Bug 2443276

Backport request for fix currently being applied to older Ubuntu releases:

Ref: https://bugs.launchpad.net/ubuntu/+source/nftables/+bug/2142552

From that bug:

[ Impact ]

 * When one uses newer & older nftables on the same host (for example from new and old containers) the old nftables can start crashing with segfaults.

 * This is very pronounced when deploying kubernetes and for example deploying istio or calico with containers that have newer nftables (newer than 1.1.1) and subsequently older nftables (on the host or another container) are used.

https://git.netfilter.org/nftables/commit/?id=be737a1986bfee0ddea4bee7863dca0123a2bcbc

Whenever a new version adds udata support to an expression, then old
versions of nft will crash when trying to list such a ruleset generated
by a more recent version of nftables.

Fix this by falling back to 'type' format.

Fixes: 6e48df5329ea ('src: add "typeof" build/parse/print support')
Signed-off-by: Florian Westphal <email address hidden>
Reviewed-by: Pablo Neira Ayuso <email address hidden>

Also see:
- https://github.com/istio/istio/issues/58492



Reproducible: Always

Steps to Reproduce:
1. Add a set using a version of nftables > 1.1.3 (eg in a container in the host network namespace
# podman run --rm -it --privileged --network host quay.io/fedora/fedora:43 /bin/bash
{container}# dnf install -y nftables
{container}# nft add table test
{container}# nft 'add set test dummy { type ipv4_addr; }'

2. Attempt to list the ruleset on the F42 host: 
# nft list ruleset


Actual Results:
Segmentation fault (core dumped)

Expected Results:
List of current ruleset

Additional Information:
Newer versions of nftables add sets/chains that have udata added, and nftables 1.1.1 on F42 currently lacks the patch to correctly handle the udata, and crashes.

The fix is linked above, but the patch is as simple as:

--- a/src/netlink.c	2024-09-26 07:17:41.004675285 -0700
+++ b/src/netlink.c	2026-02-05 11:39:31.654321471 -0800
@@ -913,7 +913,7 @@

 	etype = nftnl_udata_get_u32(ud[NFTNL_UDATA_SET_TYPEOF_EXPR]);
 	ops = expr_ops_by_type_u32(etype);
-	if (!ops)
+	if (!ops || !ops->parse_udata)
 		return NULL;

 	expr = ops->parse_udata(ud[NFTNL_UDATA_SET_TYPEOF_DATA]);
 
Without the patch, any container with a reasonably new version of nftables (including F43+) in the host namespace can lead to nftables being unavailable on the host.

NOTE: this bug probably affects F41 as well... The bug is fixed in F43, but F43+ will also trigger the problem in F42 or below in the same network namespace.