Date:	Tue, 7 Apr 2015 11:32:27 -0700
From:	Mahesh Bandewar <maheshb@...gle.com>
To:	Dan Williams <dcbw@...hat.com>
Cc:	linux-netdev <netdev@...r.kernel.org>, jbenc@...hat.com
Subject: Re: [PATCH] ipvlan: fix up broadcast MAC filtering for ARP and DHCP

On Mon, Apr 6, 2015 at 10:17 AM, Dan Williams <dcbw@...hat.com> wrote:
> On Thu, 2015-04-02 at 18:39 -0700, Mahesh Bandewar wrote:
>> On Thu, Apr 2, 2015 at 7:40 AM, Dan Williams <dcbw@...hat.com> wrote:
>> > On Wed, 2015-04-01 at 18:30 -0700, Mahesh Bandewar wrote:
>> >> On Wed, Apr 1, 2015 at 1:55 PM, Dan Williams <dcbw@...hat.com> wrote:
>> >> > On Wed, 2015-04-01 at 13:24 -0700, Mahesh Bandewar wrote:
>> >> >> On Mon, Mar 30, 2015 at 9:22 PM, Dan Williams <dcbw@...hat.com> wrote:
>> >> >> > The broadcast MAC is supposed to be allowed whenever the device
>> >> >> > has an IPv4 address, otherwise ARP requests get dropped on the
>> >> >> > floor.  If ndo_set_rx_mode (and thus
>> >> >> > ipvlan_set_multicast_mac_filter()) got called after the address
>> >> >> > was added, it would blow away the broadcast MAC address in
>> >> >> > mac_filters that was added at IPv4 address addition.
>> >> >> >
>> >> >> > Next, ipvlan currently fails DHCP addressing for two reasons:
>> >> >> >
>> >> >> I would prefer DHCPv4 instead of DHCP since it could also mean DHCPv6
>> >> >> which does not use broadcast.
>> >> >>
>> >> >> > 1) DHCP offers are typically unicast back to the device's MAC
>> >> >> > address, and at the IP layer have a destination IP address
>> >> >> > matching the new lease address.  In ipvlan unicast packets
>> >> >> > are checked against existing addresses assigned to the ipvlan
>> >> >> > interface, so clearly this fails hard because the ipvlan
>> >> >> > interface has no IP addresses yet.  Workaround: request
>> >> >> > that the server broadcast replies (-B for dhclient) which
>> >> >> > don't get checked against the address list.
>> >> >> >
>> >> >> > 2) Even when that's done, mac_filters only allows the
>> >> >> > broadcast MAC address when the interface has >= 1 IPv4
>> >> >> > addresses, so double-fail, and the incoming DHCP offer
>> >> >> > gets dropped on the floor again.
>> >> >> >
>> >> >> > To fix these issues, Watch for outgoing DHCP requests and
>> >> >> > enable the broadcast MAC address indefinitely when one is seen.
>> >> >> >
>> >> >> This approach will work but adds a performance drag for all UDP
>> >> >> traffic whether the link needs DHCPv4 or not (especially when it does
>> >> >> not need). Logic works well only when the link (always) needs DHCPv4.
>> >> >> When the link does not care about DHCPv4 the packet-inspection always
>> >> >> happens.
>> >> >
>> >> > If DHCPv4 is detected on a link, doesn't that mean the link needs the
>> >> > broadcast filter enabled?
>> >> Yes and it should stay enabled as it's going to have IPv4. Also once
>> >> broadcast bit is set there should not be any additional jugglery /
>> >> packet inspection which hurts performance.
>> >
>> > Yes, which is why my patch includes the check for dhcp4_seen and skips
>> > inspecting the packet.  I could modify it to skip the L3 header check
>> > too in that case.
>> >
>> dhcp4_seen works correctly when there is dhcpv4 happening, but
>> snooping continues if autoconf is not used.
>>
>> >> > If you start DHCPv4 on the link you obviously
>> >> > expect it to work, unless there's no DHCPv4 server available.  The patch
>> >> > should also reset the broadcast filter on close, so network management
>> >> > could simply start DHCPv4, watch it fail, close/open and set static IP.
>> >> >
>> >> > Alternatively we could have the broadcast filter on a 2 minute timer
>> >> > that starts when DHCPv4 is seen.
>> >> >
>> >> Well, but this would be error-prone. I don't remember but DHCP timeout
>> >> is not just 2 minutes.
>> >
>> > 2m is IIRC an RFC recommended timeout for initial DHCPv4 requests.  If
>> > you don't get a reply to your requests within 2m, you aren't likely to
>> > get one ever on this network and you can stop trying for a while.  You
>> > either get a reply in which case you get an IPv4 address and broadcast
>> > is always enabled, or you don't, in which case you don't really need
>> > broadcast.  Then when userspace next tries DHCP, the timer would allow
>> > broadcast for another 2 minutes.
