netdev - [PATCH bpf v3] bpf: fix "unresolved symbol" build error with resolve

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Date:   Wed, 30 Sep 2020 22:13:39 -0700
From:   Yonghong Song <yhs@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        Michal Kubecek <mkubecek@...e.cz>
Subject: [PATCH bpf v3] bpf: fix "unresolved symbol" build error with resolve_btfids

Michal reported a build failure likes below:
   BTFIDS  vmlinux
   FAILED unresolved symbol tcp_timewait_sock
   make[1]: *** [/.../linux-5.9-rc7/Makefile:1176: vmlinux] Error 255

This error can be triggered when config has CONFIG_NET enabled
but CONFIG_INET disabled. In this case, there is no user of
structs inet_timewait_sock and tcp_timewait_sock and hence vmlinux BTF
types are not generated for these two structures.

To fix the problem, let us force BTF generation for these two
structures with BTF_TYPE_EMIT.

Fixes: fce557bcef11 ("bpf: Make btf_sock_ids global")
Reported-by: Michal Kubecek <mkubecek@...e.cz>
Signed-off-by: Yonghong Song <yhs@...com>
---
 net/core/filter.c | 6 ++++++
 1 file changed, 6 insertions(+)

Changelog:
  v2 -> v3:
    . instead of avoiding generating BTF types which is more
      complex, just always generating these BTF types (under CONFIG_NET).
  v1 -> v2:
    . For unsupported socket times, using BTF_ID_UNUSED to
      keep the id as 0. (Martin)

diff --git a/net/core/filter.c b/net/core/filter.c
index 21eaf3b182f2..b5f3faac5e3b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -9558,6 +9558,12 @@ const struct bpf_func_proto bpf_skc_to_tcp_sock_proto = {
 
 BPF_CALL_1(bpf_skc_to_tcp_timewait_sock, struct sock *, sk)
 {
+	/* BTF types for tcp_timewait_sock and inet_timewait_sock are not
+	 * generated if CONFIG_INET=n. Trigger an explicit generation here.
+	 */
+	BTF_TYPE_EMIT(struct inet_timewait_sock);
+	BTF_TYPE_EMIT(struct tcp_timewait_sock);
+
 #ifdef CONFIG_INET
 	if (sk && sk->sk_prot == &tcp_prot && sk->sk_state == TCP_TIME_WAIT)
 		return (unsigned long)sk;
-- 
2.24.1