netdev - build failure (BTFIDS) with CONFIG_NET && !CONFIG

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Date:   Tue, 29 Sep 2020 12:17:37 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yonghong Song <yhs@...com>
Subject: build failure (BTFIDS) with CONFIG_NET && !CONFIG_INET

Hello,

our builds of s390x for zfcpdump fail since 5.9-rc1 with

    BTFIDS  vmlinux
  FAILED unresolved symbol tcp_timewait_sock
  make[1]: *** [/home/abuild/rpmbuild/BUILD/kernel-zfcpdump-5.9.rc7/linux-5.9-rc7/Makefile:1176: vmlinux] Error 255

I believe this is caused by commit fce557bcef11 ("bpf: Make btf_sock_ids
global") and the problem is caused by a specific configuration which has
CONFIG_NET enabled but CONFIG_INET disabled. IIUC there will be no user
of struct tcp_timewait_sock but btf_ids will try to generate BTF info
for it.

Michal