>> >
>> >> >> Do we really need to special case DHCPv4? Do you see problems in
>> >> >> inverting the current logic of adding broadcast bit (i.e. from
>> >> >> enable-if-IPv4 to disable-if-IPv6-only)?
>> >> >
>> >> > The problem with this is dual-stack configuration.  Typically both
>> >> > DHCPv4 and SLAAC are started in this case, and often SLAAC will complete
>> >> > earlier.  So now the interface only has an IPv6 address but DHCPv4 is
>> >> > still ongoing; as I understand your proposal the filter would now block
>> >> > broadcast packets that DHCPv4 is listening for.
>> >> >
>> >> That was a proposal and we can tweak it if necessary. My primary
>> >> concern is that the solution should not make us pay per packet penalty
>> >> once (and even if not) the DHCPv4 handshake is complete, which is not
>> >> the case as it stands now.
>> >
>> > I'm not sure what you mean by "not make us pay a per packet penalty once
>> > DHCPv4 is complete".  Once an IPv4 address is assigned, the broadcast
>> > filter must allow broadcast packets.  Before an IPv4 address is
>> > assigned, the driver must watch for DHCPv4 and allow broadcast packets
>> > so that the DHCP reply isn't missed.
>> >
>> If the link is not using DHCPv4 to configure, the snooping continues
>> and that is per packet cost the needs to be paid. Probably this can be
>> avoided by setting dhcp4_seen = true in add_addr4 handler.
>>
>> > If DHCPv4 gets no response, then there are two options:
>> >
>> > a) down the interface (which resets the broadcast filter to block
>> > broadcast packets) and re-up it, and don't do DHCPv4 again
>> >
>> > b) have a timer in the driver that waits a period of time for an IPv4
>> > address after a DHCPv4 request, and blocks broadcast when the timer
>> > expires
>> >
>> Probably something similar where turn on the broadcast bit and wait
>> for the interface to get configured (2 min timer) and at the expiry
>> decide what is to be done about braodcast bit. If the addresses
>> configured are all IPv6, then eliminate it and if any of them are IPv4
>> then don't change it. In this case no special casing nor snooping is
>> required and this should work for dual-stack scenario as well.
>
> This does not work for the case where, after configuring, the DHCPv4
> address lease expires and the IPv4 address is removed, and then DHCPv4
> is started again.  Possibly because the DHCP server was gone for a short
> time.  The only way to handle that is to snoop again.
>
If the DHCPv4 lease is expired, then why DHCPv4-renew wont work (also
it's unicast)? Also if the address applied is  IPv4 then the broadcast
bit will stay. In this case I don't understand why this wont work.

If the link is stripped off of address(es), then it should again go
back into the config-mode where it would turn on the broadcast bit and
enable timer to get it configured. May be I'm missing something.


> Reaching for maximum performance is great, but if that is done by
> ignoring/breaking a whole class of normal use-cases, I don't think
> that's reasonable.  It's like saying "gee, I'd love UDP to be faster, so
> I'll remove anything TCP-related in the kernel".
>
What is the amount of traffic that a link will be sending for config
(DHCPv4 in this case)? I would guarantee you that it's so negligible
to the extent that it's would be non-existent. But all the suggestions
include snooping of every packet that goes through the link. However
I'm not suggesting that this is non-important or can stay broken. But
would rather have a solution that would not affect 99.9% of the
traffic (per packet penalty that I mentioned earlier).

> Also, I don't think the snooping is as bad for performance as you may
> think.  The only relevant issue here is L2 + IPv6-only,
Also IPv4 case when the link is not using autoconf.

> and in that case
> it's 4 extra compares (dhcp4_seen, ipv4cnt, lyr3h, and addr_type) for
> the external case.  How much is that really going to slow things down,
> versus breaking a huge part of IPv4 address configuration?
