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 <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/core/filter.c

···
 	.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);
···
 		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;