bpf: add bpf_skb_cgroup_id helper

Add a new bpf_skb_cgroup_id() helper that allows to retrieve the
cgroup id from the skb's socket. This is useful in particular to
enable bpf_get_cgroup_classid()-like behavior for cgroup v1 in
cgroup v2 by allowing ID based matching on egress. This can in
particular be used in combination with applying policy e.g. from
map lookups, and also complements the older bpf_skb_under_cgroup()
interface. In user space the cgroup id for a given path can be
retrieved through the f_handle as demonstrated in [0] recently.

  [0] https://lkml.org/lkml/2018/5/22/1190

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 27 insertions, 2 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 885fb0e2f264..edbfaa613b6d 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3661,6 +3661,27 @@ static const struct bpf_func_proto bpf_skb_under_cgroup_proto = {
 	.arg3_type	= ARG_ANYTHING,
 };
 
+#ifdef CONFIG_SOCK_CGROUP_DATA
+BPF_CALL_1(bpf_skb_cgroup_id, const struct sk_buff *, skb)
+{
+	struct sock *sk = skb_to_full_sk(skb);
+	struct cgroup *cgrp;
+
+	if (!sk || !sk_fullsock(sk))
+		return 0;
+
+	cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
+	return cgrp->kn->id.id;
+}
+
+static const struct bpf_func_proto bpf_skb_cgroup_id_proto = {
+	.func           = bpf_skb_cgroup_id,
+	.gpl_only       = false,
+	.ret_type       = RET_INTEGER,
+	.arg1_type      = ARG_PTR_TO_CTX,
+};
+#endif
+
 static unsigned long bpf_xdp_copy(void *dst_buff, const void *src_buff,
 				  unsigned long off, unsigned long len)
 {
@@ -4747,12 +4768,16 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_socket_cookie_proto;
 	case BPF_FUNC_get_socket_uid:
 		return &bpf_get_socket_uid_proto;
+	case BPF_FUNC_fib_lookup:
+		return &bpf_skb_fib_lookup_proto;
 #ifdef CONFIG_XFRM
 	case BPF_FUNC_skb_get_xfrm_state:
 		return &bpf_skb_get_xfrm_state_proto;
 #endif
-	case BPF_FUNC_fib_lookup:
-		return &bpf_skb_fib_lookup_proto;
+#ifdef CONFIG_SOCK_CGROUP_DATA
+	case BPF_FUNC_skb_cgroup_id:
+		return &bpf_skb_cgroup_id_proto;
+#endif
 	default:
 		return bpf_base_func_proto(func_id);
 	}