>
> Dan
>
>> > Dan
>> >
>> >> > Dan
>> >> >
>> >> >> > Signed-off-by: Dan Williams <dcbw@...hat.com>
>> >> >> > ---
>> >> >> > NOTE: this patch supercedes my previous two ipvlan patches:
>> >> >> > [PATCH 1/2] ipvlan: don't loose broadcast MAC when setting MAC filters
>> >> >> > [PATCH 2/2] ipvlan: always allow the broadcast MAC address
>> >> >> >
>> >> >> >  drivers/net/ipvlan/ipvlan.h      |  2 ++
>> >> >> >  drivers/net/ipvlan/ipvlan_core.c | 36 ++++++++++++++++++++++++++++++++++--
>> >> >> >  drivers/net/ipvlan/ipvlan_main.c | 12 +++++++++---
>> >> >> >  3 files changed, 45 insertions(+), 5 deletions(-)
>> >> >> >
>> >> >> > diff --git a/drivers/net/ipvlan/ipvlan.h b/drivers/net/ipvlan/ipvlan.h
>> >> >> > index 924ea98..7e0b8ff 100644
>> >> >> > --- a/drivers/net/ipvlan/ipvlan.h
>> >> >> > +++ b/drivers/net/ipvlan/ipvlan.h
>> >> >> > @@ -67,6 +67,7 @@ struct ipvl_dev {
>> >> >> >         struct list_head        addrs;
>> >> >> >         int                     ipv4cnt;
>> >> >> >         int                     ipv6cnt;
>> >> >> > +       bool                    dhcp4_seen;
>> >> >> >         struct ipvl_pcpu_stats  __percpu *pcpu_stats;
>> >> >> >         DECLARE_BITMAP(mac_filters, IPVLAN_MAC_FILTER_SIZE);
>> >> >> >         netdev_features_t       sfeatures;
>> >> >> > @@ -118,4 +119,5 @@ bool ipvlan_addr_busy(struct ipvl_dev *ipvlan, void *iaddr, bool is_v6);
>> >> >> >  struct ipvl_addr *ipvlan_ht_addr_lookup(const struct ipvl_port *port,
>> >> >> >                                         const void *iaddr, bool is_v6);
>> >> >> >  void ipvlan_ht_addr_del(struct ipvl_addr *addr, bool sync);
>> >> >> > +void ipvlan_set_broadcast_mac_filter(struct ipvl_dev *ipvlan, bool set);
>> >> >> >  #endif /* __IPVLAN_H */
>> >> >> > diff --git a/drivers/net/ipvlan/ipvlan_core.c b/drivers/net/ipvlan/ipvlan_core.c
>> >> >> > index 2a17500..6dd992b 100644
>> >> >> > --- a/drivers/net/ipvlan/ipvlan_core.c
>> >> >> > +++ b/drivers/net/ipvlan/ipvlan_core.c
>> >> >> > @@ -126,6 +126,12 @@ static void *ipvlan_get_L3_hdr(struct sk_buff *skb, int *type)
>> >> >> >                 lyr3h = arph;
>> >> >> >                 break;
>> >> >> >         }
>> >> >> > +       case htons(ETH_P_ALL): {
>> >> >> > +               /* Raw sockets */
>> >> >> > +               if (eth_hdr(skb)->h_proto != htons(ETH_P_IP))
>> >> >> > +                       break;
>> >> >> > +               /* Fall through */
>> >> >> > +       }
>> >> >> >         case htons(ETH_P_IP): {
>> >> >> >                 u32 pktlen;
>> >> >> >                 struct iphdr *ip4h;
>> >> >> > @@ -454,16 +460,42 @@ out:
>> >> >> >         return ipvlan_process_outbound(skb, ipvlan);
>> >> >> >  }
>> >> >> >
>> >> >> > +#define DHCP_PACKET_MIN_LEN 282
>> >> >> > +
>> >> >> > +static bool is_dhcp(struct sk_buff *skb)
>> >> >> > +{
>> >> >> > +       struct iphdr *ip4h = ip_hdr(skb);
>> >> >> > +       struct udphdr *udph;
>> >> >> > +
>> >> >> > +       if (skb->len < DHCP_PACKET_MIN_LEN || ip4h->protocol != IPPROTO_UDP
>> >> >> > +               || ip_is_fragment(ip4h))
>> >> >> > +               return false;
>> >> >> > +
>> >> >> > +       /* In the case of RAW sockets the transport header is not set by
>> >> >> > +        * the IP stack so we must set it ourselves.
