netfilter: conntrack: make filtering by zone discoverable

Conntrack flush supports filtering by zone using CTA_ZONE, but this
attribute is really hard to use from user space applications.  The
reason is that it is not possible to tell if it's supported or not.

Older kernels silently ignore CTA_ZONE.  And in that case they just
happily flush all the entries from all zones breaking all the existing
connections.  So, applications have to infer support from the kernel
version.  While it works in most cases, it's not a particularly
reliable or desired way to check kernel capabilities from applications
that aim to be portable.  There should be a better way to probe or
discover features in the kernel.

The CTA_FILTER interface on the other hand is simple enough to probe.
We can check for NLM_F_DUMP_FILTERED in the dump to see if filtering
is supported.  And unknown sub-attributes in CTA_FILTER are rejected
explicitly since strict validation is in use there.

Let's add new CTA_FILTER_ZONE that signals that CTA_ZONE should be
filtered on.  It is a flag, since everything in the CTA_FILTER is a
bit mask, i.e., a form of a flag.  If set, it means that CTA_ZONE must
be present and be used for filtering.  If the flag is not set however,
the filtering on CTA_ZONE will still take place to ensure backwards
compatibility.  So, the flag doesn't really change the filtering
behavior, but it allows user space applications to properly discover
support for CTA_ZONE filtering without need to rely on kernel version
parsing or risk accidental flushes of the entire conntrack table,
and also without modifying the kernel state.

A new test variant is added to test with and without the new flag.
Since the setup code is moved into a shared function, expectations
replaced with assertions to bail early if the base setup fails to
avoid the cascade of secondary failures that can be misleading.
Error return is only for the SKIP cases.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/uapi/linux/netfilter/nfnetlink_conntrack.h b/include/uapi/linux/netfilter/nfnetlink_conntrack.h
index 43233af75b9d..985f9c08d3e9 100644
--- a/include/uapi/linux/netfilter/nfnetlink_conntrack.h
+++ b/include/uapi/linux/netfilter/nfnetlink_conntrack.h
@@ -285,6 +285,7 @@ enum ctattr_filter {
 	CTA_FILTER_UNSPEC,
 	CTA_FILTER_ORIG_FLAGS,
 	CTA_FILTER_REPLY_FLAGS,
+	CTA_FILTER_ZONE,
 	__CTA_FILTER_MAX
 };
 #define CTA_FILTER_MAX (__CTA_FILTER_MAX - 1)