[PATCH bpf-next v5 04/13] bpf: net_sched: Add basic bpf qdisc kfuncs

[Amery Hung <ameryhung@gmail.com>]

From: Amery Hung <amery.hung@bytedance.com>

Add basic kfuncs for working on skb in qdisc.

Both bpf_qdisc_skb_drop() and bpf_kfree_skb() can be used to release
a reference to an skb. However, bpf_qdisc_skb_drop() can only be called
in .enqueue where a to_free skb list is available from kernel to defer
the release. bpf_kfree_skb() should be used elsewhere. It is also used
in bpf_obj_free_fields() when cleaning up skb in maps and collections.

bpf_skb_get_hash() returns the flow hash of an skb, which can be used
to build flow-based queueing algorithms.

Finally, allow users to create read-only dynptr via bpf_dynptr_from_skb().

Signed-off-by: Amery Hung <amery.hung@bytedance.com>
---
 include/linux/bpf.h         |  1 +
 kernel/bpf/bpf_struct_ops.c |  2 +
 net/sched/bpf_qdisc.c       | 93 ++++++++++++++++++++++++++++++++++++-
 3 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 463e922cb0f5..d3b0c4ccaebf 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1826,6 +1826,7 @@ struct bpf_struct_ops {
 	void *cfi_stubs;
 	struct module *owner;
 	const char *name;
+	const struct btf_type *type;
 	struct btf_func_model func_models[BPF_STRUCT_OPS_MAX_NR_MEMBERS];
 };
 
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 1a5a9dee1e4a..e0a8a9319b84 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -461,6 +461,8 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
 		goto errout;
 	}
 
+	st_ops->type = t;
+
 	return 0;
 
 errout:
diff --git a/net/sched/bpf_qdisc.c b/net/sched/bpf_qdisc.c
index 00f3232f4a98..69a1d547390c 100644
--- a/net/sched/bpf_qdisc.c
+++ b/net/sched/bpf_qdisc.c
@@ -111,6 +111,80 @@ static int bpf_qdisc_btf_struct_access(struct bpf_verifier_log *log,
 	return 0;
 }
 
+__bpf_kfunc_start_defs();
+
+/* bpf_skb_get_hash - Get the flow hash of an skb.
+ * @skb: The skb to get the flow hash from.
+ */
+__bpf_kfunc u32 bpf_skb_get_hash(struct sk_buff *skb)
+{
+	return skb_get_hash(skb);
+}
+
+/* bpf_kfree_skb - Release an skb's reference and drop it immediately.
+ * @skb: The skb whose reference to be released and dropped.
+ */
+__bpf_kfunc void bpf_kfree_skb(struct sk_buff *skb)
+{
+	kfree_skb(skb);
+}
+
+/* bpf_qdisc_skb_drop - Drop an skb by adding it to a deferred free list.
+ * @skb: The skb whose reference to be released and dropped.
+ * @to_free_list: The list of skbs to be dropped.
+ */
+__bpf_kfunc void bpf_qdisc_skb_drop(struct sk_buff *skb,
+				    struct bpf_sk_buff_ptr *to_free_list)
+{
+	__qdisc_drop(skb, (struct sk_buff **)to_free_list);
+}
+
+__bpf_kfunc_end_defs();
+
+BTF_KFUNCS_START(qdisc_kfunc_ids)
+BTF_ID_FLAGS(func, bpf_skb_get_hash, KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_kfree_skb, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_qdisc_skb_drop, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_dynptr_from_skb, KF_TRUSTED_ARGS)
+BTF_KFUNCS_END(qdisc_kfunc_ids)
+
+BTF_SET_START(qdisc_common_kfunc_set)
+BTF_ID(func, bpf_skb_get_hash)
+BTF_ID(func, bpf_kfree_skb)
+BTF_ID(func, bpf_dynptr_from_skb)
+BTF_SET_END(qdisc_common_kfunc_set)
+
+BTF_SET_START(qdisc_enqueue_kfunc_set)
+BTF_ID(func, bpf_qdisc_skb_drop)
+BTF_SET_END(qdisc_enqueue_kfunc_set)
+
+static int bpf_qdisc_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id)
+{
+	if (bpf_Qdisc_ops.type != btf_type_by_id(prog->aux->attach_btf,
+						 prog->aux->attach_btf_id))
+		return 0;
+
+	/* Skip the check when prog->attach_func_name is not yet available
+	 * during check_cfg().
+	 */
+	if (!btf_id_set8_contains(&qdisc_kfunc_ids, kfunc_id) ||
+	    !prog->aux->attach_func_name)
+		return 0;
+
+	if (bpf_struct_ops_prog_moff(prog) == offsetof(struct Qdisc_ops, enqueue)) {
+		if (btf_id_set_contains(&qdisc_enqueue_kfunc_set, kfunc_id))
+			return 0;
+	}
+
+	return btf_id_set_contains(&qdisc_common_kfunc_set, kfunc_id) ? 0 : -EACCES;
+}
+
+static const struct btf_kfunc_id_set bpf_qdisc_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set   = &qdisc_kfunc_ids,
+	.filter = bpf_qdisc_kfunc_filter,
+};
+
 static const struct bpf_verifier_ops bpf_qdisc_verifier_ops = {
 	.get_func_proto		= bpf_qdisc_get_func_proto,
 	.is_valid_access	= bpf_qdisc_is_valid_access,
@@ -203,8 +277,25 @@ static struct bpf_struct_ops bpf_Qdisc_ops = {
 	.owner = THIS_MODULE,
 };
 
+BTF_ID_LIST(bpf_sk_buff_dtor_ids)
+BTF_ID(func, bpf_kfree_skb)
+
 static int __init bpf_qdisc_kfunc_init(void)
 {
-	return register_bpf_struct_ops(&bpf_Qdisc_ops, Qdisc_ops);
+	int ret;
+	const struct btf_id_dtor_kfunc skb_kfunc_dtors[] = {
+		{
+			.btf_id       = bpf_sk_buff_ids[0],
+			.kfunc_btf_id = bpf_sk_buff_dtor_ids[0]
+		},
+	};
+
+	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &bpf_qdisc_kfunc_set);
+	ret = ret ?: register_btf_id_dtor_kfuncs(skb_kfunc_dtors,
+						 ARRAY_SIZE(skb_kfunc_dtors),
+						 THIS_MODULE);
+	ret = ret ?: register_bpf_struct_ops(&bpf_Qdisc_ops, Qdisc_ops);
+
+	return ret;
 }
 late_initcall(bpf_qdisc_kfunc_init);
-- 
2.47.1