bpf: Allow bpf_get_netns_cookie in BPF_PROG_TYPE_SK_MSG

We'd like to be able to identify netns from sk_msg hooks
to accelerate local process communication form different netns.
Signed-off-by: Xu Liu <liuxu623@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210820071712.52852-2-liuxu623@gmail.com

net/core/filter.c

@@ -4688,6 +4688,18 @@ static const struct bpf_func_proto bpf_get_netns_cookie_sock_ops_proto = {
 	.arg1_type	= ARG_PTR_TO_CTX_OR_NULL,
 };
 
+BPF_CALL_1(bpf_get_netns_cookie_sk_msg, struct sk_msg *, ctx)
+{
+	return __bpf_get_netns_cookie(ctx ? ctx->sk : NULL);
+}
+
+static const struct bpf_func_proto bpf_get_netns_cookie_sk_msg_proto = {
+	.func		= bpf_get_netns_cookie_sk_msg,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX_OR_NULL,
+};
+
 BPF_CALL_1(bpf_get_socket_uid, struct sk_buff *, skb)
 {
 	struct sock *sk = sk_to_full_sk(skb->sk);
@@ -7551,6 +7563,8 @@ sk_msg_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_sk_storage_get_proto;
 	case BPF_FUNC_sk_storage_delete:
 		return &bpf_sk_storage_delete_proto;
+	case BPF_FUNC_get_netns_cookie:
+		return &bpf_get_netns_cookie_sk_msg_proto;
 #ifdef CONFIG_CGROUPS
 	case BPF_FUNC_get_current_cgroup_id:
 		return &bpf_get_current_cgroup_id_proto;