[PATCH bpf-next 4/7] bpf: allow to retrieve cgroup v1 classid from v2 hooks

[Daniel Borkmann <daniel@iogearbox.net>]

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 <daniel@iogearbox.net>
---
 include/net/cls_cgroup.h |  7 ++++++-
 net/core/filter.c        | 21 +++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/include/net/cls_cgroup.h b/include/net/cls_cgroup.h
index 4295de3e6a4b..7e78e7d6f015 100644
--- a/include/net/cls_cgroup.h
+++ b/include/net/cls_cgroup.h
@@ -45,9 +45,14 @@ static inline void sock_update_classid(struct sock_cgroup_data *skcd)
 	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'
diff --git a/net/core/filter.c b/net/core/filter.c
index e249a499cbe5..3083c7746ee0 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2642,6 +2642,19 @@ static const struct bpf_func_proto bpf_msg_pop_data_proto = {
 	.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);
@@ -6005,6 +6018,10 @@ sock_filter_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		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);
 	}
@@ -6035,6 +6052,10 @@ sock_addr_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		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;
-- 
2.21.0