>> >> >> > +        */
>> >> >> > +       if (skb->transport_header == skb->network_header)
>> >> >> > +               skb_set_transport_header(skb, ETH_HLEN + sizeof(struct iphdr));
>> >> >> > +       udph = udp_hdr(skb);
>> >> >> > +       return udph && udph->dest == htons(67) && udph->source == htons(68);
>> >> >> > +}
>> >> >> > +
>> >> >> >  static int ipvlan_xmit_mode_l2(struct sk_buff *skb, struct net_device *dev)
>> >> >> >  {
>> >> >> > -       const struct ipvl_dev *ipvlan = netdev_priv(dev);
>> >> >> > +       struct ipvl_dev *ipvlan = netdev_priv(dev);
>> >> >> >         struct ethhdr *eth = eth_hdr(skb);
>> >> >> >         struct ipvl_addr *addr;
>> >> >> >         void *lyr3h;
>> >> >> >         int addr_type;
>> >> >> >
>> >> >> > +       lyr3h = ipvlan_get_L3_hdr(skb, &addr_type);
>> >> >> > +       if (!ipvlan->dhcp4_seen && lyr3h && addr_type == IPVL_IPV4
>> >> >> > +           && is_dhcp(skb)) {
>> >> >> > +               ipvlan->dhcp4_seen = true;
>> >> >> > +               ipvlan_set_broadcast_mac_filter(ipvlan, true);
>> >> >> > +       }
>> >> >> > +
>> >> >> >         if (ether_addr_equal(eth->h_dest, eth->h_source)) {
>> >> >> > -               lyr3h = ipvlan_get_L3_hdr(skb, &addr_type);
>> >> >> >                 if (lyr3h) {
>> >> >> >                         addr = ipvlan_addr_lookup(ipvlan->port, lyr3h, addr_type, true);
>> >> >> >                         if (addr)
>> >> >> > diff --git a/drivers/net/ipvlan/ipvlan_main.c b/drivers/net/ipvlan/ipvlan_main.c
>> >> >> > index 4f4099d..a497fb9 100644
>> >> >> > --- a/drivers/net/ipvlan/ipvlan_main.c
>> >> >> > +++ b/drivers/net/ipvlan/ipvlan_main.c
>> >> >> > @@ -167,6 +167,8 @@ static int ipvlan_stop(struct net_device *dev)
>> >> >> >
>> >> >> >         dev_uc_del(phy_dev, phy_dev->dev_addr);
>> >> >> >
>> >> >> > +       ipvlan->dhcp4_seen = false;
>> >> >> > +
>> >> >> >         if (ipvlan->ipv6cnt > 0 || ipvlan->ipv4cnt > 0) {
>> >> >> >                 list_for_each_entry(addr, &ipvlan->addrs, anode)
>> >> >> >                         ipvlan_ht_addr_del(addr, !dev->dismantle);
>> >> >> > @@ -214,7 +216,7 @@ static void ipvlan_change_rx_flags(struct net_device *dev, int change)
>> >> >> >                 dev_set_allmulti(phy_dev, dev->flags & IFF_ALLMULTI? 1 : -1);
>> >> >> >  }
>> >> >> >
>> >> >> > -static void ipvlan_set_broadcast_mac_filter(struct ipvl_dev *ipvlan, bool set)
>> >> >> > +void ipvlan_set_broadcast_mac_filter(struct ipvl_dev *ipvlan, bool set)
>> >> >> >  {
>> >> >> >         struct net_device *dev = ipvlan->dev;
>> >> >> >         unsigned int hashbit = ipvlan_mac_hash(dev->broadcast);
>> >> >> > @@ -239,6 +241,9 @@ static void ipvlan_set_multicast_mac_filter(struct net_device *dev)
>> >> >> >                 netdev_for_each_mc_addr(ha, dev)
>> >> >> >                         __set_bit(ipvlan_mac_hash(ha->addr), mc_filters);
>> >> >> >
>> >> >> > +               if (ipvlan->ipv4cnt || ipvlan->dhcp4_seen)
>> >> >> > +                       __set_bit(ipvlan_mac_hash(dev->broadcast), mc_filters);
>> >> >> > +
>> >> >> >                 bitmap_copy(ipvlan->mac_filters, mc_filters,
>> >> >> >                             IPVLAN_MAC_FILTER_SIZE);
>> >> >> >         }
>> >> >> > @@ -467,6 +472,7 @@ static int ipvlan_link_new(struct net *src_net, struct net_device *dev,
>> >> >> >         INIT_LIST_HEAD(&ipvlan->addrs);
>> >> >> >         ipvlan->ipv4cnt = 0;
>> >> >> >         ipvlan->ipv6cnt = 0;
>> >> >> > +       ipvlan->dhcp4_seen = false;
>> >> >> >
>> >> >> >         /* TODO Probably put random address here to be presented to the
>> >> >> >          * world but keep using the physical-dev address for the outgoing
>> >> >> > @@ -708,8 +714,8 @@ static void ipvlan_del_addr4(struct ipvl_dev *ipvlan, struct in_addr *ip4_addr)
>> >> >> >         list_del_rcu(&addr->anode);
>> >> >> >         ipvlan->ipv4cnt--;
>> >> >> >         WARN_ON(ipvlan->ipv4cnt < 0);
>> >> >> > -       if (!ipvlan->ipv4cnt)
>> >> >> > -           ipvlan_set_broadcast_mac_filter(ipvlan, false);
>> >> >> > +       if (!ipvlan->ipv4cnt && !ipvlan->dhcp4_seen)
>> >> >> > +               ipvlan_set_broadcast_mac_filter(ipvlan, false);
>> >> >> >         kfree_rcu(addr, rcu);
>> >> >> >
>> >> >> >         return;
>> >> >> > --
>> >> >> > 2.1.0
>> >> >> >
>> >> >> >
>> >> >> --
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>> >> >
>> >> >
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