Re: [PATCH v11 1/9] netlink: add subsciption on changes in NDP/ARP table

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

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On Sat, Sep 27, 2025 at 03:25:14PM -0400, Jon Maloy wrote:
> The solution to bug https://bugs.passt.top/show_bug.cgi?id=120
> requires the ability to translate from an IP address to its
> corresponding MAC address in cases where those are present in
> the ARP or NDP tables.
> 
> To keep track of the contents of these tables we add a netlink
> based neighbour subscription feature.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Although there are some suggestions for polish below.

> 
> ---
> v3: - Added an attribute contianing NDA_DST to sent message, so
>       that we let the kernel do the filtering of the IP address
>       and return only one entry.
>     - Added interface index to the call signature. Since the only
>       interface we know is the template interface, this limits
>       the number of hosts that will be seen as 'network segment
>       local' from a PASST viewpoint.
> v4: - Made loop independent of attribute order.
>     - Ignoring L2 addresses which are not of size ETH_ALEN.
> v5: - Changed return value of new function, so caller can know if
>       a MAC address really was found.
> v6: - Removed warning printout which had ended up in the wrong
>       commit.
> v8: - Changed to neighbour event subscription model
>     - netlink: arp/ndp table subscription
> v10:- Updated according to David's latest comments on v8
>     - Added functionaly where we initially read current
>       state of ARP/NDP tables
> ---
>  epoll_type.h |   2 +
>  netlink.c    | 180 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  netlink.h    |   4 ++
>  passt.c      |   8 +++
>  4 files changed, 194 insertions(+)
> 
> diff --git a/epoll_type.h b/epoll_type.h
> index 12ac59b..a90ffb6 100644
> --- a/epoll_type.h
> +++ b/epoll_type.h
> @@ -44,6 +44,8 @@ enum epoll_type {
>  	EPOLL_TYPE_REPAIR_LISTEN,
>  	/* TCP_REPAIR helper socket */
>  	EPOLL_TYPE_REPAIR,
> +	/* Netlink neighbour subscription socket */
> +	EPOLL_TYPE_NL_NEIGH,
>  
>  	EPOLL_NUM_TYPES,
>  };
> diff --git a/netlink.c b/netlink.c
> index c436780..0dc4e9d 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -26,6 +26,7 @@
>  #include <arpa/inet.h>
>  #include <netinet/in.h>
>  #include <netinet/if_ether.h>
> +#include <net/if_arp.h>
>  
>  #include <linux/netlink.h>
>  #include <linux/rtnetlink.h>
> @@ -40,6 +41,10 @@
>  #define RTNH_NEXT_AND_DEC(rtnh, attrlen)				\
>  	((attrlen) -= RTNH_ALIGN((rtnh)->rtnh_len), RTNH_NEXT(rtnh))
>  
> +/* Convenience macro borrowed from kernel */
> +#define NUD_VALID (NUD_PERMANENT | NUD_NOARP |		\
> +		   NUD_REACHABLE | NUD_PROBE | NUD_STALE)
> +
>  /* Netlink expects a buffer of at least 8kiB or the system page size,
>   * whichever is larger.  32kiB is recommended for more efficient.
>   * Since the largest page size on any remotely common Linux setup is
> @@ -53,6 +58,7 @@
>  int nl_sock	= -1;
>  int nl_sock_ns	= -1;
>  static int nl_seq = 1;
> +static int nl_sock_neigh = -1;
>  
>  /**
>   * nl_sock_init_do() - Set up netlink sockets in init or target namespace
> @@ -1103,3 +1109,177 @@ int nl_link_set_flags(int s, unsigned int ifi,
>  
>  	return nl_do(s, &req, RTM_NEWLINK, 0, sizeof(req));
>  }
> +
> +/**
> + * nl_neigh_msg_read() - Interpret a neigbour state message from netlink
> + *
> + */
> +static void nl_neigh_msg_read(const struct ctx *c, struct nlmsghdr *nh)
> +{
> +	struct ndmsg *ndm = NLMSG_DATA(nh);
> +	struct rtattr *rta = (struct rtattr *)(ndm + 1);
> +	size_t na = NLMSG_PAYLOAD(nh, sizeof(*ndm));
> +	char ip_str[INET6_ADDRSTRLEN];
> +	char mac_str[ETH_ADDRSTRLEN];
> +	const uint8_t *lladdr = NULL;
> +	const void *dst = NULL;
> +	size_t lladdr_len = 0;
> +	uint8_t mac[ETH_ALEN];
> +	union inany_addr addr;
> +	size_t dstlen = 0;
> +
> +	if (nh->nlmsg_type == NLMSG_DONE)
> +		return;
> +
> +	if (nh->nlmsg_type == NLMSG_ERROR) {
> +		warn("NLMSG type error");

This message could be a bit more helpful, e.g. saying that it's on the
neighbour watching socket.  There's also an errno in the message (see
nl_status()).

