packet: nlmon: virtual netlink monitoring device for packet sockets

Currently, there is no good possibility to debug netlink traffic that
is being exchanged between kernel and user space. Therefore, this patch
implements a netlink virtual device, so that netlink messages will be
made visible to PF_PACKET sockets. Once there was an approach with a
similar idea [1], but it got forgotten somehow.

I think it makes most sense to accept the "overhead" of an extra netlink
net device over implementing the same functionality from PF_PACKET
sockets once again into netlink sockets. We have BPF filters that can
already be easily applied which even have netlink extensions, we have
RX_RING zero-copy between kernel- and user space that can be reused,
and much more features. So instead of re-implementing all of this, we
simply pass the skb to a given PF_PACKET socket for further analysis.

Another nice benefit that comes from that is that no code needs to be
changed in user space packet analyzers (maybe adding a dissector, but
not more), thus out of the box, we can already capture pcap files of
netlink traffic to debug/troubleshoot netlink problems.

Also thanks goes to Thomas Graf, Flavio Leitner, Jesper Dangaard Brouer.

 [1] http://marc.info/?l=linux-netdev&m=113813401516110

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 00aba08f01a96790f16a17c7bf3b4ee61bffc1d2..b45b240889f5049db48f0d90e4d7146b57985b8e 100644 (file)
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -240,6 +240,16 @@ config VIRTIO_NET
          This is the virtual network driver for virtio.  It can be used with
          lguest or QEMU based VMMs (like KVM or Xen).  Say Y or M.
 
+config NLMON
+       tristate "Virtual netlink monitoring device"
+       ---help---
+         This option enables a monitoring net device for netlink skbs. The
+         purpose of this is to analyze netlink messages with packet sockets.
+         Thus applications like tcpdump will be able to see local netlink
+         messages if they tap into the netlink device, record pcaps for further
+         diagnostics, etc. This is mostly intended for developers or support
+         to debug netlink issues. If unsure, say N.
+
 endif # NET_CORE
 
 config SUNGEM_PHY

diff --git a/drivers/net/Makefile b/drivers/net/Makefile
index ef3d090efedfdbababcd90487095c7c1e55b1d17..3fef8a81c0f69ae990579a41d9d6fd9c44aa58cf 100644 (file)
--- a/drivers/net/Makefile
+++ b/drivers/net/Makefile
@@ -22,6 +22,7 @@ obj-$(CONFIG_TUN) += tun.o
 obj-$(CONFIG_VETH) += veth.o
 obj-$(CONFIG_VIRTIO_NET) += virtio_net.o
 obj-$(CONFIG_VXLAN) += vxlan.o
+obj-$(CONFIG_NLMON) += nlmon.o
 
