriting the previous one, rather than relying
> > on the fact that we allocate seqs, well, sequentially.
> 
> I don't understand how this fits with calling nl_next() until I get
> to the last sequence number. Letting that aside, can't I simply use
> nl_foreach(), but start with the sequence of the first nl_send()
> instead of the last one?

Uh.. yeah, it's a bit fiddly.  Especially since in those foreach loops
status does double duty as the remaining data in the current message
and as the status code.

# Option 1

Assuming contiguous sequence numbers, which is true for now.

- Change the nl_send() within the first loop to
	last_seq = nl_send(...)

Then immediately after the first loop

int status2 = status;

for (seq++; seq <= last_seq; seq++) {
	nl_foreach(nh, status2, s, buf, seq)
		;
	if (status == 0)
		status = status2;
}
At this point you will have consumed all the responses and status will
have the first reported error code.

# Option 2

Refactor nl_status() to have a version that reports sequence number
instead of taking & checking it. Loop on nl_next() until
nl_status_variant() returns <= 0 *and* the last sequence number.

# Option 3

Open-coded version of (2)

ssize_t err = status;

do {
	nh = nl_next(s, buf, nh, &status);
	if (err == 0 && nh->nl_msg_type == NLMSG_ERR) {
		struct nlmsgerr *errmsg = (struct nlmsgerr *)NLMSG_DATA(nh);
		err = errmsg->error;
	}
} while (ng->nlmsg_seq != last_seq ||
	(nh->nlmsg_type != NLMSG_DONE && nh->nlmsg_type != NLMSG_ERROR));

And at this point, again, you've consumed all the responses and 'err'
has the first error code.  I think this is roughly what I was
suggesting originally, but it is messier than I thought.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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