linux-kernel - [PATCH 5.18 26/88] bridge: Do not send empty IFLA_AF

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Date:   Mon,  1 Aug 2022 13:46:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ido Schimmel <idosch@...dia.com>,
        Benjamin Poirier <bpoirier@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: [PATCH 5.18 26/88] bridge: Do not send empty IFLA_AF_SPEC attribute

From: Benjamin Poirier <bpoirier@...dia.com>

commit 9b134b1694ec8926926ba6b7b80884ea829245a0 upstream.

After commit b6c02ef54913 ("bridge: Netlink interface fix."),
br_fill_ifinfo() started to send an empty IFLA_AF_SPEC attribute when a
bridge vlan dump is requested but an interface does not have any vlans
configured.

iproute2 ignores such an empty attribute since commit b262a9becbcb
("bridge: Fix output with empty vlan lists") but older iproute2 versions as
well as other utilities have their output changed by the cited kernel
commit, resulting in failed test cases. Regardless, emitting an empty
attribute is pointless and inefficient.

Avoid this change by canceling the attribute if no AF_SPEC data was added.

Fixes: b6c02ef54913 ("bridge: Netlink interface fix.")
Reviewed-by: Ido Schimmel <idosch@...dia.com>
Signed-off-by: Benjamin Poirier <bpoirier@...dia.com>
Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>
Link: https://lore.kernel.org/r/20220725001236.95062-1-bpoirier@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/bridge/br_netlink.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -589,9 +589,13 @@ static int br_fill_ifinfo(struct sk_buff
 	}

 done:
+	if (af) {
+		if (nlmsg_get_pos(skb) - (void *)af > nla_attr_size(0))
+			nla_nest_end(skb, af);
+		else
+			nla_nest_cancel(skb, af);
+	}

-	if (af)
-		nla_nest_end(skb, af);
 	nlmsg_end(skb, nlh);
 	return 0;