> +		return;
> +	}
> +
> +	if (nh->nlmsg_type != RTM_NEWNEIGH &&
> +	    nh->nlmsg_type != RTM_DELNEIGH)
> +		return;
> +
> +	for (; RTA_OK(rta, na); rta = RTA_NEXT(rta, na)) {
> +		switch (rta->rta_type) {
> +		case NDA_DST:
> +			dst = RTA_DATA(rta);
> +			dstlen = RTA_PAYLOAD(rta);
> +			break;
> +		case NDA_LLADDR:
> +			lladdr = RTA_DATA(rta);
> +			lladdr_len = RTA_PAYLOAD(rta);
> +			break;
> +		default:
> +			break;
> +		}
> +	}
> +
> +	if (!dst)
> +		return;
> +
> +	if (!lladdr || lladdr_len != ETH_ALEN)
> +		return;
> +
> +	if (ndm->ndm_type != ARPHRD_ETHER)
> +		return;
> +
> +	memcpy(mac, lladdr, ETH_ALEN);
> +	eth_ntop(mac, mac_str, sizeof(mac_str));

It would be nice to avoid the stringification if we're not printing
the trace message, but that is kind of awkward to do.

> +	if (ndm->ndm_family == AF_INET &&
> +	    (dstlen != sizeof(struct in_addr) ||
> +	     ndm->ndm_ifindex != c->ifi4))
> +		return;
> +
> +	if (ndm->ndm_family == AF_INET6 &&
> +	    (dstlen != sizeof(struct in6_addr) ||
> +	     ndm->ndm_ifindex != c->ifi6))
> +		return;

Hmm.  These are basically correct, but make me uncomfortable, because
they're checking for two very different conditions.  ifindex != ifi is
basically an expected thing; we get a message for an interface we
don't care about, ignore it and carry on.

The other condition - addr length doesn't match addr family means the
kernel has done something really unexpected.  There's not much we can
do but ignore it (maybe log) and carry on, but lumping it together
with the expected cases seems misleading to me.

> +
> +	if (ndm->ndm_family == AF_INET)
> +		addr = inany_from_v4(*(struct in_addr *) dst);
> +	else if (ndm->ndm_family == AF_INET6)
> +		addr.a6 = *(struct in6_addr *) dst;
> +	else
> +		return;

You can use inany_from_af(&addr, ndm->ndm_family, dst)

You will need to exclude non-IP addresses beforehand, though.

> +
> +	inet_ntop(ndm->ndm_family, dst, ip_str, sizeof(ip_str));
> +
> +	if (nh->nlmsg_type == RTM_NEWNEIGH &&
> +	    ndm->ndm_state & NUD_VALID) {
> +		trace("neigh table update: %s / %s", ip_str, mac_str);
> +	} else {
> +		trace("neigh table delete: %s / %s", ip_str, mac_str);
> +	}
> +}
> +
> +/**
> + * nl_neigh_subscr_sync() - Read current contents ARP/NDP tables
> + *
> + */
> +static void nl_neigh_subscr_sync(const struct ctx *c, int proto, int ifi)
> +{
> +	struct {
> +		struct nlmsghdr nlh;
> +		struct ndmsg ndm;
> +	} req = { 0 };
> +	struct nlmsghdr *nh;
> +	char buf[NLBUFSIZ];
> +	ssize_t status;
> +	uint32_t seq;
> +
> +	req.ndm.ndm_family = proto;
> +	req.ndm.ndm_ifindex = ifi;
> +	seq = nl_send(nl_sock_neigh, &req, RTM_GETNEIGH, NLM_F_DUMP, sizeof(req));
> +	nl_foreach_oftype(nh, status, nl_sock_neigh, buf, seq, RTM_NEWNEIGH)
> +		nl_neigh_msg_read(c, nh);

Should probably check status afterwards, and log an error if necessary.

