On Tue, Aug 20, 2024 at 10:39:26PM +0200, Stefano Brivio wrote:On Tue, 20 Aug 2024 10:42:17 +1000 David Gibson <david(a)gibson.dropbear.id.au> wrote:Ok.On Mon, Aug 19, 2024 at 03:01:00PM +0200, Stefano Brivio wrote:I meant using ip(8) from the test script itself, but it doesn't actually make sense: # ip address change 2a01:4f8:222:904:c800:94ff:fe29:a8d/64 permanent dev eth0 Warning: permanent option is not mutable from userspace because (RFC 3549): IFA_F_PERMANENT For a permanent address set by the user. When this is not set, it means the address was dynamically created (e.g., by stateless autoconfiguration). So the address you used in your test _should_ have IFA_F_PERMANENT. The plot thickens. I just tried this, which confirms your hypothesis that bringing the link down is a different event: # ip addr add 2001:db8::1 dev dummy0 # ip link set dummy0 down # ip addr show dev dummy0 5: dummy0: <BROADCAST,NOARP> mtu 1280 qdisc noqueue state DOWN group default qlen 1000 link/ether 02:59:00:28:1b:5f brd ff:ff:ff:ff:ff:ff inet 1.2.3.1/24 scope global dummy0 valid_lft forever preferred_lft forever inet6 2001:db8::1/128 scope global valid_lft forever preferred_lft forever # ip link set dummy0 mtu 1279 # ip addr show dev dummy0 5: dummy0: <BROADCAST,NOARP> mtu 1279 qdisc noqueue state DOWN group default qlen 1000 link/ether 02:59:00:28:1b:5f brd ff:ff:ff:ff:ff:ff inet 1.2.3.1/24 scope global dummy0 valid_lft forever preferred_lft forever ...I just can't see that from the code.On Mon, 19 Aug 2024 19:52:49 +1000 David Gibson <david(a)gibson.dropbear.id.au> wrote:Huh. Not in the passt/VM case, though, which is where I actually encountered this.On Mon, Aug 19, 2024 at 11:27:49AM +0200, Stefano Brivio wrote: > On Mon, 19 Aug 2024 18:46:31 +1000 > David Gibson <david(a)gibson.dropbear.id.au> wrote: > > > On Fri, Aug 16, 2024 at 03:39:41PM +1000, David Gibson wrote: > > > Based on Stefano's recent patch for faster tests. > > > > > > Allow the user to specify which addresses are translated when used by > > > the guest, rather than always being the gateway address or nothing. > > > We also allow this remapping to go to the host's global address (more > > > precisely the address assigned to the guest) rather than just host > > > loopback. > > > > > > Suggestions for better names for the new options in patches 20 & 22 > > > are most welcome. > > > > > > Along the way to implementing that make many changes to clarify what > > > various addresses we track mean, fixing a number of small bugs as > > > well. > > > > > > NOTE: there is a bug in 21/22 which breaks some of the passt_tcp perf > > > tests. I haven't managed to figure out why it's causing the problem, > > > or even what the exact triggering conditions are (running the single > > > stalling iperf alone doesn't do it). Have to wrap up for today, so I > > > thought I'd get this out for review anyway. > > > > I've identified the bug here. IMO, it's a pre-existing problem that > > only works by accident at the moment. The immediate fix is pretty > > obvious, but it raises some broader questions > > > > The problem arises because of the MTU changes we make in order to test > > throughput with different packet sizes. Specifically we change the > > MTU to values < 1280, which implicitly disables IPv6 since it requires > > an MTU >= 1280. When we change the MTU back to a larger value IPv6 is > > re-enabled, but some configuration has been lost in the meantime. > > > > After the MTU is restored the guest reconfigures with NDP, but does > > not re-DHCPv6. That means the guest gets a SLAAC address in the right > > prefix but not the exact /128 address we've tried to assign to it. > > However, at least with the sequence of things we have in the tests, > > the guest never sends any packets with the new address, so passt > > doesn't update addr_seen. When the inbound connection comes we send > > it to the assigned address instead of the guest's actual address and > > the guest rejects it. > > I still have to take a closer look, but I'm fairly sure I hit a similar > issue while I was writing these tests originally. I pondered > reconfiguring the address via DHCPv6, or using the keep_addr_on_down > sysctl (net.ipv6.conf.