[PATCH conntrack,v4] conntrack: label update requires a previous label in place

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As of kernel 6.6-rc, your ruleset must use either the 'connlabel' match
in iptables or the 'ct label' statement in nftables to attach labels to
conntrack entries. Update documentation to describe this behaviour.

This patch addresses a corner case scenario: conntrack already contains
entries but ruleset that specifies connlabel did not get loaded yet.
In such case, skip ENOSPC errors for conntracks that have ct label

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1622
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
 conntrack.8     | 4 ++++
 src/conntrack.c | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/conntrack.8 b/conntrack.8
index 031eaa4e9fef..3b6a15b5152d 100644
--- a/conntrack.8
+++ b/conntrack.8
@@ -193,6 +193,10 @@ Use multiple \-l options to specify multiple labels that need to be set.
 Specify the conntrack label to add to the selected conntracks.
 This option is only available in conjunction with "\-I, \-\-create",
 "\-A, \-\-add" or "\-U, \-\-update".
+As a rule of thumb, you must use either the 'connlabel' match in your iptables
+ruleset or the 'ct label' statement in your nftables ruleset, this turns on the
+ct label support in the kernel and it allows you to update labels via
+"\-U, \-\-update", otherwise label updates are ignored.
 .BI "--label-del " "[LABEL]"
 Specify the conntrack label to delete from the selected conntracks.
diff --git a/src/conntrack.c b/src/conntrack.c
index f9758d78d39b..c1551cadbdb3 100644
--- a/src/conntrack.c
+++ b/src/conntrack.c
@@ -2195,6 +2195,11 @@ static int mnl_nfct_update_cb(const struct nlmsghdr *nlh, void *data)
 		/* the entry has vanish in middle of the update */
 		if (errno == ENOENT)
 			goto destroy_ok;
+		else if (cmd->options & (CT_OPT_ADD_LABEL | CT_OPT_DEL_LABEL) &&
+			 !nfct_attr_is_set(ct, ATTR_CONNLABELS) &&
+			 errno == ENOSPC)
+			goto destroy_ok;
 			   "Operation failed: %s",
 			   err2str(errno, CT_UPDATE));

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