bpf: Allow to retrieve cgroup v1 classid from v2 hooks

Today, Kubernetes is still operating on cgroups v1, however, it is
possible to retrieve the task's classid based on 'current' out of
connect(), sendmsg(), recvmsg() and bind-related hooks for orchestrators
which attach to the root cgroup v2 hook in a mixed env like in case
of Cilium, for example, in order to then correlate certain pod traffic
and use it as part of the key for BPF map lookups.

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

include/net/cls_cgroup.h

···
 	sock_cgroup_set_classid(skcd, classid);
 }
 
+static inline u32 __task_get_classid(struct task_struct *task)
+{
+	return task_cls_state(task)->classid;
+}
+
 static inline u32 task_get_classid(const struct sk_buff *skb)
 {
-	u32 classid = task_cls_state(current)->classid;
+	u32 classid = __task_get_classid(current);
 
 	/* Due to the nature of the classifier it is required to ignore all
 	 * packets originating from softirq context as accessing `current'

net/core/filter.c

···
 	.arg4_type	= ARG_ANYTHING,
 };
 
+#ifdef CONFIG_CGROUP_NET_CLASSID
+BPF_CALL_0(bpf_get_cgroup_classid_curr)
+{
+	return __task_get_classid(current);
+}
+
+static const struct bpf_func_proto bpf_get_cgroup_classid_curr_proto = {
+	.func		= bpf_get_cgroup_classid_curr,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+};
+#endif
+
 BPF_CALL_1(bpf_get_cgroup_classid, const struct sk_buff *, skb)
 {
 	return task_get_classid(skb);
···
 		return &bpf_get_netns_cookie_sock_proto;
 	case BPF_FUNC_perf_event_output:
 		return &bpf_event_output_data_proto;
+#ifdef CONFIG_CGROUP_NET_CLASSID
+	case BPF_FUNC_get_cgroup_classid:
+		return &bpf_get_cgroup_classid_curr_proto;
+#endif
 	default:
 		return bpf_base_func_proto(func_id);
 	}
···
 		return &bpf_get_local_storage_proto;
 	case BPF_FUNC_perf_event_output:
 		return &bpf_event_output_data_proto;
+#ifdef CONFIG_CGROUP_NET_CLASSID
+	case BPF_FUNC_get_cgroup_classid:
+		return &bpf_get_cgroup_classid_curr_proto;
+#endif
 #ifdef CONFIG_INET
 	case BPF_FUNC_sk_lookup_tcp:
 		return &bpf_sock_addr_sk_lookup_tcp_proto;