<interface>.keep_addr_on_down), which was added > around that time. > > Then: > > > This "worked" previously, because before this patch, passt would > > translate the inbound connection to have source/dest as link-local > > addresses. > > ...I realised that this worked and forgot about the whole issue. > > > We *do* have a current addr_ll_seen because (a) it won't > > change if the guest doesn't change MAC and (b) when IPv6 is re-enabled > > the NDP traffic the guest generates will have link-local addresses > > that update addr_ll_seen. With this patch, and a global address for > > --map-host-loopback, we now need to send to addr_seen instead of > > addr_ll_seen, hence exposing the bug. > > > > In the short term, the obvious fix would be to re-run dhclient -6 in > > the guest after we twiddle MTU but before running IPv6 tests. > > I guess setting keep_addr_on_down (even for "all" interfaces) should > work as well. Sounds like it. I wasn't aware of that one. /me tests.. actually, no it doesn't work.. # sysctl -a | grep keep_addr_on_down net.ipv6.conf.all.keep_addr_on_down = 1 net.ipv6.conf.default.keep_addr_on_down = 1 net.ipv6.conf.dummy0.keep_addr_on_down = 1 net.ipv6.conf.lo.keep_addr_on_down = 0 # ip addr add 2001:db8::1 dev dummy0 # ip a 1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000 link/ether c2:02:f2:79:f9:94 brd ff:ff:ff:ff:ff:ff inet6 2001:db8::1/128 scope global valid_lft forever preferred_lft forever # ip link set dummy0 mtu 1200 # ip a 1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: dummy0: <BROADCAST,NOARP> mtu 1200 qdisc noop state DOWN group default qlen 1000 link/ether c2:02:f2:79:f9:94 brd ff:ff:ff:ff:ff:ff # ip link set dummy0 mtu 1500 # ip a 1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000 link/ether c2:02:f2:79:f9:94 brd ff:ff:ff:ff:ff:ff My guess is that IPv6 being deconfigured because of an unsuitable MTU is considered a different event from a mere "down".I guess it's because they're not IFA_F_PERMANENT, because addrconf_permanent_addr() has: case NETDEV_CHANGEMTU: /* if MTU under IPV6_MIN_MTU stop IPv6 on this interface. */ if (dev->mtu < IPV6_MIN_MTU) { addrconf_ifdown(dev, dev != net->loopback_dev); break; } but addrconf_ifdown() does: if (!keep_addr || !(ifa->flags & IFA_F_PERMANENT) || addr_is_local(&ifa->addr)) { hlist_del_init_rcu(&ifa->addr_lst); goto restart; } I'm not sure about the logic behind that. We could actually set those addresses as permanent once the DHCPv6 client configures them, if it's cleaner.Yes.. that's kind of my point....but then we should have multiple addresses anyway.Oh, I see. Assuming that at some point the DHCP client will re-run.> > This kind of opens a question about how hard we should try to > > accomodate guests which don't configure themselves how we told them. > > There's a notable distinction between guests temporarily diverging (in > different ways) and guests we don't configure at all. I'm not really sure what you're getting at here.In this case, it's not true that the guest doesn't configure itself in the way we requested -- it's just a temporary diversion from that configuration.Those are different cases that we can handle in different ways, I think. If it's a glitch that will only happen during testing, let's work around that. But if the guest really ignores DHCPv6 information, I think we should keep that working.Ok.> It's probably more important to ensure we use the right type of address "type" in what sense here?Global unicast instead of link-local.Eh, maybe. Unless us trying to make sense of a nonsense situation causes some unpredictable behaviour that breaks something else.> (security) rather than ensuring we somehow manage to deliver packets at > any time (minor glitch otherwise), also because the one you describe is > something we're unlikely to hit outside of tests. > > > Personally I'd be ok with saying that nothing works if the guest > > doesn't configure itself properly, thereby removing addr_seen and > > addr_ll_seen entirely. But I think, Stefano, you've been against that > > idea in the past. > > Yes, I still think we should support guests that don't use DHCPv6 or > NDP at all, Well, you still wouldn't *need* DHCPv6 or NDP, but you'd have to manually configure the interface in the guest to match the address you've configured with -a. Just like you'd expect to have to correctly configure your address on a real network.True, but if we make correctness as optional as possible, we'll be more compatible (less time spent by users fixing situations that don't necessarily need fixing, less time spent by developers to look into reports, no matter who's at fault).If we have separate interfaces for each guest, yes. But not if we have multiple guests behind a single tap because the initial guest sets up a bridge or routing. Then we have nothing but the address.> or where related exchanges fail for any reason. It improves > reliability and compatibility at a small cost. In this case, I think > it's a nice feature that we would resume communicating as soon as the > guest shows its global unicast address. Hm, maybe. I'm not entirely convinced the cost is so small long term. It's pretty badly incompatible with having multiple guests behind the same passt instance: such as the initial guest bridging or routing to nested guests.Why? We will need to hash the interface/guest index anyway, for outbound flows.By the way, I'm not sure we'll ever be able to support that kind of configuration.I don't see why not. It would require configuration so that it's clear what each inbound forward targets. But I don't see any inherent problem here, though there are a number of current implementation details