netdev - Unused variable 'mark' in v6.1-rc7

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Date:   Sun, 27 Nov 2022 17:30:47 -0800
From:   Ivan Babrou <ivan@...n.computer>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Unused variable 'mark' in v6.1-rc7

There's 52d1aa8b8249 in v6.1-rc7:

* netfilter: conntrack: Fix data-races around ct mark

It triggers an error:

#19 355.8 /build/linux-source/net/netfilter/nf_conntrack_netlink.c: In
function '__ctnetlink_glue_build':
#19 355.8 /build/linux-source/net/netfilter/nf_conntrack_netlink.c:2674:13:
error: unused variable 'mark' [-Werror=unused-variable]
#19 355.8  2674 |         u32 mark;
#19 355.8       |             ^~~~
#19 355.8 cc1: all warnings being treated as errors

If CONFIG_NF_CONNTRACK_MARK is not enabled, as mark is declared
unconditionally, but used under ifdef:

 #ifdef CONFIG_NF_CONNTRACK_MARK
-       if ((events & (1 << IPCT_MARK) || ct->mark)
-           && ctnetlink_dump_mark(skb, ct) < 0)
+       mark = READ_ONCE(ct->mark);
+       if ((events & (1 << IPCT_MARK) || mark) &&
+           ctnetlink_dump_mark(skb, mark) < 0)
                goto nla_put_failure;
 #endif

To have NF_CONNTRACK_MARK one needs NETFILTER_ADVANCED:

config NF_CONNTRACK_MARK
        bool  'Connection mark tracking support'
        depends on NETFILTER_ADVANCED

It's supposed to be enabled by default:

config NETFILTER_ADVANCED
        bool "Advanced netfilter configuration"
        depends on NETFILTER
        default y

But it's not in defconfig (it's missing from arm64 completely):

$ rg NETFILTER_ADVANCED arch/x86/configs/x86_64_defconfig
93:# CONFIG_NETFILTER_ADVANCED is not set

I think the solution is to enclose mark definition into ifdef as well
and I'm happy to send a patch if you agree and would like me to.