[PATCH nf] netfilter: restore default behavior for nf_conntrack_events

[Nicolas Dichtel <nicolas.dichtel@6wind.com>]

Since the below commit, there are regressions for legacy setups:
1/ conntracks are created while there are no listener
2/ a listener starts and dumps all conntracks to get the current state
3/ conntracks deleted before the listener has started are not advertised

This is problematic in containers, where conntracks could be created early.
This sysctl is part of unsafe sysctl and could not be changed easily in
some environments.

Let's switch back to the legacy behavior.

CC: stable@vger.kernel.org
Fixes: 90d1daa45849 ("netfilter: conntrack: add nf_conntrack_events autodetect mode")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
---
 Documentation/networking/nf_conntrack-sysctl.rst | 10 ++++++----
 net/netfilter/nf_conntrack_ecache.c              |  2 +-
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/Documentation/networking/nf_conntrack-sysctl.rst b/Documentation/networking/nf_conntrack-sysctl.rst
index c383a394c665..edc04f99e1aa 100644
--- a/Documentation/networking/nf_conntrack-sysctl.rst
+++ b/Documentation/networking/nf_conntrack-sysctl.rst
@@ -34,13 +34,15 @@ nf_conntrack_count - INTEGER (read-only)
 
 nf_conntrack_events - BOOLEAN
 	- 0 - disabled
-	- 1 - enabled
-	- 2 - auto (default)
+	- 1 - enabled (default)
+	- 2 - auto
 
 	If this option is enabled, the connection tracking code will
 	provide userspace with connection tracking events via ctnetlink.
-	The default allocates the extension if a userspace program is
-	listening to ctnetlink events.
+	The 'auto' allocates the extension if a userspace program is
+	listening to ctnetlink events. Note that conntracks created
+	before the first listener has started won't trigger any netlink
+	event.
 
 nf_conntrack_expect_max - INTEGER
 	Maximum size of expectation table.  Default value is
diff --git a/net/netfilter/nf_conntrack_ecache.c b/net/netfilter/nf_conntrack_ecache.c
index 69948e1d6974..4c8559529e18 100644
--- a/net/netfilter/nf_conntrack_ecache.c
+++ b/net/netfilter/nf_conntrack_ecache.c
@@ -334,7 +334,7 @@ bool nf_ct_ecache_ext_add(struct nf_conn *ct, u16 ctmask, u16 expmask, gfp_t gfp
 }
 EXPORT_SYMBOL_GPL(nf_ct_ecache_ext_add);
 
-#define NF_CT_EVENTS_DEFAULT 2
+#define NF_CT_EVENTS_DEFAULT 1
 static int nf_ct_events __read_mostly = NF_CT_EVENTS_DEFAULT;
 
 void nf_conntrack_ecache_pernet_init(struct net *net)
-- 
2.43.1