AYA tc classifier does not support using tc_classid

[danielconnor]

A tc classifier eBPF program can set the tc_classid field to specify the class that should be used to handle the packet. A direct-action eBPF program only supports setting the minor 16 bits of the classid, meaning the classid/flowid attribute needs to be set on the eBPF configuration for it to work. AYA currently doesn't support setting the classid/flowid attribute(via TCA_BPF_CLASSID) on a classifier program.

Explaination

An aya tc classifier program is always direct-action
https://github.com/aya-rs/aya/blob/main/aya/src/sys/netlink.rs#L161-L162

direct-action means exts_integrated
https://github.com/torvalds/linux/blob/master/net/sched/cls_bpf.c#L480

If exts_integrated, the classid is prog->res.classid (a 32bit integer) ORed with qdisc_skb_cb(skb)->tc_classid, which is a 16bit integer - meaning the bpf program can only set the minor 16 bits.
https://github.com/torvalds/linux/blob/master/net/sched/cls_bpf.c#L110-L119

Since the program can only set the minor 16 bits and the major 16 bits need to be set, setting the tc_classid field from bpf has no effect unless the program classid/flowid is set.

Note: calling the property classid/flowid because that's what tc accepts and outputs as flowid (parsing classid/flowid
https://github.com/iproute2/iproute2/blob/d9b886d745ada3b8481e041ceca579c6f3acbea3/tc/f_bpf.c#L123-L132C26)

I've verified it works with the following change:

diff --git a/aya/src/sys/netlink.rs b/aya/src/sys/netlink.rs
index f10c283..46a2e73 100644
--- a/aya/src/sys/netlink.rs
+++ b/aya/src/sys/netlink.rs
@@ -19,6 +19,7 @@ use crate::{
     generated::{
         ifinfomsg, tcmsg, IFLA_XDP_EXPECTED_FD, IFLA_XDP_FD, IFLA_XDP_FLAGS, NLMSG_ALIGNTO,
         TCA_BPF_FD, TCA_BPF_FLAGS, TCA_BPF_FLAG_ACT_DIRECT, TCA_BPF_NAME, TCA_KIND, TCA_OPTIONS,
+        TCA_BPF_CLASSID,
         TC_H_CLSACT, TC_H_INGRESS, TC_H_MAJ_MASK, TC_H_UNSPEC, XDP_FLAGS_REPLACE,
     },
     programs::TcAttachType,
@@ -160,6 +161,8 @@ pub(crate) unsafe fn netlink_qdisc_attach(
     options.write_attr_bytes(TCA_BPF_NAME as u16, prog_name.to_bytes_with_nul())?;
     let flags: u32 = TCA_BPF_FLAG_ACT_DIRECT;
     options.write_attr(TCA_BPF_FLAGS as u16, flags)?;
+    options.write_attr(TCA_BPF_CLASSID as u16, 1 << 16)?;
+
     let options_len = options.finish()?;

     req.header.nlmsg_len += align_to(kind_len + options_len, NLA_ALIGNTO as usize) as u32;