 #
 # Networking Drivers

diff --git a/drivers/net/nlmon.c b/drivers/net/nlmon.c
new file mode 100644 (file)
index 0000000..dc364be
--- /dev/null
+++ b/drivers/net/nlmon.c
@@ -0,0 +1,170 @@
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/netdevice.h>
+#include <linux/netlink.h>
+#include <net/net_namespace.h>
+#include <linux/if_arp.h>
+
+struct pcpu_lstats {
+       u64 packets;
+       u64 bytes;
+       struct u64_stats_sync syncp;
+};
+
+static netdev_tx_t nlmon_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+       int len = skb->len;
+       struct pcpu_lstats *stats = this_cpu_ptr(dev->lstats);
+
+       u64_stats_update_begin(&stats->syncp);
+       stats->bytes += len;
+       stats->packets++;
+       u64_stats_update_end(&stats->syncp);
+
+       dev_kfree_skb(skb);
+
+       return NETDEV_TX_OK;
+}
+
+static int nlmon_is_valid_mtu(int new_mtu)
+{
+       return new_mtu >= sizeof(struct nlmsghdr) && new_mtu <= INT_MAX;
+}
+
+static int nlmon_change_mtu(struct net_device *dev, int new_mtu)
+{
+       if (!nlmon_is_valid_mtu(new_mtu))
+               return -EINVAL;
+
+       dev->mtu = new_mtu;
+       return 0;
+}
+
+static int nlmon_dev_init(struct net_device *dev)
+{
+       dev->lstats = alloc_percpu(struct pcpu_lstats);
+
+       return dev->lstats == NULL ? -ENOMEM : 0;
+}
+
+static void nlmon_dev_uninit(struct net_device *dev)
+{
+       free_percpu(dev->lstats);
+}
+
+static struct netlink_tap nlmon_tap;
+
+static int nlmon_open(struct net_device *dev)
+{
+       return netlink_add_tap(&nlmon_tap);
+}
+
+static int nlmon_close(struct net_device *dev)
+{
+       return netlink_remove_tap(&nlmon_tap);
+}
+
+static struct rtnl_link_stats64 *
+nlmon_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats)
+{
+       int i;
+       u64 bytes = 0, packets = 0;
+
+       for_each_possible_cpu(i) {
+               const struct pcpu_lstats *nl_stats;
+               u64 tbytes, tpackets;
+               unsigned int start;
+
+               nl_stats = per_cpu_ptr(dev->lstats, i);
+
+               do {
+                       start = u64_stats_fetch_begin_bh(&nl_stats->syncp);
+                       tbytes = nl_stats->bytes;
+                       tpackets = nl_stats->packets;
+               } while (u64_stats_fetch_retry_bh(&nl_stats->syncp, start));
+
+               packets += tpackets;
+               bytes += tbytes;
+       }
+
+       stats->rx_packets = packets;
+       stats->tx_packets = 0;
+
+       stats->rx_bytes = bytes;
+       stats->tx_bytes = 0;
+
+       return stats;
+}
+
+static u32 always_on(struct net_device *dev)
+{
+       return 1;
+}
+
+static const struct ethtool_ops nlmon_ethtool_ops = {
+       .get_link = always_on,
+};
+
+static const struct net_device_ops nlmon_ops = {
+       .ndo_init = nlmon_dev_init,
+       .ndo_uninit = nlmon_dev_uninit,
+       .ndo_open = nlmon_open,
+       .ndo_stop = nlmon_close,
+       .ndo_start_xmit = nlmon_xmit,
+       .ndo_get_stats64 = nlmon_get_stats64,
+       .ndo_change_mtu = nlmon_change_mtu,
+};
+
+static struct netlink_tap nlmon_tap __read_mostly = {
+       .module = THIS_MODULE,
+};
+
+static void nlmon_setup(struct net_device *dev)
+{
+       dev->type = ARPHRD_NETLINK;
+       dev->tx_queue_len = 0;
+
+       dev->netdev_ops = &nlmon_ops;
+       dev->ethtool_ops = &nlmon_ethtool_ops;
+       dev->destructor = free_netdev;
+
+       dev->features = NETIF_F_FRAGLIST | NETIF_F_HIGHDMA;
+       dev->flags = IFF_NOARP;
+
+       /* That's rather a softlimit here, which, of course,
+        * can be altered. Not a real MTU, but what is to be
+        * expected in most cases.
+        */
+       dev->mtu = NLMSG_GOODSIZE;
+}
+
+static __init int nlmon_register(void)
+{
+       int err;
+       struct net_device *nldev;
+
+       nldev = nlmon_tap.dev = alloc_netdev(0, "netlink", nlmon_setup);
+       if (unlikely(nldev == NULL))
+               return -ENOMEM;
+
+       err = register_netdev(nldev);
+       if (unlikely(err))
+               free_netdev(nldev);
+
+       return err;
+}
+
+static __exit void nlmon_unregister(void)
+{
+       struct net_device *nldev = nlmon_tap.dev;
+
+       unregister_netdev(nldev);
+}
+
+module_init(nlmon_register);
+module_exit(nlmon_unregister);
+
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
+MODULE_AUTHOR("Mathieu Geli <geli@enseirb.fr>");
+MODULE_DESCRIPTION("Netlink monitoring device");