Improve error messages for unsupported extensions

If a given extension was not supported by the kernel, iptables would
print a rather confusing error message if extension parameters were
given:

| # rm /lib/modules/$(uname -r)/kernel/net/netfilter/xt_LOG.ko
| # iptables -A FORWARD -j LOG --log-prefix foo
| iptables v1.8.7 (legacy): unknown option "--log-prefix"

Avoid this by pretending extension revision 0 is always supported. It is
the same hack as used to successfully print extension help texts as
unprivileged user, extended to all error codes to serve privileged ones
as well.

In addition, print a warning if kernel rejected revision 0 and it's not
a permissions problem. This helps users find out which extension in a
rule the kernel didn't like.

Finally, the above commands result in these messages:

| Warning: Extension LOG revision 0 not supported, missing kernel module?
| iptables: No chain/target/match by that name.

Or, for iptables-nft:

| Warning: Extension LOG revision 0 not supported, missing kernel module?
| iptables v1.8.7 (nf_tables):  RULE_APPEND failed (No such file or directory): rule in chain FORWARD

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

1 files changed, 6 insertions, 1 deletions

diff --git a/libxtables/xtables.c b/libxtables/xtables.c
index b34d62ac..87424d04 100644
--- a/libxtables/xtables.c
+++ b/libxtables/xtables.c
@@ -958,7 +958,12 @@ int xtables_compatible_revision(const char *name, uint8_t revision, int opt)
 		/* Definitely don't support this? */
 		if (errno == ENOENT || errno == EPROTONOSUPPORT) {
 			close(sockfd);
-			return 0;
+			/* Pretend revision 0 support for better error messaging */
+			if (revision == 0)
+				fprintf(stderr,
+					"Warning: Extension %s revision 0 not supported, missing kernel module?\n",
+					name);
+			return (revision == 0);
 		} else if (errno == ENOPROTOOPT) {
 			close(sockfd);
 			/* Assume only revision 0 support (old kernel) */