Re: [PATCH 1/2] netlink: Use regular request/response netlink socket for initial neighbour sync

On Thu, May 21, 2026 at 08:01:45PM +0200, Stefano Brivio wrote:

...instead of the one dedicated to the neighbour monitor, because, if neighbour notifications start coming in before or while we send the initial request to read out the neighbour tables, messages and sequence numbers will collide.

For example, if nl_neigh_sync() sends a RTM_GETNEIGH request with sequence 20, we expect a corresponding reply with sequence 20. But given that we already used the same socket to subscribe to notifications, and notifications don't correspond to any specific request we sent, we might now get a message with sequence 0.

Heh. Called it, kinda. Nice job tracking this down.

The collision between messages wouldn't actually matter, as we'll handle anyway any RTM_NEWNEIGH message in the same fashion, but we need to validate sequence numbers for robustness, and that will fail.

At the same time, we have to subscribe to neighbour notifications before calling nl_neigh_sync(), because we'll have a race condition otherwise, as we might miss neighbours that were added before the notifier is registered.

Use the regular nl_sock for nl_neigh_sync().

Drop the interface index from the request: we won't get any entry otherwise, because the Linux kernel (as of version 7.0) is unable to filter on it. Results are now filtered by interface index as we read them.

Passing along an interface index used to work when nl_neigh_sync() used the notifier socket, because NETLINK_GET_STRICT_CHK is not set on it, meaning that results weren't filtered at all (interface and IP version passed in the request were ignored altogether).

To reproduce the issue fixed here:

* detach a network and user namespace:

[terminal 0] $ unshare -rUn # echo $$ 1543307

* attach pasta to it:

[terminal 1] $ ./pasta -f --config-net 1543307 -I enp9s0

* enter that namespace from yet another terminal:

[terminal 2] $ nsenter --preserve-credentials -U -n -t 1543307

* start flooding the MAC address table of this namespace:

[terminal 1] # for i in $(seq 10 99); do for j in $(seq 10 99); do for k in $(seq 10 99); do ip ne add dev enp9s0 10.$i.$j.$k lladdr 00:11:22:$i:$j:$k; done; done; done

* and now start another instance of pasta in this namespace:

[terminal 2] # ./pasta -d --config-net

which will eventually result in pasta exiting with a message like:

0.0253: netlink: Unexpected sequence number (0 != 34)

Reported-by: Paul Holzinger Link: https://bugs.passt.top/show_bug.cgi?id=203 Fixes: 3c469013cfaa ("netlink: add subscription on changes in NDP/ARP table") Signed-off-by: Stefano Brivio

Reviewed-by: David Gibson

--- netlink.c | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/netlink.c b/netlink.c index c3c830e..0863734 100644 --- a/netlink.c +++ b/netlink.c @@ -1206,24 +1206,23 @@ static void nl_neigh_msg_read(const struct ctx *c, struct nlmsghdr *nh) * @proto: Protocol, AF_INET or AF_INET6 * @ifi: Interface index */ -static void nl_neigh_sync(const struct ctx *c, int proto, int ifi) +static void nl_neigh_sync(const struct ctx *c, int proto) { struct { struct nlmsghdr nlh; struct ndmsg ndm; } req = { - .ndm.ndm_family = proto, - .ndm.ndm_ifindex = ifi, + .ndm.ndm_family = proto, }; struct nlmsghdr *nh; char buf[NLBUFSIZ]; ssize_t status; uint32_t seq;

- 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) + seq = nl_send(nl_sock, &req, RTM_GETNEIGH, NLM_F_DUMP, sizeof(req)); + nl_foreach_oftype(nh, status, nl_sock, buf, seq, RTM_NEWNEIGH) nl_neigh_msg_read(c, nh); + if (status < 0) warn("netlink: RTM_GETNEIGH failed: %s", strerror_(-status)); } @@ -1298,8 +1297,8 @@ int nl_neigh_notify_init(const struct ctx *c) return -1; }

- nl_neigh_sync(c, AF_INET, c->ifi4); - nl_neigh_sync(c, AF_INET6, c->ifi6); + nl_neigh_sync(c, AF_INET); + nl_neigh_sync(c, AF_INET6);

return 0; } -- 2.43.0

-- 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