which prevent it (addr_seen is one, replying to all arps is another).How does a guest set up a bridge and use passt at the same time?I'm not thinking of a bridge shared with the host, but a bridge (or routing) between nested guests or namespaces. This is essentially the "private switch with pasta uplink" case we've discussed occasionally before. It doesn't technically have to be nested guests - the guest could bridge between its uplink and a tunnel, but nested guests is the likely use case.Ah, ok. So again, assuming a static configuration of known guests, rather than a local bridge established by a guest at runtime.In the set of all addr_seen and addr, we would have at least a non-unique value. Or, practically speaking, we should refuse to set addr_seen if it matches addr for another guest.And for inbound flows, if a guest steals the address of another guest, we'll give priority to the normal 'addr' versions instead of the '_seen' ones, to decide how to direct traffic.I don't see how we'd know we're in this situation, so when to prioritise which address over the other.Right. I forgot the NS went out with :: as source. Snooping the NS that way again assumes that there's only one logical machine on the guest side. But since this is for addr_seen which fundamentally assumes that anyway, I guess it doesn't make anything worse.It avoids substantial effort and frustration for everybody involved though. The practical problem with lacking predictability is if it makes things harder to debug, I guess, which shouldn't be the case here.Because predictability is good, and working _most_ of the time is a failure of predictability.I'm actually not sure if encountering this bug makes me more or less in favour of addr_seen. On the one hand I think it highlights the flakiness of this approach; there are situations where we just won't know the right address.I don't understand this argument: indeed, there are such situations, and they are annoying. Why should we make them more common?It probably did, but we ignored that anyway because DAD is done by sending neighbour solicitations with an unspecified address as source, for example (the "change" here drops "nodad"): $ ./pasta --config-net -p dad.pcap Saving packet capture to dad.pcap # ip addr change dev enp9s0 fe80::3882:b5ff:fe01:e9a1/64 # tshark -r dad.pcap |grep Neigh Running as user "root" and group "root". This could be dangerous. 10 2.642467 :: → ff02::1:ff01:e9a1 ICMPv6 86 Neighbor Solicitation for fe80::3882:b5ff:fe01:e9a1 and in tap6_handler() we do: } else if (!IN6_IS_ADDR_UNSPECIFIED(saddr)){ c->ip6.addr_seen = *saddr; } ...then, in ndp(): if (IN6_IS_ADDR_UNSPECIFIED(saddr)) return 1; we could set addr_seen by looking at the *target* address of the neighbour solicitation when the source address is ::, but it's not implemented yet.Oh, good point. Hrm... then I'm unsure why the guest wasn't re-DADing its new address.On the other hand if shows a relatively plausible case where the guest won't get exactly the address we want it to (it uses NDP but not DHCPv6) Hrm... actually this also shows a potential danger in the recent patches to disable DAD in the guest. With DAD enabled, when the guest grabs a new address, we'd expect it to emit DAD messages, which would have the side effect of updating our addr_seen (although I'm pretty sure I hit this patch before the nodad patches were applied, so that doesn't seem to be foolproof).Well, but we do that for containers with --config-net only. In that case, the addresses we configure have infinite lifetime anyway.Yes, but I think snopping the NS from DAD is probably a better idea. -- 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/~dgibsonContainers running actual applications are noisy. I've only seen this kind of problem (addr_seen not set/matching) in particularly crafted test environments.Besides, I don't think we need to have addr_seen updated as quickly and correctly as possible just for the sake of it, we can also update it when we get any other neighbour solicitation because the guest is actually using the network. It's not meant to be perfect.If the guest is a pure server (a common case for containers AFAICT), then I don't know that we can expect NS messages for anything other than the default gateway, which is (typically) link-local and so won't help us to learn the new global address.Oh, I see. I think it makes sense, even though we'll set addr_seen a bit too early, but not enough to be a practical issue, I think.A case just like the one in the tests: the interface bounces, and we get NDP traffic on the link-local address, but nothing on the global address before an inbound connection.We could maybe update addr_seen when we send RA messages to the guest - assuming that it will use the same host part (low 64-bits) for both link-local and global addresses. Not sure if that's a widely safe assumption or not.I don't understand: what case are you trying to cover with this?