tc_classid access in skb bpf context

[Matthew Cover]

__sk_buff has a member tc_classid which I'm interested in accessing from the skb bpf context.

A bpf program which accesses skb->tc_classid compiles, but fails verification; the specific failure is "invalid bpf_context access".

if (skb->tc_classid != 0)
 return 1;
return 0;

Some of the tests in tools/testing/selftests/bpf/verifier/ (those on tc_classid) further confirm that this is, in all likelihood, intentional behavior.

The very similar bpf program which instead accesses skb->mark works as desired.

if (skb->mark != 0)
 return 1;
return 0;

I built a kernel (v5.1) with 4 instances of the following line removed from net/core/filter.c to test the behavior when the instructions pass verification.

    switch (off) {
-    case bpf_ctx_range(struct __sk_buff, tc_classid):
...
        return false;

It appears skb->tc_classid is always zero within my bpf program, even when I verify by other means (e.g. netfilter) that the value is set non-zero.

I gather that sk_buff proper sometimes (i.e. at some layers) has qdisc_skb_cb stored in skb->cb, but not always.

I suspect that the tc_classid is available at l3 (and therefore to utils like netfilter, ip route, tc), but not at l2 (and not to AF_PACKET).

Is it impractical to make skb->tc_classid available in this bpf context or is there just some plumbing which hasn't been connected yet?

Is my suspicion that skb->cb no longer contains qdisc_skb_cb due to crossing a layer boundary well founded?

I'm willing to look into hooking things together as time permits if it's a feasible task.

It's trivial to have iptables match on tc_classid and set a mark which is available to bpf at l2, but I'd like to better understand this.

Thanks,
Matt C.