> +}
> +
> +/**
> + * nl_neigh_subscr_handler() - Non-blocking drain of pending neighbour updates
> + * @c:	       Execution context
> + */
> +void nl_neigh_subscr_handler(struct ctx *c)
> +{
> +	struct nlmsghdr *nh;
> +	char buf[NLBUFSIZ];
> +	ssize_t n;
> +
> +	for (;;) {
> +		n = recv(nl_sock_neigh, buf, sizeof(buf), MSG_DONTWAIT);
> +		if (n < 0) {
> +			if (errno != EAGAIN)
> +				warn_perror("Failed to read from netlink sock");
> +			return;
> +		}
> +		nh = (struct nlmsghdr *)buf;
> +		for (; NLMSG_OK(nh, n); nh = NLMSG_NEXT(nh, n))
> +			nl_neigh_msg_read(c, nh);
> +	}
> +}
> +
> +/**
> + * nl_neigh_subscr_init() - Open netlink sock and subscribe to neighbour events
> + *
> + * Return: 0 on success, -1 on failure
> + */
> +int nl_neigh_subscr_init(struct ctx *c)
> +{
> +	union epoll_ref ref = {
> +		.type = EPOLL_TYPE_NL_NEIGH
> +	};
> +	struct epoll_event ev = {
> +		.events = EPOLLIN
> +	};
> +	struct sockaddr_nl addr = {
> +		.nl_family = AF_NETLINK,
> +		.nl_groups = RTMGRP_NEIGH,
> +	};
> +
> +	if (nl_sock_neigh >= 0)
> +		return 0;
> +
> +	nl_sock_neigh = socket(AF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, NETLINK_ROUTE);
> +	if (nl_sock_neigh < 0)
> +		return -1;
> +
> +	if (bind(nl_sock_neigh, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
> +		close(nl_sock_neigh);
> +		nl_sock_neigh = -1;
> +		return -1;
> +	}
> +
> +	ref.fd = nl_sock_neigh;
> +	ev.data.u64 = ref.u64;
> +	if (epoll_ctl(c->epollfd, EPOLL_CTL_ADD, nl_sock_neigh, &ev) == -1) {
> +		close(nl_sock_neigh);
> +		nl_sock_neigh = -1;
> +		return -1;
> +	}
> +
> +	nl_neigh_subscr_sync(c, AF_INET, c->ifi4);
> +	nl_neigh_subscr_sync(c, AF_INET6, c->ifi6);
> +
> +	return 0;
> +}
> diff --git a/netlink.h b/netlink.h
> index b51e99c..a7d3506 100644
> --- a/netlink.h
> +++ b/netlink.h
> @@ -17,6 +17,8 @@ int nl_route_dup(int s_src, unsigned int ifi_src,
>  		 int s_dst, unsigned int ifi_dst, sa_family_t af);
>  int nl_addr_get(int s, unsigned int ifi, sa_family_t af,
>  		void *addr, int *prefix_len, void *addr_l);
> +bool nl_neigh_mac_get(int s, const union inany_addr *addr, int ifi,
> +		      unsigned char *mac);
>  int nl_addr_set(int s, unsigned int ifi, sa_family_t af,
>  		const void *addr, int prefix_len);
>  int nl_addr_get_ll(int s, unsigned int ifi, struct in6_addr *addr);
> @@ -28,5 +30,7 @@ int nl_link_set_mac(int s, unsigned int ifi, const void *mac);
>  int nl_link_set_mtu(int s, unsigned int ifi, int mtu);
>  int nl_link_set_flags(int s, unsigned int ifi,
>  		      unsigned int set, unsigned int change);
> +int nl_neigh_subscr_init(struct ctx *c);
> +void nl_neigh_subscr_handler(struct ctx *c);
>  
>  #endif /* NETLINK_H */
> diff --git a/passt.c b/passt.c
> index 31fbb75..e20bbad 100644
> --- a/passt.c
> +++ b/passt.c
> @@ -53,6 +53,7 @@
>  #include "vu_common.h"
>  #include "migrate.h"
>  #include "repair.h"
> +#include "netlink.h"
>  
>  #define EPOLL_EVENTS		8
>  
> @@ -79,6 +80,7 @@ char *epoll_type_str[] = {
>  	[EPOLL_TYPE_VHOST_KICK]		= "vhost-user kick socket",
>  	[EPOLL_TYPE_REPAIR_LISTEN]	= "TCP_REPAIR helper listening socket",
>  	[EPOLL_TYPE_REPAIR]		= "TCP_REPAIR helper socket",
> +	[EPOLL_TYPE_NL_NEIGH]		= "netlink neighbour subscription socket",
>  };
>  static_assert(ARRAY_SIZE(epoll_type_str) == EPOLL_NUM_TYPES,
>  	      "epoll_type_str[] doesn't match enum epoll_type");
> @@ -322,6 +324,9 @@ int main(int argc, char **argv)
>  
>  	pcap_init(&c);
>  
> +	if (nl_neigh_subscr_init(&c) < 0)
> +		warn("Failed to subscribe to RTMGRP_NEIGH");

I'd be inclined to put the error message inside
nl_neigh_subscr_init(), where it can be a bit more detailed as to what
went wrong where.

> +
>  	if (!c.foreground) {
>  		if ((devnull_fd = open("/dev/null", O_RDWR | O_CLOEXEC)) < 0)
>  			die_perror("Failed to open /dev/null");
> @@ -414,6 +419,9 @@ loop:
>  		case EPOLL_TYPE_REPAIR:
>  			repair_handler(&c, eventmask);
>  			break;
> +		case EPOLL_TYPE_NL_NEIGH:
> +			nl_neigh_subscr_handler(&c);
> +			break;
>  		default:
>  			/* Can't happen */
>  			ASSERT(0);
> -- 
> 2.50.1
> 

-- 
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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