[PATCH bpf-next 3/3] bpf: Fix ksock build with BPF_LSM=y and SECURITY_NETWORK=n

[Sasha Levin]

bpf_ksock.c references bpf_lsm_socket_sendmsg() whenever
CONFIG_BPF_LSM=y, but the socket LSM hooks, and with them the BPF LSM
socket_sendmsg hook, only exist with CONFIG_SECURITY_NETWORK=y. Without
it resolve_btfids warns:

  WARN: resolve_btfids: unresolved symbol bpf_lsm_socket_sendmsg

and since commit 201a454df948 ("resolve_btfids: Fix the --fatal_warnings
exit status") that fails the build with CONFIG_WERROR=y.

Only reference the hook with CONFIG_SECURITY_NETWORK=y, and use
BTF_ID_UNUSED otherwise. The ID stays zero, as it was when unresolved,
so bpf_ksock_kfunc_filter() keeps allowing LSM programs: none can be
attached to socket_sendmsg without CONFIG_SECURITY_NETWORK.

Fixes: 7ae4eb14c5f9 ("bpf: Add ksock kfuncs")
Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 net/core/bpf_ksock.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/core/bpf_ksock.c b/net/core/bpf_ksock.c
index 7bf796193ab61..48a2e2327acef 100644
--- a/net/core/bpf_ksock.c
+++ b/net/core/bpf_ksock.c
@@ -258,7 +258,12 @@ BTF_ID_FLAGS(func, bpf_ksock_send, KF_SLEEPABLE)
 BTF_KFUNCS_END(ksock_kfunc_btf_ids)
 
 #ifdef CONFIG_BPF_LSM
-BTF_ID_LIST_SINGLE(bpf_lsm_socket_sendmsg_id, func, bpf_lsm_socket_sendmsg)
+BTF_ID_LIST(bpf_lsm_socket_sendmsg_id)
+#ifdef CONFIG_SECURITY_NETWORK
+BTF_ID(func, bpf_lsm_socket_sendmsg)
+#else
+BTF_ID_UNUSED
+#endif
 #endif
 
 static int bpf_ksock_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id)
-- 
2.53.0