nft: Check base-chain compatibility when adding to cache

With introduction of dedicated base-chain slots, a selection process was
established as no longer all base-chains ended in the same chain list
for later searching/checking but only the first one found for each hook
matching criteria is kept and the rest discarded.

A side-effect of the above is that table compatibility checking started
to omit consecutive base-chains, making iptables-nft less restrictive as
long as the expected base-chains were returned first from kernel when
populating the cache.

Make behaviour consistent and warn users about the possibly disturbing
chains found by:

* Run all base-chain checks from nft_is_chain_compatible() before
  allowing a base-chain to occupy its slot.
* If an unfit base-chain was found (and discarded), flag the table's
  cache as tainted and warn about it if the remaining ruleset is
  otherwise compatible.

Since base-chains that remain in cache would pass
nft_is_chain_compatible() checking, remove that and reduce it to rule
inspection.

Signed-off-by: Phil Sutter <phil@nwl.cc>

1 files changed, 3 insertions, 0 deletions

diff --git a/iptables/xtables-save.c b/iptables/xtables-save.c
index 98cd0ed3..f794e3ff 100644
--- a/iptables/xtables-save.c
+++ b/iptables/xtables-save.c
@@ -78,6 +78,9 @@ __do_output(struct nft_handle *h, const char *tablename, void *data)
 		printf("# Table `%s' is incompatible, use 'nft' tool.\n",
 		       tablename);
 		return 0;
+	} else if (nft_is_table_tainted(h, tablename)) {
+		printf("# Table `%s' contains incompatible base-chains, use 'nft' tool to list them.\n",
+		       tablename);
 	}
 
 	now = time(NULL);