Re: [RFC PATCH v9 06/11] bpf: net_sched: Add bpf qdisc kfuncs

On Thu, Jul 25, 2024 at 3:39 PM Martin KaFai Lau [off-list ref] wrote:

On 7/14/24 10:51 AM, Amery Hung wrote:

quoted

Add kfuncs for working on skb in qdisc.

Both bpf_qdisc_skb_drop() and bpf_skb_release() 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

Enforcing the bpf_qdisc_skb_drop() kfunc only available to the ".enqueue" is
achieved by the  "struct bpf_sk_buff_ptr" pointer type only available to the
".enqueue" ops ?

Yes. I assume it will be better to make this availability check
explicit using the .filter you mentioned.

quoted

the release. Otherwise, bpf_skb_release() should be used elsewhere. It
is also used in bpf_obj_free_fields() when cleaning up skb in maps and
collections.

bpf_qdisc_schedule() can be used to schedule the execution of the qdisc.
An example use case is to throttle a qdisc if the time to dequeue the
next packet is known.

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

Signed-off-by: Amery Hung <redacted>
---
  net/sched/bpf_qdisc.c | 74 ++++++++++++++++++++++++++++++++++++++++++-
  1 file changed, 73 insertions(+), 1 deletion(-)

diff --git a/net/sched/bpf_qdisc.c b/net/sched/bpf_qdisc.c
index a68fc115d8f8..eff7559aa346 100644
--- a/net/sched/bpf_qdisc.c
+++ b/net/sched/bpf_qdisc.c

@@ -148,6 +148,64 @@ 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_skb_release - Release an skb reference acquired on an skb immediately.
+ * @skb: The skb on which a reference is being released.
+ */
+__bpf_kfunc void bpf_skb_release(struct sk_buff *skb)
+{
+     consume_skb(skb);

snippet from the comment of consume_skb():

  *      Functions identically to kfree_skb, but kfree_skb assumes that the frame
  *      is being dropped after a failure and notes that

consume_skb() has a different tracepoint from the kfree_skb also. It is better
not to confuse the tracing.

I think at least the Qdisc_ops.reset and the btf_id_dtor_kfunc don't fall into
the consume_skb(). May be useful to add the kfree_skb[_reason?]() kfunc also?

I see. I will change bpf_skb_release() from using consume_skb() to
kfree_skb() (existing qdiscs are not using skb_drop_reason). The skb
cleanup mechanism when .reset is called can use rtnl_kfree_skbs().

quoted

+}
+
+/* bpf_qdisc_skb_drop - Add an skb to be dropped later to a list.
+ * @skb: The skb on which a reference is being 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_qdisc_watchdog_schedule - Schedule a qdisc to a later time using a timer.
+ * @sch: The qdisc to be scheduled.
+ * @expire: The expiry time of the timer.
+ * @delta_ns: The slack range of the timer.
+ */
+__bpf_kfunc void bpf_qdisc_watchdog_schedule(struct Qdisc *sch, u64 expire, u64 delta_ns)
+{
+     struct bpf_sched_data *q = qdisc_priv(sch);
+
+     qdisc_watchdog_schedule_range_ns(&q->watchdog, expire, delta_ns);
+}
+
+__bpf_kfunc_end_defs();
+
+BTF_KFUNCS_START(bpf_qdisc_kfunc_ids)
+BTF_ID_FLAGS(func, bpf_skb_get_hash)

Add KF_TRUSTED_ARGS. Avoid cases like getting a skb from walking the skb->next
for now.

Good point. Will do.

quoted

+BTF_ID_FLAGS(func, bpf_skb_release, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_qdisc_skb_drop, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_qdisc_watchdog_schedule)

Also add KF_TRUSTED_ARGS here.

quoted

+BTF_KFUNCS_END(bpf_qdisc_kfunc_ids)
+
+static const struct btf_kfunc_id_set bpf_qdisc_kfunc_set = {
+     .owner = THIS_MODULE,
+     .set   = &bpf_qdisc_kfunc_ids,
+};
+
+BTF_ID_LIST(skb_kfunc_dtor_ids)
+BTF_ID(struct, sk_buff)
+BTF_ID_FLAGS(func, bpf_skb_release, KF_RELEASE)
+
  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,

@@ -347,6 +405,20 @@ static struct bpf_struct_ops bpf_Qdisc_ops = {

  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       = skb_kfunc_dtor_ids[0],
+                     .kfunc_btf_id = skb_kfunc_dtor_ids[1]
+             },
+     };
+
+     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);